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Temporal Junctures in the Mind
Please cite this article in press as: Redshaw and Suddendorf, Temporal Junctures in the Mind, Trends in Cognitive Sciences (2019), https:// doi.org/10.1016/j.tics.2019.10.009
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Opinion
Temporal Junctures in the Mind Jonathan Redshaw1,* and Thomas Suddendorf1 Humans can imagine what happened in the past and what will happen in the future, but also what did not happen and what might happen. We reflect on envisioned events from alternative timelines, while knowing that we only ever live on one timeline. Considering alternative timelines rests on representations of temporal junctures, or points in time at which possible versions of reality diverge. These representations become increasingly sophisticated over childhood, first enabling preparation for mutually exclusive future possibilities and later the experience of counterfactual emotions like regret. By contrast, it remains unclear whether non-human animals represent temporal junctures at all. The emergence of these representations may have been a prime mover in human evolution.
Possible Worlds: Past, Present, and Future [For] an intellect which at a certain moment would know all forces that set nature in motion, and all positions of all items of which nature was composed . nothing would be uncertain and the future just like the past would be present before its eyes. – Pierre-Simon Laplace, A Philosophical Essay on Probabilities (1814) A cat is penned up in a steel chamber, along with . a tiny bit of radioactive substance . If one has left this entire system to itself for an hour, one would say that the cat still lives if meanwhile no atom has decayed. The first atomic decay would have poisoned it. The psi-function of the entire system would express this by having in it the living and dead cat mixed or smeared out in equal parts. – Erwin Schro¨dinger, The Present Situation in Quantum Mechanics (1935) Laplace’s Demon [1,2] and Schro¨dinger’s Cat [3,4] continue to be bones of contention among philosophers and physicists alike. Do we live in a deterministic universe, or is there room for chance and choice? Does quantum mechanics really entail that multiple versions of unitary phenomena exist simultaneously? For most humans, however, the implications of these thought experiments are preposterous. ‘Of course the future is uncertain; of course the cat is alive or dead.’ The reason these hypotheticals evoke such visceral denials is because they fundamentally violate our folk psychological notions of reality. Humans readily envision alternative possible versions of single future events, often stemming from alternative volitional actions, and evaluate them in terms of likelihood and desirability [5–7]. Likewise, we readily bring to mind alternative possible versions of past events and consider how they might have led to alternative versions of the present [8–10]. Yet, we also appreciate that these alternative timelines are mutually exclusive. Our minds might visit other possible worlds, but our bodies remain forever confined to one. The capacity to imagine and evaluate events from mutually exclusive timelines is currently a hot topic of research in developmental [11–16], comparative [15,17–21], and clinical psychology [22–26], as well as in cognitive neuroscience [7,8,27–29]. In this opinion article, we propose that thinking about alternative timelines principally rests on representations of temporal junctures (see Glossary), or points in time at which subjectively possible worlds branch off from one another. We first briefly describe these representations and situate our perspective in relation to other recent conceptualisations of reasoning about possibility and time. We then summarise children’s protracted development in this domain, from initially representing temporal junctures located in the future to later representing temporal junctures located in the past, and propose a representational hierarchy to explain this trajectory. We examine the question of whether representations of temporal junctures are uniquely human, and outline some of the many adaptive benefits these representations can provide.
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Highlights Many recent studies have examined the development, evolution, dysfunctions, and neurocognitive mechanisms of the capacity to consider events from alternative timelines. Children begin to consistently prepare for alternative versions of immediate future events around 4 years of age, but do not accurately consider alternative versions of past events and present situations until around 6 years of age. There is no compelling evidence that non-human animals consider and compare events from alternative timelines, although interpretations of some results remain contentious. Representing alternative versions of the future enables humans to form contingency plans and otherwise compensate for their inability to perfectly predict future outcomes. Representing alternative versions of the past enables humans to learn associations not only between actual behaviours and outcomes, but also counterfactual behaviours and outcomes.
1School of Psychology, University of Queensland, Brisbane, QLD 4072, Australia
*Correspondence: [email protected]
https://doi.org/10.1016/j.tics.2019.10.009 ª 2019 Elsevier Ltd. All rights reserved.
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Temporal Junctures: Live and Expired Our perspective on temporal junctures shares some similarities with Byrne’s notion of counterfactual fault lines [30,31], or the aspects of past events that people tend to mentally adjust to create alternative versions of the present. You may, for instance, spend an inordinate amount of time wondering what the present would look like if you had stayed together with a former romantic partner, or if an alternative candidate had won a crucial election. Whereas Byrne emphasises past divergence points, however, one can of course also think about future divergence points. Furthermore, whereas Byrne narrows her focus to the particular biases that people have when imagining past alternatives, here we refer to the more general capacity to represent temporal junctures as being located at any time point and for any event outcome. Representing a temporal juncture includes the critical understanding that all subjectively possible outcomes of the juncture are alternatives of precisely the same event: we understand that a single flipped coin must land on heads or tails, but never on both. With representations of temporal junctures, therefore, we acquire McCormack and Hoerl’s notion of event-independent time [32–34]. Generally speaking, we can think of each moment in time as a single ‘slot’ [32] that can only ever be filled by one particular state of the world. Yet, because we can represent temporal junctures, we also understand that this one particular state of the world is only a sample of the many possible worlds that could fill (or could have filled) that slot. In one sense, representations of temporal junctures may simply be construed as a first-person, temporally oriented form of false-belief understanding: people understand that they can imagine both true and false versions of past and future events. However, temporal junctures also have a characteristic asymmetry: true versions of past events are typically represented as being fixed, whereas true versions of future events are often represented as being open. In other words, only in the future is a temporal juncture ‘live’ [35], in the sense that a person experiences that each subjectively possible outcome of an event both could and could not happen. Once the outcome has become known, however, a person recognises that the temporal juncture is ‘expired’, in the sense that each previously possible outcome either did or did not happen. To summarise, we consider representations of temporal junctures to have three key qualities: (i) they can be represented as being located at any point in time in both the past and the future; (ii) they denote single points in time at which alternative represented outcomes of the same event diverge; and (iii) they are typically asymmetrical, in that future junctures are live and past junctures are expired.
Children’s Representations of Temporal Junctures In developmental psychology [36–39], as in comparative psychology [40–42], the basic capacity for mental time travel is often examined using variations of the two-rooms task [36] (also known as the spoon test [43]). In brief, children are first exposed to a problem in one room, before being taken to a second room where they later have the chance to select the solution (from among distractors) to take back to the first room. Experiments with two-rooms tasks have provided strong evidence for both memory of unique past events and preparation for specific future situations in young children [37,44]. Yet, some versions of the task (especially those used with animals) have been subject to considerable controversy, arguably failing to control for simple associative learning explanations [44–49]. Be that as it may, for present purposes it should be noted that the two-rooms task is aimed only at assessing the capacity to imagine and link events from a single timeline: children have reason to believe that, as long as they accurately recall the problem from the first room, then selecting the appropriate item in the second room will enable them to solve the problem later on [50]. Passing does not, however, necessarily require children to represent the alternative: that if they do not select the item they will not solve the problem upon return later on. Instead, children’s capacities to represent temporal junctures and alternative timelines have been assessed by different lines of research.
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Glossary Bayesian priors: data-driven inferences about the likelihood of particular uncertain outcomes occurring in the future. In cognition, Bayesian priors may be experienced as intuitions about uncertain outcomes rather than rationally derived predictions. Causally overdetermined events: events that were in fact caused by one antecedent but would have been caused by a second antecedent in the absence of the first. Counterfactual thinking: accurately representing how the present version of reality would look had an alternative, nonveridical version of the past transpired. Mental time travel: the capacity to mentally re-experience past episodes and pre-experience future episodes, and to situate these imagined episodes in relation to the present. Metaforesight: the overarching capacity to reflect on alternative future possibilities and optimise behaviour accordingly. Metaforesight may express itself in various intelligent behaviours, such as contingency planning to compensate for one’s inherent inability to predict some future outcomes with certainty. Metarepresentation: representing representational relations. That is, representing that representations are just representations that need not perfectly correspond to their referents. Regret: the negative emotion experienced when one believes that an alternative, nonveridical past choice would have led to a more desirable present than the actual past choice. Relief: the positive emotion experienced when one believes that an alternative, nonveridical past choice would have led to a less desirable present than the actual past choice. Sense of free will: the sense that one has genuine choice between alternative courses of future action, regardless of whether such choice in fact exists. Subjectively possible outcome: from the perspective of the agent, there is a non-zero chance that the represented outcome may correspond to the true outcome. Note that in some cases the outcome may have already eventuated and
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Live Temporal Junctures We recently developed a minimalist, principally nonverbal task to assess children’s (and non-human primates’) ability to simultaneously prepare for two mutually exclusive versions of a single future event. In the original ‘forked tube’ task [15], children aged between 2 and 4 years were invited to catch a reward that an experimenter dropped into an inverted Y-shaped tube with one opening at the top and two possible exits at the bottom (Box 1). Children could ensure they would always catch the reward by simply covering both exits when preparing for the drop, whereas those who prepared by covering only one exit missed the reward approximately half of the time. This task is plainly future-oriented, in the minimal sense that children are given the opportunity to prepare for an imminent event. Yet, it also greatly relaxes demands on other capacities normally associated with thinking about the future. Unlike the two-rooms task, for example, the forked tube task places no demands on episodic memory or spatial scene construction. Instead, it isolates the basic capacity to represent alternative future timelines for which the temporal juncture will occur in one’s immediate spatial proximity. In the original study, we found that most 4-year-olds and some 3-year-olds spontaneously and consistently covered both exits of the forked tube, whereas 2-year-olds typically covered one exit only. And although some 2-year-olds and many 3-year-olds covered two exits on some occasions across 12 trials, a substantial proportion of these children regressed to covering only one exit on subsequent trials. This response pattern is consistent with the possibility that many younger children were weakly conditioned into showing the optimal response over their experience with the task [15]. The 4year-olds, however, did not regress after first covering both exits, suggesting they had insight into the optimality of that response.
yet the agent remains unaware of it (e.g., when watching a prerecorded sporting match). In such cases, the temporal juncture may nonetheless be represented as live, as multiple outcomes still remain possible from the agent’s perspective. Temporal embedding: a representational perspective with a particular temporal orientation (forward-looking or backwardlooking) is represented as being located within another representational perspective with its own temporal orientation. Regret and relief, for instance, arise when a forward-looking representational perspective is embedded within a backward-looking representational perspective. Temporal junctures: represented points in time at which multiple subjectively possible versions of reality diverge from each other.
To examine the generalisability of these findings, we subsequently administered the forked tube task to 3- to 6-year-old children from three diverse cultures [16]. One sample was from the same industrialised western city as in the original study (Brisbane, Australia), whereas the other two were from remote indigenous communities in the Australian Outback and the Kalahari Desert. Some 3-year-olds from all groups spontaneously and consistently covered two exits, and the majority of the oldest children from each sample passed every trial. This pattern of results indicates that humans may universally acquire the capacity to prepare for alternative futures in early childhood (also see [51,52]). In another study, we developed a version of the paradigm where the uncertain outcome was visibly controlled by a social agent rather than by a hidden physical mechanism. We reasoned that children may perform better under conditions of social uncertainty, and when the spatial point of the temporal juncture is in plain sight [17]. The results again replicated the finding of improved performance from 2 to 4 years of age, but children failed to perform better than on the original forked tube task. Critically, in two other recent studies [18,53], we found that 2.5- and 3-year-olds who struggled to prepare for alternative possibilities nevertheless performed close to ceiling on control tasks requiring them to use two hands to catch two rewards dropped simultaneously on certain trajectories. This pattern suggests that young children’s difficulties with the original tasks were due to the uncertainty of the single outcome rather than with manual motor coordination. On the whole, these findings suggest that many 4-year-olds but few younger children can mentally represent and prepare for alternative possible outcomes of an uncertain future event. Notably, several studies [15,17–19] have found that non-human great apes and monkeys, like young children, tend to prepare for only a single version of the future on uncertain versions of the task. Interpretations of these negative results, however, remain contentious (Box 1). Curiously, most younger children and primates switch between covering the left and right exit across trials (and some switch during trials), suggesting that they can indeed envision both possible outcomes in some basic sense. Yet, when 4-year-olds spontaneously and consistently cover two exits to catch the single reward, they appear to also demonstrate the critical understanding that these two outcomes are alternatives of the same event. That is, these children represent the live temporal juncture ahead of them.
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Box 1. Can Non-human Primates Represent Alternative Futures? Using several variations of the forked tube paradigm [15,17,18], we have found that non-human primates typically cover only one exit when preparing to catch a single food item (Figure I). Although negative findings are difficult to interpret, these results raise the possibility that other primates cannot represent live temporal junctures and alternative future timelines. Recently, Lambert and Osvath [19] replicated chimpanzees’ poor performance but also found that, in contrast to our results with young children [18,53], the apes performed relatively poorly when given the opportunity to catch two rewards with certain trajectories (also see our recent data from monkeys [18]). The authors suggested that, rather than being unable to conceive of future alternatives, nonhuman primates may simply lack the motor coordination required to cover two tube exits simultaneously. However, although chimpanzees’ performance in this study was indeed relatively low across tasks, in the main experiment they were seven times more likely to pass certain-trajectory trials (74/301: 24.4%) than uncertaintrajectory trials (5/144: 3.4%). The apes never passed more than one uncertain-trajectory trial in a row, whereas there were several long streaks of passing certain-trajectory trials (i.e., 8, 8, 13, 16, and 18 in a row). These results suggest that chimpanzees’ struggles on the original tasks may be at least partly explained by future uncertainty. Notably, covering one exit on certain-trajectory trials does guarantee some food reward, whereas on uncertain-trajectory trials it does not; and only on certain-trajectory trials must apes attend (through cage mesh) to two separate locations to notice the two rewards. The apes’ overall poor performance on certain-trajectory trials may be related to these factors. We disagree with these authors’ suggestion that eye-tracking methods could make an important contribution, as an animal that simply glances from one tube exit to the other does not necessarily possess the critical understanding that both possibilities are alternatives of the same event [67]. Nonetheless, we do agree that ruling out problems deriving from motor coordination is important, and so future studies should incorporate further types of simultaneous response options in preparation for mutually exclusive future alternatives. We reassert that strong conclusions about non-human primates’ capacities in this domain are not yet appropriate [15], as the negative evidence remains confined to variations of a single paradigm. After all, absence of evidence is not evidence of absence (see [75,110]).
Figure I. Depiction of the Original Forked Tube Task. Many 4-year-old children spontaneously and consistently cover both exits, whereas younger children and nonhuman primates typically cover one exit only. Reprinted, with permission, from [15].
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Expired Temporal Junctures Children’s representations of expired temporal junctures may not begin to mature until around 2 years later than their understanding of live temporal junctures [50]. This developmental trajectory has been illuminated by studies of children’s understanding of causally overdetermined events. A person may, for instance, miss her train on the way to an important meeting, only to discover that the train broke down and so infer that she would have missed the meeting even if she had caught the train. Two recent, superficially similar studies [11,12] aimed at measuring children’s reasoning about such events have uncovered contrasting patterns of results, implicating a critical role for representations of expired temporal junctures. In the first study [11], children were introduced to a toy pig that could be independently knocked over by each of two disks – red and yellow – that were dropped into an apparatus one at a time. Afterwards, the two disks were dropped at the same time but the red disk knocked the pig over because of its shorter path to the pig. Critically, children were then asked what would have happened if the red disk was not rolled or if its path to the pig had been blocked. Children aged 6 years and older, but not 4- and 5-year-olds, were able to identify that the pig would still have been knocked over by the yellow disk. In the second study [12], by contrast, children were introduced to two blocks – red and blue – and shown that each of them independently caused a box to light up when they were placed atop the box one at a time. Afterwards, the two blocks were simultaneously placed on the box and children were asked what would have happened if the red block was not placed. In this case, even 4-year-olds could correctly identify that the box would still have lit up. Why the difference in results across studies? One critical variation between the two tasks is in temporal perspective. In the disk task, the event in question is in the past, such that children have to actively reimagine this event and mentally undo it in order to envision the realistic alternative; in the block task, however, the event in question is still occurring in the present and so there is no need to reimagine the past at all [50,54]. That is, only in the disk task must children represent an expired temporal juncture. In a follow-up version of the block task [12], two causal blocks were placed on the box one after the other – first blue, then green – with only the blue block directly causing the box to light up. The blocks were then hidden by a blank screen and children were asked what would have happened if the green block was not placed. Again, 4-year-olds passed above chance levels. Even in this task, however, passing does not necessitate representing an expired temporal juncture. Rather, the children could have come to the correct conclusion by simply remembering the true past state of the world where the box was already lit up (by the blue block) before the green block was placed. By contrast, children would fail the disk task [11] if they relied on memory alone to answer the critical question, as the pig remained upright before the red disk was rolled. Only from 6 years do children appear to first reimagine this true version of the past and then move forward into the counterfactual alternative where the yellow disk knocks over the pig. Although it remains possible that other factors can explain the differential patterns of results (see [12]), our interpretation is consistent with a long line of broader research on the development of counterfactual thinking. This research typically indicates that children only begin to impose realistic constraints on their imaginings of alternative past events from around age 6 onwards [14,55,56]. That is, only around this age do children appear to accurately consider alternative outcomes that once were true possibilities stemming from an expired temporal juncture. This developmental trajectory closely corresponds to the emergence of children’s experience of regret, which likewise requires the ability to simulate alternative outcomes of an expired temporal juncture. Although a detailed discussion is beyond the scope of this opinion article, studies have consistently indicated that, when controlling for more basic negative emotions such as disappointment and frustration, children typically begin to experience regret from approximately 6 years of age [57–62]. Around this time, children also begin to show signs of regret’s positive counterpart, relief [61,62].
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Why the Protracted Development? A Representational Hierarchy It may seem counterintuitive that children appear to represent future temporal junctures around 2 years before they can represent past temporal junctures and alternative versions of the present. After all, children have direct access to the present (via perception) and indirect access to the past (via memory), and yet they can only infer the future. One potential explanation for this developmental trajectory is that envisioning past and present alternatives requires greater inhibitory control than imagining multiple future possibilities. In particular, children have to inhibit what they know to be true in order to envision a counterfactual past and present [63,64], whereas in order to consider future alternatives they need only to imagine what at least could be true. Although we agree that inhibitory control likely plays a central role, our perspective also highlights an important difference in representational complexity between future and past temporal junctures. In order to represent future (live) temporal junctures, children may first have to acquire the recursive capacity for metarepresentation [65,66]. In particular, children may have to be able to represent that any single representation of a future outcome will not necessarily correspond to the actual outcome [67,68,114]. Adults readily demonstrate such a capacity: we may, for example, imagine that a tossed coin will land on heads, but we also understand that this particular represented version of the future may not actually transpire. To understand that an alternative version of the past could have been the true version of the past, children have to represent an expired temporal juncture as if it were live. That is, they must recognise that two different versions of a single past event, which now have the distinct propositional values of true and false, previously had the shared propositional value of being merely possible versions of the future. Or, as Hoerl and McCormack ([69], p. 249) put it when writing on the function of regret: ‘Experiencing regret seems to hinge on being aware of the availability of these different options, which are now in the past but were once possible ways the future could have unfolded’. This may necessitate a second hierarchical level of representation: one metarepresents that, at a particular time in the past (level 2), any single representation of a particular future outcome (or choice) did not necessarily have to correspond to the actual outcome (level 1). The additional hierarchical level of representation may explain why children do not appear to reason accurately about past temporal junctures until much later than future temporal junctures (Figure 1). Notably, although children appear to experience regret from around 6 years of age [57–62], they do not seem to anticipate experiencing regret (and relief) in the future until approximately 8 years. That is, only from around this age do children typically predict they will feel sad in the future if a choice they have made turns out to have been suboptimal [70,71]. The late emergence of such anticipated counterfactual thinking may be explained by the fact that it requires a third hierarchical level of representation [72]: one metarepresents that, at a particular time in the future (level 3), one will reflect on the relative past (level 2) and represent that – at that time – any single representation of a future outcome (or choice) did not necessarily have to correspond to the actual outcome (level 1). Although perhaps difficult to comprehend when formulated in this manner, the cognitive process becomes readily apparent when considering the intertemporal choices regularly faced by humans (e.g., ‘if I eat that cake now, I’ll think to myself later that I shouldn’t have done so’). Anticipated regret stops us from pursuing many behaviours that we would very much enjoy in the moment but we foresee we will wish we had not pursued after the fact [73–75]. Anticipated relief, by contrast, can motivate us to push through difficult circumstances on our way to achieving a goal (e.g., ‘when I graduate university, I’ll be glad that I stuck with it instead of quitting’). Although recursive operations are in principle unbounded [76,77], this form of temporal embedding is likely to be constrained by working memory capacity [50,72]. It seems fitting then, that, alongside linear increases in working memory capacity [78], children initially become capable of representing first-order (live) temporal junctures around age 4, before second-order (expired) temporal junctures around age 6, and then third-order (to be expired) temporal junctures around age 8 – albeit with much
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Figure 1. How Humans Represent Events from Alternative Timelines. Representing alternative future possibilities requires a basic understanding of live temporal junctures, at which subjectively possible versions of the future branch off. It is more cognitively demanding to think about alternative past and present possibilities, because in these cases an expired temporal juncture must be represented as if it were live. In other words, one must represent that – in the now fixed past – there were multiple ways the future could have branched off. Regret and relief subsequently result from comparing an alternative present situation to the true present situation.
individual variation [16,61,70,79]. Adults are capable of yet further orders of temporal embedding, which may be equivalent to the recursive processes involved in higher-order theory of mind and syntactic language (Box 2).
Are Representations of Temporal Junctures Uniquely Human? The basic ability to predict uncertain future outcomes is evolutionarily ancient [80,81], and several influential unifying theories of neuroscience posit that animal brains are essentially prediction machines [82–86]. Common to these theories is the notion that brains function to register and minimise prediction error in an a posteriori manner: the future is predicted, and any discrepancies between the expected and actual outcome change the parameters of subsequent predictions. Yet, humans can do much more than merely predict outcomes and register error. Indeed, we can recursively turn this predictive process back on itself, in a sense, and predict that our predictions may themselves be incorrect. When about to have a baby, for example, you may think: ‘I predict it will be a girl, but I also know there is a good chance my prediction will turn out to be wrong; I’m often wrong with these’. We not only imagine the future outcome but also reflect on whether our prediction will be accurate. In other words, human brains additionally function to represent prediction error in an a priori manner, such that we infer live temporal junctures leading to alternative future possibilities. A critical question, then, is whether other animals can also predict that their predictions about uncertain future events may be incorrect. At least some animals clearly behave as if they experience uncertainty about many future outcomes [87,88], and this behaviour may indeed sometimes be accompanied by simultaneous representations of mutually exclusive possibilities. Yet, as detailed in Box 3, simply being uncertain about a future outcome is not the same as being aware of one’s uncertainty [89–92] and of the possibility of making a false prediction [67]. To our knowledge, such awareness has yet to be demonstrated in any non-human animal. As described in Box 1, studies using variations of the forked tube paradigm have so far consistently found that non-human primates typically prepare for only one version of even immediate uncertain future events [15,17–19]. Although this basic pattern needs replication with other primate individuals, other species, and other behavioural paradigms, generalising from the current results we may cautiously predict that non-human primates cannot represent first-order, live temporal junctures. However, in deploying our own further capacity for representing third-order temporal junctures, we also accept the alternative possibility that future findings may cause us to look back on that prediction and recognise its inaccuracy. Instead, it may turn out that only this very third-order capacity – or second-order, or fourth-order – lies beyond the reach of non-human primates.
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Box 2. Recursion, Recursion, Recursion Humans can hold in mind several levels of recursive temporal embedding. A prisoner serving a life sentence for a crime committed in the heat of the moment, for instance, may think to himself: ‘for the rest of my life (level 5), I will look back on my past (level 4) and regret the fact that I failed to anticipate (level 3) that I would regret my past decision (level 2) to commit that crime instead of not committing the crime (level 1 temporal juncture)’. Yet, temporal embedding does not necessarily have to involve junctures. One can, for example, recall a past occasion in which one made a prediction about the future (e.g., who would win the election) without considering that one could have predicted otherwise. Furthermore, temporal embedding does not always involve back-and-forth switches between future and past perspectives [72]. One can, for instance, recall past moments (level 2) of reminiscence (level 1); worry that in the future (level 2) one will continue to worry about the future (level 1); or indeed hope (level 2) that one will not worry in the future (level 1). Some studies have suggested that humans can typically represent up to five orders of embedded reasoning about minds (e.g., ‘you believe that I suppose that you know that I want you to believe that.’) and similar orders of embedded syntactic language [111,112]. Future studies may wish to examine associations between these types of recursive processes and recursive temporal embedding within individuals. Studies may also want to assess whether such relationships are mediated by working memory capacity. One recent study [113] found that children’s second-order false-belief understanding was strongly predicted not by simple working memory span, but rather by performance on a more complex working memory task that required participants to: (i) remember and correctly order the final words from multiple sentences, while also (ii) judging each sentence as true or false. We predict similar relationships between such complex working memory measures and children’s understanding of higher-order temporal junctures, which likewise involve serial ordering of representations and judgements of representational veracity (see Figure 1 in main text). Briefly, we also note that, unlike theory of mind and syntactic language, respectively, representations of temporal junctures and temporal embedding require no inferences about the existence of other minds and no understanding of arbitrary symbols (i.e., words). Given this relative representational simplicity, mental time travel is a strong candidate for the original context in which recursive cognition evolved (also see [109]).
Curiously, the strongest claims that non-human animals can represent alternative timelines have come from work with rodents. In particular, two recent studies have claimed that rodents can both experience [20] and anticipate experiencing [21] regret. In the first study [20], rats were given a series of choices between two options: (i) waiting for a food reward to become available in one location, or (ii) skipping the wait and instead moving to a second location where a different food reward would become available after another wait. On some trials, rats skipped a low-cost offer (e.g., a short wait for a desired reward) only to find themselves subsequently confronted with a high-cost offer (i.e., a longer wait for a less desired reward). In these situations, the rats were more likely than normal to pause their movement and orient themselves towards the previous, skipped option, and this behaviour was associated with neural activity suggesting that the rats may have been mentally simulating the event of accepting the low-cost offer. The authors interpreted these findings as indicating that the rats were experiencing regret at their decision to reject the low-cost offer. In the follow-up study [21], mice that initially accepted a food offer were given a second chance to skip that offer and move on to the next one. When they skipped the offer at this second stage, the mice incurred a cost of ‘wasted time’ that they could have spent moving towards the next offer. Over a test period of 70 days, the mice became increasingly less likely to skip rewards at the second stage. The authors interpreted this pattern to indicate that the mice had learned to anticipate and eventually avoid the experience of regret at wasting their time. If these authors’ interpretations are correct, then it follows that rodents can represent second- and even third-order temporal junctures that human children do not seem to represent until around 6 (regret) and 8 years of age (anticipated regret) respectively. On the basis of these two studies alone, however, one need not necessarily draw this conclusion. In the first study [20], the rats may have simply been comparing the rejected offer to the current offer and orienting themselves towards the rejected offer because it was clearly more preferred. As others have pointed out [33], this need not entail that the rats wound back the clock, so to speak, to mentally undo their previous action of rejecting the
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Box 3. Being Uncertain Does Not Require Awareness of Uncertainty A large body of work has demonstrated that non-human primates and other animals often act adaptively in situations of uncertainty [87,88]. In typical ‘uncertainty monitoring’ tasks, for instance, monkeys are required to discriminate between pairs of stimuli (e.g., in deciding which of two lines is longer). Correct responses are rewarded with food, whereas incorrect responses are punished by requiring the animal to wait for a period before they can move onto the next trial. Critically, throughout the task the monkeys are given the opportunity to ‘opt out’ of any given trial, which immediately moves them onto the next (potentially easier) trial without incurring the cost of waiting. Typically, after many trials, monkeys begin to opt out of difficult trials far more frequently than easy trials. One interpretation of this behaviour is that the monkeys are aware of their own uncertainty on difficult trials and thus opt out in order to maximise their overall reward frequency. Yet, non-human animals need not necessarily be aware of their own uncertainty in order to opt out appropriately in such tasks [89–92]. As schematised in Figure I, an uncertain animal may in some sense represent mutually exclusive possibilities, with each of these alternatives differing in degree of ‘representational strength’ (perhaps akin to Bayesian priors). And in an uncertainty monitoring task, an animal may simply learn that when the two possibilities are represented with roughly equal strength, committing to one outcome over the other often leads to punishment. In other words, failing to opt out when in a state of high uncertainty would become associated with punishment and the opt-out behaviour would be shaped by operant conditioning principles. (For a related discussion of rich and lean interpretations of uncertainty monitoring in human infants, see the companion review article in this issue [92].) A similar process may explain why some younger children and non-human primates cover two exits on some trials of the forked tube paradigm (Box 1) but then regress to covering only one exit on subsequent trials. That is, if the two possible outcomes are represented with roughly equal strength, then the two-exit response may briefly appear. After further experience, however, the representational strength (e.g., Bayesian prior) of each outcome shifts, such that one representation dominates the other and the one-exit response returns. This process could also explain why a small number of children pass the very first trial before regressing [15–17].
UNCERTAINTY
AWARENESS OF UNCERTAINTY Trends in Cognitive Sciences
Figure I. Representational Demands of Uncertainty and Awareness of Uncertainty. As on the left, many agents may experience uncertainty about future outcomes. Some possible outcomes may be strongly predicted (denoted by the opaque bubble) and others may be weakly predicted (denoted by the translucent bubble). As on the right, however, an agent that is aware of this uncertainty can metarepresent that each represented future outcome is an alternative of the same event following a temporal juncture, with the occurrence of any one outcome precluding the occurrence of all others.
offer (note that this same alternative explanation applies to a study suggesting that apes may regret their suboptimal choices [93]). Similarly, in the second study [21], the mice may have simply learned over time that the experience of waiting for a low-desirability reward is aversive, and thus eventually avoided actions that had previously led to such an experience. That is, one only needs to appeal to operant conditioning principles to explain why such a change in the mice’s behaviour would be shaped over the long period of the study.
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Nevertheless, we cannot rule out that the rodents were in fact experiencing and anticipating regret in these studies. And if these emotions are truly within rodents’ capacity, then according to our perspective they should also be capable of representing first-order, live temporal junctures. Rodents may therefore be ideal candidates for administering species-appropriate adaptations of the forked tube task and related paradigms. Other non-primate candidates include corvids [94,95], parrots [96,97], and cetaceans [98,99], given their impressive cognitive skills demonstrated in other domains. If animals are indeed to be shown to be capable of representing first-order temporal junctures, let alone higher-order junctures, then the ethical implications could be monumental. After all, the ability to consider alternative futures is one criterion used to bestow personhood [100,101].
Concluding Remarks Many complex human behaviours are underscored by representations of future temporal junctures. Recently, we proposed the term metaforesight [68] to encapsulate the overarching capacity to reflect on alternative future possibilities and optimise behaviour accordingly. Perhaps the most obvious benefit of metaforesight is contingency planning: an agent that represents mutually exclusive future possibilities can try to ‘cover all bases’ by preparing not only for likely and desirable outcomes, but also for unlikely and undesirable outcomes [15,67,102]. Metaforesight also encompasses self-reflection on predicted outcomes of potential behaviours [103]. While thin ice may at first seem like a good surface to skate on, taking an extra moment to consider the possible outcomes (or to ask someone else) can lead one to think better of it [5]. To paraphrase Karl Popper [104], we let our hypotheses die in our stead. Representations of past temporal junctures also provide numerous benefits [9,30,31,105]. Many of these benefits flow from the ability to learn associations between particular behaviours and outcomes by simply imagining such pairings [106], an ability that has been recently documented in humans [107]. When we feel regret, for instance, we not only experience punishment for our actual behaviour, but we also typically envision the reinforcement that would have been associated with an alternative behaviour. A job candidate, for example, may not only feel anguish after providing a weak answer to a critical interview question, but also imagine the satisfaction (and new job) that may have followed if she had provided a stronger answer. This is a particularly efficient means of learning, because from a single event one can learn both: (i) what not to do, and (ii) what to do instead when similar situations arise in the future. Regardless of the original context in which they may have first evolved, representations of temporal junctures likely supported the emergence of a powerful suite of entirely novel behaviours in our hominin ancestors. These representations would have been critical to individuals forming complex if-then plans [5,108] and to human groups determining how best to proceed in cooperative endeavours [75,109]. Preparations for a hunt, a conflict, or a migration would have undoubtedly been much more efficient once our ancestors could think through (and discuss) various alternative ways the future might unfold. These ancestors could ask themselves ‘what could possibly go wrong?’, and thus prevent catastrophe before it eventuated. As in modern times, representing temporal junctures would have also enabled early humans to evaluate alternative answers to the questions of ‘who do I want to be?’ and ‘who do we want to be?’. These representations bestowed us with an intuitive sense of free will; Laplace’s Demon be damned. And with this sense of free will, along with the eventual ability to represent second-order temporal junctures, we could make increasingly sophisticated moral judgements about the courses of action taken by ourselves and others in the past. Judgements of past intent, of course, remain central to many moral systems, such that certain actions demand punishment if, and only if, the relevant outcomes of one’s past actions are deemed to have been foreseeable at the time. On an even broader note, modern discussions about the future of the human species often invoke third-order notions of legacy: ‘what will our great-grandchildren have to say about the courses of action we choose tomorrow?’ Until now, however, research on the development, evolution, functions, dysfunctions, neurocognitive mechanisms, and broader implications of the capacity to envision and evaluate events from
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Outstanding Questions In people with psychological disorders and age-related cognitive decline, is the capacity for counterfactual thinking more likely to be absent or dysfunctional than the capacity to consider alternative future possibilities? Such a pattern would be consistent with the representational hierarchy outlined herein. In children, adults, and clinical populations, are there associations between performances on working memory tasks requiring judgements of representational veracity (Box 2) and tasks requiring representations of higher-order temporal junctures? Although the shared developmental trajectory between working memory and levels of represented temporal junctures is clear, there is currently no compelling evidence of correlations between these faculties. How do children and animals perform on (ethically appropriate) tasks in which they must act to simultaneously prevent two mutually exclusive losses, or two mutually exclusive harmful outcomes? The evolutionary imperative may be stronger in these cases than in the forked tube paradigm, which requires children and non-human primates to simultaneously prepare for two mutually exclusive rewarding outcomes. What type of archaeological evidence would indicate the appearance of representations of temporal junctures, be they first-, second-, or third-order? If these representations are indeed uniquely human, then they likely emerged after the split of the hominin and chimpanzee lineages in the last 5–7 million years. Have representations of temporal junctures evolved in non-primate lineages? Corvids, parrots, and cetaceans, in particular, have demonstrated intelligence in many domains and may be ideal candidates for adaptations of the forked tube task and related paradigms.
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alternative timelines has lacked an overarching framework with which to guide predictions and situate patterns of results. Here we have provided such a framework by introducing a representational hierarchy of temporal junctures in the mind. Future studies in each domain of inquiry may benefit from attending to findings and interpretations from the other domains, and from borrowing and adapting behavioural paradigms that have been shown to be particularly fruitful (see Outstanding Questions). Research in this area is itself at a critical juncture, and the best way forward is most likely collaborative.
Acknowledgements This work was supported by a University of Queensland Development Fellowship (UQFEL1832633) awarded to J.R.
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Opinion
Temporal Junctures in the Mind Jonathan Redshaw1,* and Thomas Suddendorf1 Humans can imagine what happened in the past and what will happen in the future, but also what did not happen and what might happen. We reflect on envisioned events from alternative timelines, while knowing that we only ever live on one timeline. Considering alternative timelines rests on representations of temporal junctures, or points in time at which possible versions of reality diverge. These representations become increasingly sophisticated over childhood, first enabling preparation for mutually exclusive future possibilities and later the experience of counterfactual emotions like regret. By contrast, it remains unclear whether non-human animals represent temporal junctures at all. The emergence of these representations may have been a prime mover in human evolution.
Possible Worlds: Past, Present, and Future [For] an intellect which at a certain moment would know all forces that set nature in motion, and all positions of all items of which nature was composed . nothing would be uncertain and the future just like the past would be present before its eyes. – Pierre-Simon Laplace, A Philosophical Essay on Probabilities (1814) A cat is penned up in a steel chamber, along with . a tiny bit of radioactive substance . If one has left this entire system to itself for an hour, one would say that the cat still lives if meanwhile no atom has decayed. The first atomic decay would have poisoned it. The psi-function of the entire system would express this by having in it the living and dead cat mixed or smeared out in equal parts. – Erwin Schro¨dinger, The Present Situation in Quantum Mechanics (1935) Laplace’s Demon [1,2] and Schro¨dinger’s Cat [3,4] continue to be bones of contention among philosophers and physicists alike. Do we live in a deterministic universe, or is there room for chance and choice? Does quantum mechanics really entail that multiple versions of unitary phenomena exist simultaneously? For most humans, however, the implications of these thought experiments are preposterous. ‘Of course the future is uncertain; of course the cat is alive or dead.’ The reason these hypotheticals evoke such visceral denials is because they fundamentally violate our folk psychological notions of reality. Humans readily envision alternative possible versions of single future events, often stemming from alternative volitional actions, and evaluate them in terms of likelihood and desirability [5–7]. Likewise, we readily bring to mind alternative possible versions of past events and consider how they might have led to alternative versions of the present [8–10]. Yet, we also appreciate that these alternative timelines are mutually exclusive. Our minds might visit other possible worlds, but our bodies remain forever confined to one. The capacity to imagine and evaluate events from mutually exclusive timelines is currently a hot topic of research in developmental [11–16], comparative [15,17–21], and clinical psychology [22–26], as well as in cognitive neuroscience [7,8,27–29]. In this opinion article, we propose that thinking about alternative timelines principally rests on representations of temporal junctures (see Glossary), or points in time at which subjectively possible worlds branch off from one another. We first briefly describe these representations and situate our perspective in relation to other recent conceptualisations of reasoning about possibility and time. We then summarise children’s protracted development in this domain, from initially representing temporal junctures located in the future to later representing temporal junctures located in the past, and propose a representational hierarchy to explain this trajectory. We examine the question of whether representations of temporal junctures are uniquely human, and outline some of the many adaptive benefits these representations can provide.
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Highlights Many recent studies have examined the development, evolution, dysfunctions, and neurocognitive mechanisms of the capacity to consider events from alternative timelines. Children begin to consistently prepare for alternative versions of immediate future events around 4 years of age, but do not accurately consider alternative versions of past events and present situations until around 6 years of age. There is no compelling evidence that non-human animals consider and compare events from alternative timelines, although interpretations of some results remain contentious. Representing alternative versions of the future enables humans to form contingency plans and otherwise compensate for their inability to perfectly predict future outcomes. Representing alternative versions of the past enables humans to learn associations not only between actual behaviours and outcomes, but also counterfactual behaviours and outcomes.
1School of Psychology, University of Queensland, Brisbane, QLD 4072, Australia
*Correspondence: [email protected]
https://doi.org/10.1016/j.tics.2019.10.009 ª 2019 Elsevier Ltd. All rights reserved.
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Temporal Junctures: Live and Expired Our perspective on temporal junctures shares some similarities with Byrne’s notion of counterfactual fault lines [30,31], or the aspects of past events that people tend to mentally adjust to create alternative versions of the present. You may, for instance, spend an inordinate amount of time wondering what the present would look like if you had stayed together with a former romantic partner, or if an alternative candidate had won a crucial election. Whereas Byrne emphasises past divergence points, however, one can of course also think about future divergence points. Furthermore, whereas Byrne narrows her focus to the particular biases that people have when imagining past alternatives, here we refer to the more general capacity to represent temporal junctures as being located at any time point and for any event outcome. Representing a temporal juncture includes the critical understanding that all subjectively possible outcomes of the juncture are alternatives of precisely the same event: we understand that a single flipped coin must land on heads or tails, but never on both. With representations of temporal junctures, therefore, we acquire McCormack and Hoerl’s notion of event-independent time [32–34]. Generally speaking, we can think of each moment in time as a single ‘slot’ [32] that can only ever be filled by one particular state of the world. Yet, because we can represent temporal junctures, we also understand that this one particular state of the world is only a sample of the many possible worlds that could fill (or could have filled) that slot. In one sense, representations of temporal junctures may simply be construed as a first-person, temporally oriented form of false-belief understanding: people understand that they can imagine both true and false versions of past and future events. However, temporal junctures also have a characteristic asymmetry: true versions of past events are typically represented as being fixed, whereas true versions of future events are often represented as being open. In other words, only in the future is a temporal juncture ‘live’ [35], in the sense that a person experiences that each subjectively possible outcome of an event both could and could not happen. Once the outcome has become known, however, a person recognises that the temporal juncture is ‘expired’, in the sense that each previously possible outcome either did or did not happen. To summarise, we consider representations of temporal junctures to have three key qualities: (i) they can be represented as being located at any point in time in both the past and the future; (ii) they denote single points in time at which alternative represented outcomes of the same event diverge; and (iii) they are typically asymmetrical, in that future junctures are live and past junctures are expired.
Children’s Representations of Temporal Junctures In developmental psychology [36–39], as in comparative psychology [40–42], the basic capacity for mental time travel is often examined using variations of the two-rooms task [36] (also known as the spoon test [43]). In brief, children are first exposed to a problem in one room, before being taken to a second room where they later have the chance to select the solution (from among distractors) to take back to the first room. Experiments with two-rooms tasks have provided strong evidence for both memory of unique past events and preparation for specific future situations in young children [37,44]. Yet, some versions of the task (especially those used with animals) have been subject to considerable controversy, arguably failing to control for simple associative learning explanations [44–49]. Be that as it may, for present purposes it should be noted that the two-rooms task is aimed only at assessing the capacity to imagine and link events from a single timeline: children have reason to believe that, as long as they accurately recall the problem from the first room, then selecting the appropriate item in the second room will enable them to solve the problem later on [50]. Passing does not, however, necessarily require children to represent the alternative: that if they do not select the item they will not solve the problem upon return later on. Instead, children’s capacities to represent temporal junctures and alternative timelines have been assessed by different lines of research.
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Glossary Bayesian priors: data-driven inferences about the likelihood of particular uncertain outcomes occurring in the future. In cognition, Bayesian priors may be experienced as intuitions about uncertain outcomes rather than rationally derived predictions. Causally overdetermined events: events that were in fact caused by one antecedent but would have been caused by a second antecedent in the absence of the first. Counterfactual thinking: accurately representing how the present version of reality would look had an alternative, nonveridical version of the past transpired. Mental time travel: the capacity to mentally re-experience past episodes and pre-experience future episodes, and to situate these imagined episodes in relation to the present. Metaforesight: the overarching capacity to reflect on alternative future possibilities and optimise behaviour accordingly. Metaforesight may express itself in various intelligent behaviours, such as contingency planning to compensate for one’s inherent inability to predict some future outcomes with certainty. Metarepresentation: representing representational relations. That is, representing that representations are just representations that need not perfectly correspond to their referents. Regret: the negative emotion experienced when one believes that an alternative, nonveridical past choice would have led to a more desirable present than the actual past choice. Relief: the positive emotion experienced when one believes that an alternative, nonveridical past choice would have led to a less desirable present than the actual past choice. Sense of free will: the sense that one has genuine choice between alternative courses of future action, regardless of whether such choice in fact exists. Subjectively possible outcome: from the perspective of the agent, there is a non-zero chance that the represented outcome may correspond to the true outcome. Note that in some cases the outcome may have already eventuated and
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Live Temporal Junctures We recently developed a minimalist, principally nonverbal task to assess children’s (and non-human primates’) ability to simultaneously prepare for two mutually exclusive versions of a single future event. In the original ‘forked tube’ task [15], children aged between 2 and 4 years were invited to catch a reward that an experimenter dropped into an inverted Y-shaped tube with one opening at the top and two possible exits at the bottom (Box 1). Children could ensure they would always catch the reward by simply covering both exits when preparing for the drop, whereas those who prepared by covering only one exit missed the reward approximately half of the time. This task is plainly future-oriented, in the minimal sense that children are given the opportunity to prepare for an imminent event. Yet, it also greatly relaxes demands on other capacities normally associated with thinking about the future. Unlike the two-rooms task, for example, the forked tube task places no demands on episodic memory or spatial scene construction. Instead, it isolates the basic capacity to represent alternative future timelines for which the temporal juncture will occur in one’s immediate spatial proximity. In the original study, we found that most 4-year-olds and some 3-year-olds spontaneously and consistently covered both exits of the forked tube, whereas 2-year-olds typically covered one exit only. And although some 2-year-olds and many 3-year-olds covered two exits on some occasions across 12 trials, a substantial proportion of these children regressed to covering only one exit on subsequent trials. This response pattern is consistent with the possibility that many younger children were weakly conditioned into showing the optimal response over their experience with the task [15]. The 4year-olds, however, did not regress after first covering both exits, suggesting they had insight into the optimality of that response.
yet the agent remains unaware of it (e.g., when watching a prerecorded sporting match). In such cases, the temporal juncture may nonetheless be represented as live, as multiple outcomes still remain possible from the agent’s perspective. Temporal embedding: a representational perspective with a particular temporal orientation (forward-looking or backwardlooking) is represented as being located within another representational perspective with its own temporal orientation. Regret and relief, for instance, arise when a forward-looking representational perspective is embedded within a backward-looking representational perspective. Temporal junctures: represented points in time at which multiple subjectively possible versions of reality diverge from each other.
To examine the generalisability of these findings, we subsequently administered the forked tube task to 3- to 6-year-old children from three diverse cultures [16]. One sample was from the same industrialised western city as in the original study (Brisbane, Australia), whereas the other two were from remote indigenous communities in the Australian Outback and the Kalahari Desert. Some 3-year-olds from all groups spontaneously and consistently covered two exits, and the majority of the oldest children from each sample passed every trial. This pattern of results indicates that humans may universally acquire the capacity to prepare for alternative futures in early childhood (also see [51,52]). In another study, we developed a version of the paradigm where the uncertain outcome was visibly controlled by a social agent rather than by a hidden physical mechanism. We reasoned that children may perform better under conditions of social uncertainty, and when the spatial point of the temporal juncture is in plain sight [17]. The results again replicated the finding of improved performance from 2 to 4 years of age, but children failed to perform better than on the original forked tube task. Critically, in two other recent studies [18,53], we found that 2.5- and 3-year-olds who struggled to prepare for alternative possibilities nevertheless performed close to ceiling on control tasks requiring them to use two hands to catch two rewards dropped simultaneously on certain trajectories. This pattern suggests that young children’s difficulties with the original tasks were due to the uncertainty of the single outcome rather than with manual motor coordination. On the whole, these findings suggest that many 4-year-olds but few younger children can mentally represent and prepare for alternative possible outcomes of an uncertain future event. Notably, several studies [15,17–19] have found that non-human great apes and monkeys, like young children, tend to prepare for only a single version of the future on uncertain versions of the task. Interpretations of these negative results, however, remain contentious (Box 1). Curiously, most younger children and primates switch between covering the left and right exit across trials (and some switch during trials), suggesting that they can indeed envision both possible outcomes in some basic sense. Yet, when 4-year-olds spontaneously and consistently cover two exits to catch the single reward, they appear to also demonstrate the critical understanding that these two outcomes are alternatives of the same event. That is, these children represent the live temporal juncture ahead of them.
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Box 1. Can Non-human Primates Represent Alternative Futures? Using several variations of the forked tube paradigm [15,17,18], we have found that non-human primates typically cover only one exit when preparing to catch a single food item (Figure I). Although negative findings are difficult to interpret, these results raise the possibility that other primates cannot represent live temporal junctures and alternative future timelines. Recently, Lambert and Osvath [19] replicated chimpanzees’ poor performance but also found that, in contrast to our results with young children [18,53], the apes performed relatively poorly when given the opportunity to catch two rewards with certain trajectories (also see our recent data from monkeys [18]). The authors suggested that, rather than being unable to conceive of future alternatives, nonhuman primates may simply lack the motor coordination required to cover two tube exits simultaneously. However, although chimpanzees’ performance in this study was indeed relatively low across tasks, in the main experiment they were seven times more likely to pass certain-trajectory trials (74/301: 24.4%) than uncertaintrajectory trials (5/144: 3.4%). The apes never passed more than one uncertain-trajectory trial in a row, whereas there were several long streaks of passing certain-trajectory trials (i.e., 8, 8, 13, 16, and 18 in a row). These results suggest that chimpanzees’ struggles on the original tasks may be at least partly explained by future uncertainty. Notably, covering one exit on certain-trajectory trials does guarantee some food reward, whereas on uncertain-trajectory trials it does not; and only on certain-trajectory trials must apes attend (through cage mesh) to two separate locations to notice the two rewards. The apes’ overall poor performance on certain-trajectory trials may be related to these factors. We disagree with these authors’ suggestion that eye-tracking methods could make an important contribution, as an animal that simply glances from one tube exit to the other does not necessarily possess the critical understanding that both possibilities are alternatives of the same event [67]. Nonetheless, we do agree that ruling out problems deriving from motor coordination is important, and so future studies should incorporate further types of simultaneous response options in preparation for mutually exclusive future alternatives. We reassert that strong conclusions about non-human primates’ capacities in this domain are not yet appropriate [15], as the negative evidence remains confined to variations of a single paradigm. After all, absence of evidence is not evidence of absence (see [75,110]).
Figure I. Depiction of the Original Forked Tube Task. Many 4-year-old children spontaneously and consistently cover both exits, whereas younger children and nonhuman primates typically cover one exit only. Reprinted, with permission, from [15].
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Expired Temporal Junctures Children’s representations of expired temporal junctures may not begin to mature until around 2 years later than their understanding of live temporal junctures [50]. This developmental trajectory has been illuminated by studies of children’s understanding of causally overdetermined events. A person may, for instance, miss her train on the way to an important meeting, only to discover that the train broke down and so infer that she would have missed the meeting even if she had caught the train. Two recent, superficially similar studies [11,12] aimed at measuring children’s reasoning about such events have uncovered contrasting patterns of results, implicating a critical role for representations of expired temporal junctures. In the first study [11], children were introduced to a toy pig that could be independently knocked over by each of two disks – red and yellow – that were dropped into an apparatus one at a time. Afterwards, the two disks were dropped at the same time but the red disk knocked the pig over because of its shorter path to the pig. Critically, children were then asked what would have happened if the red disk was not rolled or if its path to the pig had been blocked. Children aged 6 years and older, but not 4- and 5-year-olds, were able to identify that the pig would still have been knocked over by the yellow disk. In the second study [12], by contrast, children were introduced to two blocks – red and blue – and shown that each of them independently caused a box to light up when they were placed atop the box one at a time. Afterwards, the two blocks were simultaneously placed on the box and children were asked what would have happened if the red block was not placed. In this case, even 4-year-olds could correctly identify that the box would still have lit up. Why the difference in results across studies? One critical variation between the two tasks is in temporal perspective. In the disk task, the event in question is in the past, such that children have to actively reimagine this event and mentally undo it in order to envision the realistic alternative; in the block task, however, the event in question is still occurring in the present and so there is no need to reimagine the past at all [50,54]. That is, only in the disk task must children represent an expired temporal juncture. In a follow-up version of the block task [12], two causal blocks were placed on the box one after the other – first blue, then green – with only the blue block directly causing the box to light up. The blocks were then hidden by a blank screen and children were asked what would have happened if the green block was not placed. Again, 4-year-olds passed above chance levels. Even in this task, however, passing does not necessitate representing an expired temporal juncture. Rather, the children could have come to the correct conclusion by simply remembering the true past state of the world where the box was already lit up (by the blue block) before the green block was placed. By contrast, children would fail the disk task [11] if they relied on memory alone to answer the critical question, as the pig remained upright before the red disk was rolled. Only from 6 years do children appear to first reimagine this true version of the past and then move forward into the counterfactual alternative where the yellow disk knocks over the pig. Although it remains possible that other factors can explain the differential patterns of results (see [12]), our interpretation is consistent with a long line of broader research on the development of counterfactual thinking. This research typically indicates that children only begin to impose realistic constraints on their imaginings of alternative past events from around age 6 onwards [14,55,56]. That is, only around this age do children appear to accurately consider alternative outcomes that once were true possibilities stemming from an expired temporal juncture. This developmental trajectory closely corresponds to the emergence of children’s experience of regret, which likewise requires the ability to simulate alternative outcomes of an expired temporal juncture. Although a detailed discussion is beyond the scope of this opinion article, studies have consistently indicated that, when controlling for more basic negative emotions such as disappointment and frustration, children typically begin to experience regret from approximately 6 years of age [57–62]. Around this time, children also begin to show signs of regret’s positive counterpart, relief [61,62].
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Why the Protracted Development? A Representational Hierarchy It may seem counterintuitive that children appear to represent future temporal junctures around 2 years before they can represent past temporal junctures and alternative versions of the present. After all, children have direct access to the present (via perception) and indirect access to the past (via memory), and yet they can only infer the future. One potential explanation for this developmental trajectory is that envisioning past and present alternatives requires greater inhibitory control than imagining multiple future possibilities. In particular, children have to inhibit what they know to be true in order to envision a counterfactual past and present [63,64], whereas in order to consider future alternatives they need only to imagine what at least could be true. Although we agree that inhibitory control likely plays a central role, our perspective also highlights an important difference in representational complexity between future and past temporal junctures. In order to represent future (live) temporal junctures, children may first have to acquire the recursive capacity for metarepresentation [65,66]. In particular, children may have to be able to represent that any single representation of a future outcome will not necessarily correspond to the actual outcome [67,68,114]. Adults readily demonstrate such a capacity: we may, for example, imagine that a tossed coin will land on heads, but we also understand that this particular represented version of the future may not actually transpire. To understand that an alternative version of the past could have been the true version of the past, children have to represent an expired temporal juncture as if it were live. That is, they must recognise that two different versions of a single past event, which now have the distinct propositional values of true and false, previously had the shared propositional value of being merely possible versions of the future. Or, as Hoerl and McCormack ([69], p. 249) put it when writing on the function of regret: ‘Experiencing regret seems to hinge on being aware of the availability of these different options, which are now in the past but were once possible ways the future could have unfolded’. This may necessitate a second hierarchical level of representation: one metarepresents that, at a particular time in the past (level 2), any single representation of a particular future outcome (or choice) did not necessarily have to correspond to the actual outcome (level 1). The additional hierarchical level of representation may explain why children do not appear to reason accurately about past temporal junctures until much later than future temporal junctures (Figure 1). Notably, although children appear to experience regret from around 6 years of age [57–62], they do not seem to anticipate experiencing regret (and relief) in the future until approximately 8 years. That is, only from around this age do children typically predict they will feel sad in the future if a choice they have made turns out to have been suboptimal [70,71]. The late emergence of such anticipated counterfactual thinking may be explained by the fact that it requires a third hierarchical level of representation [72]: one metarepresents that, at a particular time in the future (level 3), one will reflect on the relative past (level 2) and represent that – at that time – any single representation of a future outcome (or choice) did not necessarily have to correspond to the actual outcome (level 1). Although perhaps difficult to comprehend when formulated in this manner, the cognitive process becomes readily apparent when considering the intertemporal choices regularly faced by humans (e.g., ‘if I eat that cake now, I’ll think to myself later that I shouldn’t have done so’). Anticipated regret stops us from pursuing many behaviours that we would very much enjoy in the moment but we foresee we will wish we had not pursued after the fact [73–75]. Anticipated relief, by contrast, can motivate us to push through difficult circumstances on our way to achieving a goal (e.g., ‘when I graduate university, I’ll be glad that I stuck with it instead of quitting’). Although recursive operations are in principle unbounded [76,77], this form of temporal embedding is likely to be constrained by working memory capacity [50,72]. It seems fitting then, that, alongside linear increases in working memory capacity [78], children initially become capable of representing first-order (live) temporal junctures around age 4, before second-order (expired) temporal junctures around age 6, and then third-order (to be expired) temporal junctures around age 8 – albeit with much
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Figure 1. How Humans Represent Events from Alternative Timelines. Representing alternative future possibilities requires a basic understanding of live temporal junctures, at which subjectively possible versions of the future branch off. It is more cognitively demanding to think about alternative past and present possibilities, because in these cases an expired temporal juncture must be represented as if it were live. In other words, one must represent that – in the now fixed past – there were multiple ways the future could have branched off. Regret and relief subsequently result from comparing an alternative present situation to the true present situation.
individual variation [16,61,70,79]. Adults are capable of yet further orders of temporal embedding, which may be equivalent to the recursive processes involved in higher-order theory of mind and syntactic language (Box 2).
Are Representations of Temporal Junctures Uniquely Human? The basic ability to predict uncertain future outcomes is evolutionarily ancient [80,81], and several influential unifying theories of neuroscience posit that animal brains are essentially prediction machines [82–86]. Common to these theories is the notion that brains function to register and minimise prediction error in an a posteriori manner: the future is predicted, and any discrepancies between the expected and actual outcome change the parameters of subsequent predictions. Yet, humans can do much more than merely predict outcomes and register error. Indeed, we can recursively turn this predictive process back on itself, in a sense, and predict that our predictions may themselves be incorrect. When about to have a baby, for example, you may think: ‘I predict it will be a girl, but I also know there is a good chance my prediction will turn out to be wrong; I’m often wrong with these’. We not only imagine the future outcome but also reflect on whether our prediction will be accurate. In other words, human brains additionally function to represent prediction error in an a priori manner, such that we infer live temporal junctures leading to alternative future possibilities. A critical question, then, is whether other animals can also predict that their predictions about uncertain future events may be incorrect. At least some animals clearly behave as if they experience uncertainty about many future outcomes [87,88], and this behaviour may indeed sometimes be accompanied by simultaneous representations of mutually exclusive possibilities. Yet, as detailed in Box 3, simply being uncertain about a future outcome is not the same as being aware of one’s uncertainty [89–92] and of the possibility of making a false prediction [67]. To our knowledge, such awareness has yet to be demonstrated in any non-human animal. As described in Box 1, studies using variations of the forked tube paradigm have so far consistently found that non-human primates typically prepare for only one version of even immediate uncertain future events [15,17–19]. Although this basic pattern needs replication with other primate individuals, other species, and other behavioural paradigms, generalising from the current results we may cautiously predict that non-human primates cannot represent first-order, live temporal junctures. However, in deploying our own further capacity for representing third-order temporal junctures, we also accept the alternative possibility that future findings may cause us to look back on that prediction and recognise its inaccuracy. Instead, it may turn out that only this very third-order capacity – or second-order, or fourth-order – lies beyond the reach of non-human primates.
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Box 2. Recursion, Recursion, Recursion Humans can hold in mind several levels of recursive temporal embedding. A prisoner serving a life sentence for a crime committed in the heat of the moment, for instance, may think to himself: ‘for the rest of my life (level 5), I will look back on my past (level 4) and regret the fact that I failed to anticipate (level 3) that I would regret my past decision (level 2) to commit that crime instead of not committing the crime (level 1 temporal juncture)’. Yet, temporal embedding does not necessarily have to involve junctures. One can, for example, recall a past occasion in which one made a prediction about the future (e.g., who would win the election) without considering that one could have predicted otherwise. Furthermore, temporal embedding does not always involve back-and-forth switches between future and past perspectives [72]. One can, for instance, recall past moments (level 2) of reminiscence (level 1); worry that in the future (level 2) one will continue to worry about the future (level 1); or indeed hope (level 2) that one will not worry in the future (level 1). Some studies have suggested that humans can typically represent up to five orders of embedded reasoning about minds (e.g., ‘you believe that I suppose that you know that I want you to believe that.’) and similar orders of embedded syntactic language [111,112]. Future studies may wish to examine associations between these types of recursive processes and recursive temporal embedding within individuals. Studies may also want to assess whether such relationships are mediated by working memory capacity. One recent study [113] found that children’s second-order false-belief understanding was strongly predicted not by simple working memory span, but rather by performance on a more complex working memory task that required participants to: (i) remember and correctly order the final words from multiple sentences, while also (ii) judging each sentence as true or false. We predict similar relationships between such complex working memory measures and children’s understanding of higher-order temporal junctures, which likewise involve serial ordering of representations and judgements of representational veracity (see Figure 1 in main text). Briefly, we also note that, unlike theory of mind and syntactic language, respectively, representations of temporal junctures and temporal embedding require no inferences about the existence of other minds and no understanding of arbitrary symbols (i.e., words). Given this relative representational simplicity, mental time travel is a strong candidate for the original context in which recursive cognition evolved (also see [109]).
Curiously, the strongest claims that non-human animals can represent alternative timelines have come from work with rodents. In particular, two recent studies have claimed that rodents can both experience [20] and anticipate experiencing [21] regret. In the first study [20], rats were given a series of choices between two options: (i) waiting for a food reward to become available in one location, or (ii) skipping the wait and instead moving to a second location where a different food reward would become available after another wait. On some trials, rats skipped a low-cost offer (e.g., a short wait for a desired reward) only to find themselves subsequently confronted with a high-cost offer (i.e., a longer wait for a less desired reward). In these situations, the rats were more likely than normal to pause their movement and orient themselves towards the previous, skipped option, and this behaviour was associated with neural activity suggesting that the rats may have been mentally simulating the event of accepting the low-cost offer. The authors interpreted these findings as indicating that the rats were experiencing regret at their decision to reject the low-cost offer. In the follow-up study [21], mice that initially accepted a food offer were given a second chance to skip that offer and move on to the next one. When they skipped the offer at this second stage, the mice incurred a cost of ‘wasted time’ that they could have spent moving towards the next offer. Over a test period of 70 days, the mice became increasingly less likely to skip rewards at the second stage. The authors interpreted this pattern to indicate that the mice had learned to anticipate and eventually avoid the experience of regret at wasting their time. If these authors’ interpretations are correct, then it follows that rodents can represent second- and even third-order temporal junctures that human children do not seem to represent until around 6 (regret) and 8 years of age (anticipated regret) respectively. On the basis of these two studies alone, however, one need not necessarily draw this conclusion. In the first study [20], the rats may have simply been comparing the rejected offer to the current offer and orienting themselves towards the rejected offer because it was clearly more preferred. As others have pointed out [33], this need not entail that the rats wound back the clock, so to speak, to mentally undo their previous action of rejecting the
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Box 3. Being Uncertain Does Not Require Awareness of Uncertainty A large body of work has demonstrated that non-human primates and other animals often act adaptively in situations of uncertainty [87,88]. In typical ‘uncertainty monitoring’ tasks, for instance, monkeys are required to discriminate between pairs of stimuli (e.g., in deciding which of two lines is longer). Correct responses are rewarded with food, whereas incorrect responses are punished by requiring the animal to wait for a period before they can move onto the next trial. Critically, throughout the task the monkeys are given the opportunity to ‘opt out’ of any given trial, which immediately moves them onto the next (potentially easier) trial without incurring the cost of waiting. Typically, after many trials, monkeys begin to opt out of difficult trials far more frequently than easy trials. One interpretation of this behaviour is that the monkeys are aware of their own uncertainty on difficult trials and thus opt out in order to maximise their overall reward frequency. Yet, non-human animals need not necessarily be aware of their own uncertainty in order to opt out appropriately in such tasks [89–92]. As schematised in Figure I, an uncertain animal may in some sense represent mutually exclusive possibilities, with each of these alternatives differing in degree of ‘representational strength’ (perhaps akin to Bayesian priors). And in an uncertainty monitoring task, an animal may simply learn that when the two possibilities are represented with roughly equal strength, committing to one outcome over the other often leads to punishment. In other words, failing to opt out when in a state of high uncertainty would become associated with punishment and the opt-out behaviour would be shaped by operant conditioning principles. (For a related discussion of rich and lean interpretations of uncertainty monitoring in human infants, see the companion review article in this issue [92].) A similar process may explain why some younger children and non-human primates cover two exits on some trials of the forked tube paradigm (Box 1) but then regress to covering only one exit on subsequent trials. That is, if the two possible outcomes are represented with roughly equal strength, then the two-exit response may briefly appear. After further experience, however, the representational strength (e.g., Bayesian prior) of each outcome shifts, such that one representation dominates the other and the one-exit response returns. This process could also explain why a small number of children pass the very first trial before regressing [15–17].
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AWARENESS OF UNCERTAINTY Trends in Cognitive Sciences
Figure I. Representational Demands of Uncertainty and Awareness of Uncertainty. As on the left, many agents may experience uncertainty about future outcomes. Some possible outcomes may be strongly predicted (denoted by the opaque bubble) and others may be weakly predicted (denoted by the translucent bubble). As on the right, however, an agent that is aware of this uncertainty can metarepresent that each represented future outcome is an alternative of the same event following a temporal juncture, with the occurrence of any one outcome precluding the occurrence of all others.
offer (note that this same alternative explanation applies to a study suggesting that apes may regret their suboptimal choices [93]). Similarly, in the second study [21], the mice may have simply learned over time that the experience of waiting for a low-desirability reward is aversive, and thus eventually avoided actions that had previously led to such an experience. That is, one only needs to appeal to operant conditioning principles to explain why such a change in the mice’s behaviour would be shaped over the long period of the study.
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Nevertheless, we cannot rule out that the rodents were in fact experiencing and anticipating regret in these studies. And if these emotions are truly within rodents’ capacity, then according to our perspective they should also be capable of representing first-order, live temporal junctures. Rodents may therefore be ideal candidates for administering species-appropriate adaptations of the forked tube task and related paradigms. Other non-primate candidates include corvids [94,95], parrots [96,97], and cetaceans [98,99], given their impressive cognitive skills demonstrated in other domains. If animals are indeed to be shown to be capable of representing first-order temporal junctures, let alone higher-order junctures, then the ethical implications could be monumental. After all, the ability to consider alternative futures is one criterion used to bestow personhood [100,101].
Concluding Remarks Many complex human behaviours are underscored by representations of future temporal junctures. Recently, we proposed the term metaforesight [68] to encapsulate the overarching capacity to reflect on alternative future possibilities and optimise behaviour accordingly. Perhaps the most obvious benefit of metaforesight is contingency planning: an agent that represents mutually exclusive future possibilities can try to ‘cover all bases’ by preparing not only for likely and desirable outcomes, but also for unlikely and undesirable outcomes [15,67,102]. Metaforesight also encompasses self-reflection on predicted outcomes of potential behaviours [103]. While thin ice may at first seem like a good surface to skate on, taking an extra moment to consider the possible outcomes (or to ask someone else) can lead one to think better of it [5]. To paraphrase Karl Popper [104], we let our hypotheses die in our stead. Representations of past temporal junctures also provide numerous benefits [9,30,31,105]. Many of these benefits flow from the ability to learn associations between particular behaviours and outcomes by simply imagining such pairings [106], an ability that has been recently documented in humans [107]. When we feel regret, for instance, we not only experience punishment for our actual behaviour, but we also typically envision the reinforcement that would have been associated with an alternative behaviour. A job candidate, for example, may not only feel anguish after providing a weak answer to a critical interview question, but also imagine the satisfaction (and new job) that may have followed if she had provided a stronger answer. This is a particularly efficient means of learning, because from a single event one can learn both: (i) what not to do, and (ii) what to do instead when similar situations arise in the future. Regardless of the original context in which they may have first evolved, representations of temporal junctures likely supported the emergence of a powerful suite of entirely novel behaviours in our hominin ancestors. These representations would have been critical to individuals forming complex if-then plans [5,108] and to human groups determining how best to proceed in cooperative endeavours [75,109]. Preparations for a hunt, a conflict, or a migration would have undoubtedly been much more efficient once our ancestors could think through (and discuss) various alternative ways the future might unfold. These ancestors could ask themselves ‘what could possibly go wrong?’, and thus prevent catastrophe before it eventuated. As in modern times, representing temporal junctures would have also enabled early humans to evaluate alternative answers to the questions of ‘who do I want to be?’ and ‘who do we want to be?’. These representations bestowed us with an intuitive sense of free will; Laplace’s Demon be damned. And with this sense of free will, along with the eventual ability to represent second-order temporal junctures, we could make increasingly sophisticated moral judgements about the courses of action taken by ourselves and others in the past. Judgements of past intent, of course, remain central to many moral systems, such that certain actions demand punishment if, and only if, the relevant outcomes of one’s past actions are deemed to have been foreseeable at the time. On an even broader note, modern discussions about the future of the human species often invoke third-order notions of legacy: ‘what will our great-grandchildren have to say about the courses of action we choose tomorrow?’ Until now, however, research on the development, evolution, functions, dysfunctions, neurocognitive mechanisms, and broader implications of the capacity to envision and evaluate events from
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Outstanding Questions In people with psychological disorders and age-related cognitive decline, is the capacity for counterfactual thinking more likely to be absent or dysfunctional than the capacity to consider alternative future possibilities? Such a pattern would be consistent with the representational hierarchy outlined herein. In children, adults, and clinical populations, are there associations between performances on working memory tasks requiring judgements of representational veracity (Box 2) and tasks requiring representations of higher-order temporal junctures? Although the shared developmental trajectory between working memory and levels of represented temporal junctures is clear, there is currently no compelling evidence of correlations between these faculties. How do children and animals perform on (ethically appropriate) tasks in which they must act to simultaneously prevent two mutually exclusive losses, or two mutually exclusive harmful outcomes? The evolutionary imperative may be stronger in these cases than in the forked tube paradigm, which requires children and non-human primates to simultaneously prepare for two mutually exclusive rewarding outcomes. What type of archaeological evidence would indicate the appearance of representations of temporal junctures, be they first-, second-, or third-order? If these representations are indeed uniquely human, then they likely emerged after the split of the hominin and chimpanzee lineages in the last 5–7 million years. Have representations of temporal junctures evolved in non-primate lineages? Corvids, parrots, and cetaceans, in particular, have demonstrated intelligence in many domains and may be ideal candidates for adaptations of the forked tube task and related paradigms.
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alternative timelines has lacked an overarching framework with which to guide predictions and situate patterns of results. Here we have provided such a framework by introducing a representational hierarchy of temporal junctures in the mind. Future studies in each domain of inquiry may benefit from attending to findings and interpretations from the other domains, and from borrowing and adapting behavioural paradigms that have been shown to be particularly fruitful (see Outstanding Questions). Research in this area is itself at a critical juncture, and the best way forward is most likely collaborative.
Acknowledgements This work was supported by a University of Queensland Development Fellowship (UQFEL1832633) awarded to J.R.
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