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Meeting of minds: Tacit coordination in adolescent and adult twins
Personality and Individual Differences 58 (2014) 31–36
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Meeting of minds: Tacit coordination in adolescent and adult twins Nancy L. Segal a,⇑, Jaimee E. Munson a, William D. Marelich a, Aaron T. Goetz a, Shirley A. McGuire b a b
California State University, Fullerton, Department of Psychology, Fullerton, CA, USA University of San Francisco, Department of Psychology, San Francisco, CA, USA
a r t i c l e
i n f o
Article history: Received 25 June 2013 Received in revised form 17 September 2013 Accepted 24 September 2013 Available online 18 October 2013 Keywords: Twins Monozygotic Dizygotic Tacit coordination
a b s t r a c t Kin selection theory predicts that cooperative acts should occur more often between individuals sharing a higher proportion of genes than those sharing fewer. This expectation is supported, in part, by the greater social closeness of monozygotic (MZ) twins than dizygotic (DZ) twins. Previous research in 2008, using young twins, suggested that the synchronized MZ twin relationship may partly reflect non-negotiated consensus, occurring as a function of the twins’ shared behavioral similarities. The present study sought to determine if tacit coordination also varies with genetic relatedness in older twins. The sample included 43 MZ twin pairs and 34 DZ twin pairs, ages 12–59 years, who completed a Tacit Coordination Questionnaire under Individual and Coordination conditions. Significant Zygosity and Condition effects were found, with MZ twins outscoring DZ twins, and Coordination matches exceeding Individual matches. Logistic regression analysis identified specific classes of items that discriminated between twin types better than others. The present findings are compared with those from the 2008 study, refining theories concerning genetic contributions to coordination and partner success. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction Researchers have long been interested in identifying factors making some partners more successful than others in achieving mutual goals. The study of tacit coordination (TC) concerns an individual’s ability to recognize courses of action leading to desired outcomes. TC refers to circumstances in which ‘‘two parties have identical interests and face the problem not of reconciling interests, but only of coordinating their actions for their mutual benefit when communication is impossible’’ (Schelling, 1960, p. 54). If two hikers became separated they would need to synchronize their actions (in the absence of communication) in order to reunite. Given the complexity of the social environment in which decision-making occurs, various theoretical approaches have offered explanations for such behaviors. Two approaches include evolutionary psychological and behavioral-genetic perspectives, both of which have been underutilized in experimental game research. 1.1. Evolutionary psychology Evolutionary psychology (EP) examines why the mind is designed as it is, and how this design interacts with environmental events to produce observable behaviors (Buss, 2004). Cognitive structure, like physiological structure, undergoes evo⇑ Corresponding author. Address: CSUF, Psychology Department, 800 N. State College Blvd., Fullerton, CA 92834, USA. Tel.: +1 657 278 2142; fax: +1 714 278 4843. E-mail address: [email protected] (N.L. Segal). 0191-8869/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.paid.2013.09.028
lution by natural selection, in which processes that are seemingly adapted to the functional roles they perform are preserved and reproduced (Cosmides & Tooby, 1987). Therefore, it is argued that persisting psychological mechanisms generally led to the successful solution of specific adaptive problems for the organisms’ ancestors. However, cooperative behavior such as altruism contradicts these assumptions, as altruistic acts reflect behavior benefitting a recipient at some cost to the actor (Kurland & Gaulin, 2005). This dilemma was resolved by kin selection theory, namely the concept of inclusive fitness (Hamilton, 1964). Hamilton asserted that behaviors incurring a cost to the self (e.g., altruism) could evolve if such costs were outweighed by the benefit, multiplied by the coefficient of relatedness. Stated differently, natural selection favors alleles predisposing individuals to act in ways favoring the transmission of those alleles. Thus, individuals are biologically predisposed to act altruistically if the beneficiary (1) shares genes with the benefactor, providing an indirect ‘‘vehicle’’ for the benefactor to transmit his or her genes, and/or (2) appears likely to reciprocate the altruistic act (Trivers, 1971). Accurate identification of relatives is requisite to this process. Lieberman, Tooby, and Cosmides (2007) described converging lines of evidence for human kin detection mechanisms. These mechanisms provide individuals with information to assess kinship, thereby expressing behaviors consistent with that degree of relatedness (i.e., greater levels of cooperative acts would be expected between the self and another, estimated to be a close relative). The authors emphasized the impact of maternal perinatal
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association and duration of coresidence as primary informative cues responsible for estimating kinship. However, these factors would not account for the greater observed prosocial behavior between MZ co-twins relative to DZ co-twins. Twins pose a curious situation regarding factors affecting differential social relatedness. Co-twins in both MZ and DZ pairs witness one another in perinatal association with their mother during development and generally share the same home since birth. However, studies consistently find greater social closeness between MZ than DZ twins regardless of age and gender (Neyer, 2002; Segal, 2000). More compelling, this same social closeness difference has been observed between MZ and DZ twins reared apart and reunited as adults (Segal, 2012). These differences must, therefore, be associated with factors beyond the maternal rearing situation. The comparison of MZ and DZ twins’ interactional patterns in the present study may help identify these factors. 1.2. Behavior genetics ehavioral-genetic theory (BG) concerns the genetic transmission of behavioral traits. BG research utilizes genetically and environmentally informative designs, such as those involving twins or adoptees. The twin design compares the similarity of MZ and DZ co-twins on a specified task or trait. MZ twins share 100% of their genes, while DZ twins share, on average, 50% of their genes by descent, making them genetically equivalent to full siblings. By comparing trait similarity between MZ and DZ twins, researchers can estimate the extent to which genetic and environmental influences underlie variation in behavioral and physical characteristics (Segal, 2000). A subfield of BG, social genetics (SG), asserts that most behaviors of highly social species are expressed within social relationships and, therefore, are rarely independent or individualized (Scott, 1977). SG research examines the effects of interactants’ genotypes on social behavior. Segal (1984, 2002) showed that young MZ twin pairs were more cooperative during puzzle completion than DZ twin pairs. Loh and Elliott (1998) observed that MZ twin pairs cooperated more than DZ twin pairs when reward equality was guaranteed. However, MZ twins competed to a greater degree than DZ twins when conditions promised the same outcome for both cotwins. The investigators suggested that the second situation gave MZ twins opportunities for ‘‘dominance testing’’ (p. 408), because there was little to challenge their relationship.
questions under two different conditions. In the Individual condition, participants simply answered the questions, while in the Coordination condition, participants were instructed to independently provide the same answer as their twin. Across both settings, MZ twin pairs showed greater agreement, followed by DZ and VT twin pairs, in that order. It was suggested that MZ twins’ demonstrated capacities for TC may imply an active gene-environment correlation, in which an individual’s decisions reflect a genetic propensity towards certain activities, such that he or she seeks out, or modifies, his or her environment in selected ways. In this sense, more closely related individuals should exhibit more similar thought processes and render more similar decisions. 1.4. Tacit coordination According to Mehta, Starmer, and Sugden (1994), circumstances which promote successful tacit coordination (non-negotiated consensus) rely on partners’ mutual recognition of signals that align each partner’s expectations with the other’s. ‘‘Thus, the aim is. . .to identify a course of action upon which each partner’s expectations converge [Schelling salience]. For example, if two people became separated, it would be advantageous for each to go to a place that both prefer, not to a place each person knows the other would individually prefer’’ (Segal et al., 2008, p. 608). Mehta et al. (1994) distinguished Schelling salience from primary salience (choosing a course of action that is more likely than any other) and secondary salience (choosing a course of action that is more likely than any other due to its primary salience for the partner). 1.5. The present study The present study aimed to replicate and expand the TC findings reported by Segal et al. (2008). The goal was to determine if the same positive relationship between genetic relatedness and TC found in the young twin sample would be found in an adolescent and adult twin sample. As in the previous study, Schelling salience was the focus of the present study. The issue under investigation was whether genetic factors underlie decisions and strategies resulting in the achievement of mutual goals (i.e., independently producing the same answers to given questions). Several hypotheses involving tacit coordination were examined. The methodology of this study is described first to provide understanding of the hypotheses listed below.
1.3. Twin studies of experimental games 2. Methods A surprisingly small number of studies have included genetically informative participants in experimental games (Segal, McGuire, Miller, & Havlena, 2008). A smaller number have paired co-twins together in experimental games, consequently missing opportunities to assess the role of genetic relatedness in such settings. The first study to compare MZ and DZ co-twins’ performance on a prisoner’s dilemma game found greater cooperation within MZ pairs than DZ pairs (Segal & Hershberger, 1999). Subsequent research in this area compared MZ and DZ twins in two independent studies, using an economic trust game (Wallace, Cesarini, Lichtenstein, & Johannesson, 2007). Results, based on data gathered in the United States and in Sweden, showed that genetic variation influenced decisions to invest and to reciprocate investment. Follow-up studies found similar patterns of behavior among Swedish MZ and DZ twins who participated in economic games, including trust, dictator and ultimatum games (Cesarini, Dawes, Johannesson, Lichtenstein, & Wallace, 2009; Cesarini et al., 2008). Segal et al. (2008) examined TC among young MZ twins (7–13 years of age), DZ twins and virtual twins (VTs, i.e., sameage genetically unrelated siblings reared together since infancy). Co-twins were given a questionnaire and asked to answer the same
2.1. Participants Participants were adolescent and adult twins sampled mostly through referrals to the CSUF Twin Studies Center. However, data for 16 pairs were collected by a Florida school teacher who sought research participation for her students. Data were available for 77 twin pairs (43 MZ and 34 DZ), 12–59 years of age. Nearly 60% of the participants were female; it is not uncommon in twin research to have a higher proportion of MZ and female twin volunteers than DZ and male twin volunteers (Lykken, McGue, & Tellegen, 1987). Significant twin group differences in age (MZ: M = 24.47, SD = 10.27; DZ: M = 19.50, SD = 5.67) [t (67.80) = 2.69, p < .01] and age variance [F = 5.23, p < .05] were found. Table 1 displays the composition of the sample. 2.2. Materials 2.2.1. Physical Resemblance Questionnaire A standard questionnaire was administered to each twin to determine pair zygosity (Nichols & Bilbro, 1966). This form shows
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Hypothesis 3. Part 1 > Part II. The first set of questions was expected to show a higher number of exact matches than the second set of questions.
Table 1 Zygosity, age and gender of the sample. Zygosity
N (pairs)
MZ
43
DZ
34
Age (years)
Gender (N = Pairs)
Mean (SD) range
Male (N)
Female (N)
MF (N)
24.47 (10.27) 12–59 19.50 (5.67) 14–40
16
27
–
11
13
10
93% agreement with serological analyses. One DZ female pair that could not be classified in this way was diagnosed as DZ by DNA testing. Another female twin pair could not be classified so was omitted from the study. 2.2.2. Tacit Coordination Questionnaire (TCQ) A twenty-two item questionnaire, modeled after an inventory by Mehta et al. (1994), was identical to the one used by Segal et al. (2008). Sample items include: ‘‘Name any US town/city’’ and ‘‘Write down any color’’; see the Appendix. 2.3. Procedure The study took place at CSUF’s Twin Studies Center and at a Florida high school. Each twin signed a consent form that outlined the scope of the study and described participants’ rights. Each twin then completed the TCQ independently and in separate rooms, under the Individual and Coordination conditions described by Segal et al. (2008). The Individual condition questions came first for all participants, followed immediately by the Coordination questions. Young twins in the 2008 study completed questions 1–12 (Part I) under the Individual and Coordination conditions in that order, then finished questions 13–22 (Part II) after completing other interviews in between. Individual condition: Twins were instructed to answer the questions ‘‘any way that you like.’’ Coordination condition: Twins were instructed to answer the same set of questions, with the goal of ‘‘providing the same answer as your twin’’. All responses were reviewed for completion and clarity by two assistants. Participant responses were scored as one of two match types. A Broad match was considered an approximate match. For example, in response to the item: ‘‘Name a color,’’ if one twin wrote ‘‘red’’ and the other wrote ‘‘ruby,’’ judges classified the response as a Broad match. However, if both twins answered ‘‘red,’’ the response would be classified as an Exact match. (Broad match scores equaled the sum of the Broad and Exact matches). Responses that did not meet either of these criteria were considered non-matches. Scoring required that two judges agree; disagreements were resolved by a third judge who rendered an opinion independently. 2.4. Hypotheses Hypotheses were generated by theoretical considerations and by findings from the 2008 study. Hypothesis 1. Zygosity: MZ > DZ. MZ twin pairs were expected to obtain a higher number of matches than DZ twin pairs.
Hypothesis 2. Condition: Coordination > Individual. The Coordination condition was expected to produce a higher number of matches than the Individual condition, given instructions to provide the same answer as the co-twin.
Hypothesis 4. MZ Coordination vs. DZ Coordination > MZ Individual vs. DZ Individual. The MZ–DZ difference in the Coordination condition was expected to exceed the MZ–DZ difference in the Individual condition.
3. Results 3.1. Correlational analyses Twins in this study showed a wide age range (12–59 years). However, none of the correlations between age or gender and the TC scores (rs = .17 to .19, n = 77) were statistically significant, based on the entire twin sample; thus, the age range did not appear problematic. Correlations between Broad and Exact matches (n = 77) were high and significant for Part I (Individual: r = .91, p < .001, Coordination: r = .91, p < .001) and Part II (Individual: r = .96, p < .001, Coordination: r = .94, p < .001). Given the consistency across coding schemes and the demonstrated greater sensitivity of exact matches to twin group differences, all analyses used Exact match data only. 3.2. General linear model The primary analysis was a repeated measures MANOVA (General Linear Model) with one between-group factor: Zygosity (MZ, DZ) and two within-subject factors: Condition (Individual, Coordination) and Questions (Part 1, Part II). Questions was included as a within-subjects factor to maintain comparability with the 2008 study. (Hypothesis 1): A significant multivariate effect was found for Zygosity [F(1, 75) = 17.98, p < .001]. As expected, MZ twins obtained more matches than DZ twins. (Hypotheses 2 and 3): Both Condition [F(1, 75) = 50.54, p < .001] and Questions [F(1, 75) = 37.39, p < .001] showed significant multivariate effects. Scores were higher under the Coordination than the Individual condition, and higher for Part I than Part II questions. These findings replicated the 2008 analysis of young twins. (Hypothesis 4): The Zygosity Condition interaction was not statistically significant, although the MZ twins outperformed the DZ twins to some extent in the Coordination condition. In contrast, the MZ– DZ difference in the 2008 study was nearly identical across conditions. The present findings are summarized in Tables 2 and 3. An unanticipated significant effect was found for the Questions Condition interaction [F(1, 75) = 17.24, p < .001]. A higher number of matches was obtained for Part I questions in the Coordination condition (4.47, SD = 1.79) than in the Individual condition (2.59, 2.62, SD = 2.04). None of the two-way interactions were significant in the earlier study.
Table 2 Mean TC scores for twins, organized by Condition (Individual and Coordination). Zygosity
Mean
SD
Range
Individual MZ DZ Total
5.65 3.85 4.85
2.71 2.18 2.63
0–12 1–8 0–12
Coordination MZ DZ Total
8.35 6.00 7.31
2.72 2.69 2.93
2–14 1–11 1–14
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Table 3 Mean TC scores for twins, organized by questions (Parts I, II).
80
Mean
SD
Range
Part I MZ DZ Total
7.95 6.00 7.09
2.89 2.88 3.03
2–14 1–14 1–14
Part II MZ DZ Total
6.05 3.85 5.08
2.39 1.99 2.46
1–13 0–8 0–13
70 Percent of Exact Matches
Zygosity
60 50 40
MZ
30
DZ
20 10
3.3. Individual item evaluation
0
3.3.1. Adolescent and adult twins Graphs were created to depict how the MZ and DZ pairs responded to each TCQ item; see Fig. 1a (Individual) and Fig. 1b (Coordination). The Hosmer–Lemeshow Test indicated a good fit to the data (i.e., twin type discrimination) in the Individual condition [X2 (8) = 7.05, p < .53]. Seven items successfully assigned 75% of the pairs. A somewhat higher percentage of MZ (77.5%) than DZ twin pairs (71.9%) was identified correctly, using a cutoff value of .50. The Hosmer–Lemeshow Test also indicated a good fit to the data for the Coordination condition [X2 (5) = 1.53, p < .91]. Four items successfully assigned 75.8% of the twin pairs. A somewhat higher percentage of DZ (80.6%) than MZ twin pairs (71.4%) was identified correctly. The Wald statistic and significance levels for each item in both conditions are displayed in Table 4.
Percent of Exact Matches
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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 Items on TC Questionnaire Fig. 1b. Percent of matches by Zygosity in the Coordination condition (Adult).
Table 4 Wald statistics for items in the individual and coordination conditions: adolescents/ adults.
a
Item
Wald statistic
DF
p<
Individual Boy’s name Favorite thing to do Sport Place in a park to meet Book Day of the week Yeara
3.82 6.71 4.46 3.99 5.77 5.43 3.71
1 1 1 1 1 1 1
.051 .010 .035 .046 .016 .020 .054
Coordination Car Positive number Movie Musical group
5.11 6.40 3.28 3.44
1 1 1 1
.024 .011 .070 .064
Part of month–day–year (items 4–6).
50 45 Percent of Exact Matches
It was possible that some items or classes of items better distinguished between MZ and DZ twin pairs than others. Mehta et al. (1994) noted that in a pure coordination game most people answer ‘‘1’’ when asked to provide a positive number. Similarly, 66.7% of randomly paired individuals in her study answered ‘‘rose’’ when asked to name a flower. Such items, which elicit similar responses from people due to shared experiential or cultural factors, would not be expected to distinguish between MZ and DZ twins. However, questions that draw forth a range of responses, such as ‘‘name a color,’’ might discriminate more effectively between twin types. This analysis was not undertaken in 2008, but is reported below for both twin samples. Logistic regression was performed to determine if some items better discriminated between MZ and DZ pairs than others. A WALD forward stepwise approach, with a variable entry cutoff of .15, and .20 for removal, was used (see Hosmer & Lemeshow, 2000 for justification). Predictor variables were the twenty-two TCQ items. This analysis was done twice, using data from the Individual and Coordination conditions, respectively.
40 35 30 25
MZ
20
DZ
15 10 5 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 Items on TC Questionnaire
Fig. 2a. Percent of exact matches by Zygosity in the Individual condition (Child).
50 40 30
MZ DZ
20 10 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 Items on TC Questionnaire
Fig. 1a. Percent of matches by Zygosity in the Individual condition (Adult). Note: Items corresponding to item numbers are listed in the Appendix.
3.3.2. Young twins Graphs were created to depict how the young MZ and DZ pairs responded to each TCQ item; see Fig. 2a (Individual) and Fig. 2b (Coordination). The Hosmer–Lemeshow Test showed a good fit to the data in the Individual condition [X2 (6) = 2.91, p < .82]. Five items successfully assigned 73.4% of the twin pairs. A higher percentage of DZ (89.5%) than MZ twin pairs (47.2%) was identified correctly. The Hosmer–Lemeshow Test also showed a good fit to the data in the Coordination condition [X2 (7) = 8.10, p < .32]. Four items successfully assigned 69.1% of the twin pairs. A higher percentage of DZ (90.7%) than MZ twin pairs (34.0%) was identified correctly. These findings are displayed in Table 5.
N.L. Segal et al. / Personality and Individual Differences 58 (2014) 31–36
60
Percent of Exact Matches
50 40 30
MZ DZ
20 10 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 Items on TC Questionnaire
Fig. 2b. Percent of exact matches by Zygosity in the Coordination condition (Child).
Table 5 Wald statistics for items in the individual and coordination conditions: young twins.
a
Item
Wald statistic
DF
p<
Individual Boy’s name Movie Place in a park to meet Country Yeara
9.42 2.93 4.54 4.54 3.72
1 1 1 1 1
.002 .087 .033 .033 .054
Coordination Car Ice cream flavor Sport Musical group
2.59 4.06 3.10 2.61
1 1 1 1
.108 .044 .078 .106
Single question (item 1): name a year.
4. Discussion Findings from the present study demonstrated greater success by adolescent and adult MZ twin pairs compared to DZ twin pairs on the TC task. These results support those of Segal et al. (2008) in showing a positive relationship between genetic relatedness and coordination. When presented with opportunities to choose, closer genetic relatives make more similar decisions than do more distant genetic relatives. The Zygosity Condition interaction was not statistically significant, yet somewhat greater success among MZ twins, relative to DZ twins, in the Coordination condition was observed. This trend may reflect MZ twins’ greater ease in the synchronization of goals. Note that both MZ and DZ twins improved their scores in the Coordination condition, but the MZ–DZ difference widened. These results are consistent with EP predictions that prosocial behavior should vary with the genetic relatedness of the interactants. Close genetic relatives may be predisposed to work together successfully given psychological mechanisms facilitating their ability to do so. Kin selection theory offers explanations for how individuals are able to assess the degree of genetic relatedness between themselves and others. Thus, the results of this study should contribute to existing models of human kin detection. Perhaps increased physical resemblance between two individuals (as is characteristic of MZ compared to DZ twin pairs), may act as an initial cue when assessing degree of relatedness. This kinship estimation, together with perceived behavioral similarities, may provide a mechanism by which individuals regulate their decisions to cooperate or compete with others. This process may be part of a broader class of mechanisms by which individuals develop and maintain close social relations (Segal et al., 2008).
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Significant multivariate effects were found for Condition (Coordination higher) and Questions (Part I higher), as in the 2008 study. Apparently, both MZ and DZ twins put forth greater effort to match their co-twin when instructed to do so, yielding a higher number of matches in the Coordination condition than in the Individual condition. Part I included two more questions (1–12) than Part II (13–22), most likely explaining the higher number of Part I matches. The unanticipated significant Condition Questions interaction may reflect both the higher number of Part I questions and/or the additional efforts made by participants. The item analysis revealed some informative findings. First, a larger number of items distinguished between MZ and DZ adolescent and adult twins (7), as compared with the young twins (5), in the Individual condition, but not in the Coordination condition. In the absence of pressure to conform, MZ twins appear to match more easily than DZ twins, possibly accounting for the discrepancy. The fewer distinguishing items found for the younger sample may reflect their relatively reduced range of experience. Distinguishing items common to both twin samples were the boy’s name and a place to meet in the park. It appears that these items have many possible answers on which MZ twins, especially, concur. Four items distinguished between MZ and DZ twins in both studies during Coordination, two of which (car and musical group) were common to both. These items showed higher agreement among the MZ twins, compared to the DZ twins. Somewhat surprisingly, interests and preferences in these domains appear well established even in childhood. A much higher percentage of young DZ twins were correctly classified in both the Individual (89.5% vs. 47.2%) and Coordination conditions (90.7% vs. 34.0%). Perhaps this is because the distinguishing items showed such low percentages of agreement among the DZ twin children, relative to the MZ twin children, making DZ twins easier to classify. Young MZ twins, while more successful in TC than DZ twins, may be somewhat variable in their coordination, relative to older MZ twins. It is possible that as twins age, MZ twins are better able to coordinate their responses as their interests and preferences increasingly converge. 4.1. Limitations Findings from the present study provide additional support for kin selection theories of social interaction, yet the Zygosity Condition interaction was non-significant. This may be partly due to the small sample size (n = 77 pairs). Alternatively, it is likely that as individuals mature they spend less time in decision-making situations with their families and more time with their colleagues and friends. These professional and social experiences could conceivably affect decision-making processes with family members. Regardless, future researchers are encouraged to include larger and more comparable twin group sizes to address these possibilities, and to arrange repeated experimental sessions. 4.2. Future research Future research is needed to explore possible associations between selected behavioral and physical similarities and TC, more specifically the role that perceived similarities play. However, physical similarity and shared genes are confounded in MZ twin pairs. Therefore, MZ pairs would have to be organized according to both perceived and measured physical and behavioral similarities to obtain a more comprehensive understanding of TC processes and outcomes. Behavioral similarities would be expected to play a larger role in TC, given the lack of personality similarity and lack of close social relatedness found in unrelated look-alike pairs (Segal, Graham, & Ettinger, 2013). Future analyses should also consider comparing TC between each twin and their close friends
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and colleagues to investigate the nature and effect of other relationships in twins’ expanding social networks. Overall, the present study of tacit coordination provides an informative view of human social interaction. Successful TC and its associated factors enhance our understanding of why some individuals work better together than others. The use of genetically sensitive designs provides an informative approach to understanding the nature and origin of this behavior. Acknowledgments The twins are gratefully acknowledged for their time and interest in this project. Jorge L. Torres, Jamie L. Graham, Ammar Altowaji and Jaime M. Velazquez provided research assistance. This work was partly supported by a CSUF Faculty Research Award to Dr. Segal and a CSUF Graduate Equity Fellowship to Jaimee Munson. Appendix Tacit Coordination Questionnaire. Write/name: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22
Year (past, present or future) Flower Car manufacturer Day of the year/week Year Month/date US town/city Positive number Color Boy’s name A coin was tossed. It came down_______________ The doctor asked for the patient’s records. The nurse gave them to__________ Favorite thing to do Ice cream flavor Movie Sport Clothing article You are meeting your twin at a big park. Where in the park will you meet? You are looking for your twin at a big department store. Where will you look for him/her? Musical group Country Book
Items 4–6 presented as one three-part item
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Meeting of minds: Tacit coordination in adolescent and adult twins Nancy L. Segal a,⇑, Jaimee E. Munson a, William D. Marelich a, Aaron T. Goetz a, Shirley A. McGuire b a b
California State University, Fullerton, Department of Psychology, Fullerton, CA, USA University of San Francisco, Department of Psychology, San Francisco, CA, USA
a r t i c l e
i n f o
Article history: Received 25 June 2013 Received in revised form 17 September 2013 Accepted 24 September 2013 Available online 18 October 2013 Keywords: Twins Monozygotic Dizygotic Tacit coordination
a b s t r a c t Kin selection theory predicts that cooperative acts should occur more often between individuals sharing a higher proportion of genes than those sharing fewer. This expectation is supported, in part, by the greater social closeness of monozygotic (MZ) twins than dizygotic (DZ) twins. Previous research in 2008, using young twins, suggested that the synchronized MZ twin relationship may partly reflect non-negotiated consensus, occurring as a function of the twins’ shared behavioral similarities. The present study sought to determine if tacit coordination also varies with genetic relatedness in older twins. The sample included 43 MZ twin pairs and 34 DZ twin pairs, ages 12–59 years, who completed a Tacit Coordination Questionnaire under Individual and Coordination conditions. Significant Zygosity and Condition effects were found, with MZ twins outscoring DZ twins, and Coordination matches exceeding Individual matches. Logistic regression analysis identified specific classes of items that discriminated between twin types better than others. The present findings are compared with those from the 2008 study, refining theories concerning genetic contributions to coordination and partner success. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction Researchers have long been interested in identifying factors making some partners more successful than others in achieving mutual goals. The study of tacit coordination (TC) concerns an individual’s ability to recognize courses of action leading to desired outcomes. TC refers to circumstances in which ‘‘two parties have identical interests and face the problem not of reconciling interests, but only of coordinating their actions for their mutual benefit when communication is impossible’’ (Schelling, 1960, p. 54). If two hikers became separated they would need to synchronize their actions (in the absence of communication) in order to reunite. Given the complexity of the social environment in which decision-making occurs, various theoretical approaches have offered explanations for such behaviors. Two approaches include evolutionary psychological and behavioral-genetic perspectives, both of which have been underutilized in experimental game research. 1.1. Evolutionary psychology Evolutionary psychology (EP) examines why the mind is designed as it is, and how this design interacts with environmental events to produce observable behaviors (Buss, 2004). Cognitive structure, like physiological structure, undergoes evo⇑ Corresponding author. Address: CSUF, Psychology Department, 800 N. State College Blvd., Fullerton, CA 92834, USA. Tel.: +1 657 278 2142; fax: +1 714 278 4843. E-mail address: [email protected] (N.L. Segal). 0191-8869/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.paid.2013.09.028
lution by natural selection, in which processes that are seemingly adapted to the functional roles they perform are preserved and reproduced (Cosmides & Tooby, 1987). Therefore, it is argued that persisting psychological mechanisms generally led to the successful solution of specific adaptive problems for the organisms’ ancestors. However, cooperative behavior such as altruism contradicts these assumptions, as altruistic acts reflect behavior benefitting a recipient at some cost to the actor (Kurland & Gaulin, 2005). This dilemma was resolved by kin selection theory, namely the concept of inclusive fitness (Hamilton, 1964). Hamilton asserted that behaviors incurring a cost to the self (e.g., altruism) could evolve if such costs were outweighed by the benefit, multiplied by the coefficient of relatedness. Stated differently, natural selection favors alleles predisposing individuals to act in ways favoring the transmission of those alleles. Thus, individuals are biologically predisposed to act altruistically if the beneficiary (1) shares genes with the benefactor, providing an indirect ‘‘vehicle’’ for the benefactor to transmit his or her genes, and/or (2) appears likely to reciprocate the altruistic act (Trivers, 1971). Accurate identification of relatives is requisite to this process. Lieberman, Tooby, and Cosmides (2007) described converging lines of evidence for human kin detection mechanisms. These mechanisms provide individuals with information to assess kinship, thereby expressing behaviors consistent with that degree of relatedness (i.e., greater levels of cooperative acts would be expected between the self and another, estimated to be a close relative). The authors emphasized the impact of maternal perinatal
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association and duration of coresidence as primary informative cues responsible for estimating kinship. However, these factors would not account for the greater observed prosocial behavior between MZ co-twins relative to DZ co-twins. Twins pose a curious situation regarding factors affecting differential social relatedness. Co-twins in both MZ and DZ pairs witness one another in perinatal association with their mother during development and generally share the same home since birth. However, studies consistently find greater social closeness between MZ than DZ twins regardless of age and gender (Neyer, 2002; Segal, 2000). More compelling, this same social closeness difference has been observed between MZ and DZ twins reared apart and reunited as adults (Segal, 2012). These differences must, therefore, be associated with factors beyond the maternal rearing situation. The comparison of MZ and DZ twins’ interactional patterns in the present study may help identify these factors. 1.2. Behavior genetics ehavioral-genetic theory (BG) concerns the genetic transmission of behavioral traits. BG research utilizes genetically and environmentally informative designs, such as those involving twins or adoptees. The twin design compares the similarity of MZ and DZ co-twins on a specified task or trait. MZ twins share 100% of their genes, while DZ twins share, on average, 50% of their genes by descent, making them genetically equivalent to full siblings. By comparing trait similarity between MZ and DZ twins, researchers can estimate the extent to which genetic and environmental influences underlie variation in behavioral and physical characteristics (Segal, 2000). A subfield of BG, social genetics (SG), asserts that most behaviors of highly social species are expressed within social relationships and, therefore, are rarely independent or individualized (Scott, 1977). SG research examines the effects of interactants’ genotypes on social behavior. Segal (1984, 2002) showed that young MZ twin pairs were more cooperative during puzzle completion than DZ twin pairs. Loh and Elliott (1998) observed that MZ twin pairs cooperated more than DZ twin pairs when reward equality was guaranteed. However, MZ twins competed to a greater degree than DZ twins when conditions promised the same outcome for both cotwins. The investigators suggested that the second situation gave MZ twins opportunities for ‘‘dominance testing’’ (p. 408), because there was little to challenge their relationship.
questions under two different conditions. In the Individual condition, participants simply answered the questions, while in the Coordination condition, participants were instructed to independently provide the same answer as their twin. Across both settings, MZ twin pairs showed greater agreement, followed by DZ and VT twin pairs, in that order. It was suggested that MZ twins’ demonstrated capacities for TC may imply an active gene-environment correlation, in which an individual’s decisions reflect a genetic propensity towards certain activities, such that he or she seeks out, or modifies, his or her environment in selected ways. In this sense, more closely related individuals should exhibit more similar thought processes and render more similar decisions. 1.4. Tacit coordination According to Mehta, Starmer, and Sugden (1994), circumstances which promote successful tacit coordination (non-negotiated consensus) rely on partners’ mutual recognition of signals that align each partner’s expectations with the other’s. ‘‘Thus, the aim is. . .to identify a course of action upon which each partner’s expectations converge [Schelling salience]. For example, if two people became separated, it would be advantageous for each to go to a place that both prefer, not to a place each person knows the other would individually prefer’’ (Segal et al., 2008, p. 608). Mehta et al. (1994) distinguished Schelling salience from primary salience (choosing a course of action that is more likely than any other) and secondary salience (choosing a course of action that is more likely than any other due to its primary salience for the partner). 1.5. The present study The present study aimed to replicate and expand the TC findings reported by Segal et al. (2008). The goal was to determine if the same positive relationship between genetic relatedness and TC found in the young twin sample would be found in an adolescent and adult twin sample. As in the previous study, Schelling salience was the focus of the present study. The issue under investigation was whether genetic factors underlie decisions and strategies resulting in the achievement of mutual goals (i.e., independently producing the same answers to given questions). Several hypotheses involving tacit coordination were examined. The methodology of this study is described first to provide understanding of the hypotheses listed below.
1.3. Twin studies of experimental games 2. Methods A surprisingly small number of studies have included genetically informative participants in experimental games (Segal, McGuire, Miller, & Havlena, 2008). A smaller number have paired co-twins together in experimental games, consequently missing opportunities to assess the role of genetic relatedness in such settings. The first study to compare MZ and DZ co-twins’ performance on a prisoner’s dilemma game found greater cooperation within MZ pairs than DZ pairs (Segal & Hershberger, 1999). Subsequent research in this area compared MZ and DZ twins in two independent studies, using an economic trust game (Wallace, Cesarini, Lichtenstein, & Johannesson, 2007). Results, based on data gathered in the United States and in Sweden, showed that genetic variation influenced decisions to invest and to reciprocate investment. Follow-up studies found similar patterns of behavior among Swedish MZ and DZ twins who participated in economic games, including trust, dictator and ultimatum games (Cesarini, Dawes, Johannesson, Lichtenstein, & Wallace, 2009; Cesarini et al., 2008). Segal et al. (2008) examined TC among young MZ twins (7–13 years of age), DZ twins and virtual twins (VTs, i.e., sameage genetically unrelated siblings reared together since infancy). Co-twins were given a questionnaire and asked to answer the same
2.1. Participants Participants were adolescent and adult twins sampled mostly through referrals to the CSUF Twin Studies Center. However, data for 16 pairs were collected by a Florida school teacher who sought research participation for her students. Data were available for 77 twin pairs (43 MZ and 34 DZ), 12–59 years of age. Nearly 60% of the participants were female; it is not uncommon in twin research to have a higher proportion of MZ and female twin volunteers than DZ and male twin volunteers (Lykken, McGue, & Tellegen, 1987). Significant twin group differences in age (MZ: M = 24.47, SD = 10.27; DZ: M = 19.50, SD = 5.67) [t (67.80) = 2.69, p < .01] and age variance [F = 5.23, p < .05] were found. Table 1 displays the composition of the sample. 2.2. Materials 2.2.1. Physical Resemblance Questionnaire A standard questionnaire was administered to each twin to determine pair zygosity (Nichols & Bilbro, 1966). This form shows
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Hypothesis 3. Part 1 > Part II. The first set of questions was expected to show a higher number of exact matches than the second set of questions.
Table 1 Zygosity, age and gender of the sample. Zygosity
N (pairs)
MZ
43
DZ
34
Age (years)
Gender (N = Pairs)
Mean (SD) range
Male (N)
Female (N)
MF (N)
24.47 (10.27) 12–59 19.50 (5.67) 14–40
16
27
–
11
13
10
93% agreement with serological analyses. One DZ female pair that could not be classified in this way was diagnosed as DZ by DNA testing. Another female twin pair could not be classified so was omitted from the study. 2.2.2. Tacit Coordination Questionnaire (TCQ) A twenty-two item questionnaire, modeled after an inventory by Mehta et al. (1994), was identical to the one used by Segal et al. (2008). Sample items include: ‘‘Name any US town/city’’ and ‘‘Write down any color’’; see the Appendix. 2.3. Procedure The study took place at CSUF’s Twin Studies Center and at a Florida high school. Each twin signed a consent form that outlined the scope of the study and described participants’ rights. Each twin then completed the TCQ independently and in separate rooms, under the Individual and Coordination conditions described by Segal et al. (2008). The Individual condition questions came first for all participants, followed immediately by the Coordination questions. Young twins in the 2008 study completed questions 1–12 (Part I) under the Individual and Coordination conditions in that order, then finished questions 13–22 (Part II) after completing other interviews in between. Individual condition: Twins were instructed to answer the questions ‘‘any way that you like.’’ Coordination condition: Twins were instructed to answer the same set of questions, with the goal of ‘‘providing the same answer as your twin’’. All responses were reviewed for completion and clarity by two assistants. Participant responses were scored as one of two match types. A Broad match was considered an approximate match. For example, in response to the item: ‘‘Name a color,’’ if one twin wrote ‘‘red’’ and the other wrote ‘‘ruby,’’ judges classified the response as a Broad match. However, if both twins answered ‘‘red,’’ the response would be classified as an Exact match. (Broad match scores equaled the sum of the Broad and Exact matches). Responses that did not meet either of these criteria were considered non-matches. Scoring required that two judges agree; disagreements were resolved by a third judge who rendered an opinion independently. 2.4. Hypotheses Hypotheses were generated by theoretical considerations and by findings from the 2008 study. Hypothesis 1. Zygosity: MZ > DZ. MZ twin pairs were expected to obtain a higher number of matches than DZ twin pairs.
Hypothesis 2. Condition: Coordination > Individual. The Coordination condition was expected to produce a higher number of matches than the Individual condition, given instructions to provide the same answer as the co-twin.
Hypothesis 4. MZ Coordination vs. DZ Coordination > MZ Individual vs. DZ Individual. The MZ–DZ difference in the Coordination condition was expected to exceed the MZ–DZ difference in the Individual condition.
3. Results 3.1. Correlational analyses Twins in this study showed a wide age range (12–59 years). However, none of the correlations between age or gender and the TC scores (rs = .17 to .19, n = 77) were statistically significant, based on the entire twin sample; thus, the age range did not appear problematic. Correlations between Broad and Exact matches (n = 77) were high and significant for Part I (Individual: r = .91, p < .001, Coordination: r = .91, p < .001) and Part II (Individual: r = .96, p < .001, Coordination: r = .94, p < .001). Given the consistency across coding schemes and the demonstrated greater sensitivity of exact matches to twin group differences, all analyses used Exact match data only. 3.2. General linear model The primary analysis was a repeated measures MANOVA (General Linear Model) with one between-group factor: Zygosity (MZ, DZ) and two within-subject factors: Condition (Individual, Coordination) and Questions (Part 1, Part II). Questions was included as a within-subjects factor to maintain comparability with the 2008 study. (Hypothesis 1): A significant multivariate effect was found for Zygosity [F(1, 75) = 17.98, p < .001]. As expected, MZ twins obtained more matches than DZ twins. (Hypotheses 2 and 3): Both Condition [F(1, 75) = 50.54, p < .001] and Questions [F(1, 75) = 37.39, p < .001] showed significant multivariate effects. Scores were higher under the Coordination than the Individual condition, and higher for Part I than Part II questions. These findings replicated the 2008 analysis of young twins. (Hypothesis 4): The Zygosity Condition interaction was not statistically significant, although the MZ twins outperformed the DZ twins to some extent in the Coordination condition. In contrast, the MZ– DZ difference in the 2008 study was nearly identical across conditions. The present findings are summarized in Tables 2 and 3. An unanticipated significant effect was found for the Questions Condition interaction [F(1, 75) = 17.24, p < .001]. A higher number of matches was obtained for Part I questions in the Coordination condition (4.47, SD = 1.79) than in the Individual condition (2.59, 2.62, SD = 2.04). None of the two-way interactions were significant in the earlier study.
Table 2 Mean TC scores for twins, organized by Condition (Individual and Coordination). Zygosity
Mean
SD
Range
Individual MZ DZ Total
5.65 3.85 4.85
2.71 2.18 2.63
0–12 1–8 0–12
Coordination MZ DZ Total
8.35 6.00 7.31
2.72 2.69 2.93
2–14 1–11 1–14
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Table 3 Mean TC scores for twins, organized by questions (Parts I, II).
80
Mean
SD
Range
Part I MZ DZ Total
7.95 6.00 7.09
2.89 2.88 3.03
2–14 1–14 1–14
Part II MZ DZ Total
6.05 3.85 5.08
2.39 1.99 2.46
1–13 0–8 0–13
70 Percent of Exact Matches
Zygosity
60 50 40
MZ
30
DZ
20 10
3.3. Individual item evaluation
0
3.3.1. Adolescent and adult twins Graphs were created to depict how the MZ and DZ pairs responded to each TCQ item; see Fig. 1a (Individual) and Fig. 1b (Coordination). The Hosmer–Lemeshow Test indicated a good fit to the data (i.e., twin type discrimination) in the Individual condition [X2 (8) = 7.05, p < .53]. Seven items successfully assigned 75% of the pairs. A somewhat higher percentage of MZ (77.5%) than DZ twin pairs (71.9%) was identified correctly, using a cutoff value of .50. The Hosmer–Lemeshow Test also indicated a good fit to the data for the Coordination condition [X2 (5) = 1.53, p < .91]. Four items successfully assigned 75.8% of the twin pairs. A somewhat higher percentage of DZ (80.6%) than MZ twin pairs (71.4%) was identified correctly. The Wald statistic and significance levels for each item in both conditions are displayed in Table 4.
Percent of Exact Matches
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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 Items on TC Questionnaire Fig. 1b. Percent of matches by Zygosity in the Coordination condition (Adult).
Table 4 Wald statistics for items in the individual and coordination conditions: adolescents/ adults.
a
Item
Wald statistic
DF
p<
Individual Boy’s name Favorite thing to do Sport Place in a park to meet Book Day of the week Yeara
3.82 6.71 4.46 3.99 5.77 5.43 3.71
1 1 1 1 1 1 1
.051 .010 .035 .046 .016 .020 .054
Coordination Car Positive number Movie Musical group
5.11 6.40 3.28 3.44
1 1 1 1
.024 .011 .070 .064
Part of month–day–year (items 4–6).
50 45 Percent of Exact Matches
It was possible that some items or classes of items better distinguished between MZ and DZ twin pairs than others. Mehta et al. (1994) noted that in a pure coordination game most people answer ‘‘1’’ when asked to provide a positive number. Similarly, 66.7% of randomly paired individuals in her study answered ‘‘rose’’ when asked to name a flower. Such items, which elicit similar responses from people due to shared experiential or cultural factors, would not be expected to distinguish between MZ and DZ twins. However, questions that draw forth a range of responses, such as ‘‘name a color,’’ might discriminate more effectively between twin types. This analysis was not undertaken in 2008, but is reported below for both twin samples. Logistic regression was performed to determine if some items better discriminated between MZ and DZ pairs than others. A WALD forward stepwise approach, with a variable entry cutoff of .15, and .20 for removal, was used (see Hosmer & Lemeshow, 2000 for justification). Predictor variables were the twenty-two TCQ items. This analysis was done twice, using data from the Individual and Coordination conditions, respectively.
40 35 30 25
MZ
20
DZ
15 10 5 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 Items on TC Questionnaire
Fig. 2a. Percent of exact matches by Zygosity in the Individual condition (Child).
50 40 30
MZ DZ
20 10 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 Items on TC Questionnaire
Fig. 1a. Percent of matches by Zygosity in the Individual condition (Adult). Note: Items corresponding to item numbers are listed in the Appendix.
3.3.2. Young twins Graphs were created to depict how the young MZ and DZ pairs responded to each TCQ item; see Fig. 2a (Individual) and Fig. 2b (Coordination). The Hosmer–Lemeshow Test showed a good fit to the data in the Individual condition [X2 (6) = 2.91, p < .82]. Five items successfully assigned 73.4% of the twin pairs. A higher percentage of DZ (89.5%) than MZ twin pairs (47.2%) was identified correctly. The Hosmer–Lemeshow Test also showed a good fit to the data in the Coordination condition [X2 (7) = 8.10, p < .32]. Four items successfully assigned 69.1% of the twin pairs. A higher percentage of DZ (90.7%) than MZ twin pairs (34.0%) was identified correctly. These findings are displayed in Table 5.
N.L. Segal et al. / Personality and Individual Differences 58 (2014) 31–36
60
Percent of Exact Matches
50 40 30
MZ DZ
20 10 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 Items on TC Questionnaire
Fig. 2b. Percent of exact matches by Zygosity in the Coordination condition (Child).
Table 5 Wald statistics for items in the individual and coordination conditions: young twins.
a
Item
Wald statistic
DF
p<
Individual Boy’s name Movie Place in a park to meet Country Yeara
9.42 2.93 4.54 4.54 3.72
1 1 1 1 1
.002 .087 .033 .033 .054
Coordination Car Ice cream flavor Sport Musical group
2.59 4.06 3.10 2.61
1 1 1 1
.108 .044 .078 .106
Single question (item 1): name a year.
4. Discussion Findings from the present study demonstrated greater success by adolescent and adult MZ twin pairs compared to DZ twin pairs on the TC task. These results support those of Segal et al. (2008) in showing a positive relationship between genetic relatedness and coordination. When presented with opportunities to choose, closer genetic relatives make more similar decisions than do more distant genetic relatives. The Zygosity Condition interaction was not statistically significant, yet somewhat greater success among MZ twins, relative to DZ twins, in the Coordination condition was observed. This trend may reflect MZ twins’ greater ease in the synchronization of goals. Note that both MZ and DZ twins improved their scores in the Coordination condition, but the MZ–DZ difference widened. These results are consistent with EP predictions that prosocial behavior should vary with the genetic relatedness of the interactants. Close genetic relatives may be predisposed to work together successfully given psychological mechanisms facilitating their ability to do so. Kin selection theory offers explanations for how individuals are able to assess the degree of genetic relatedness between themselves and others. Thus, the results of this study should contribute to existing models of human kin detection. Perhaps increased physical resemblance between two individuals (as is characteristic of MZ compared to DZ twin pairs), may act as an initial cue when assessing degree of relatedness. This kinship estimation, together with perceived behavioral similarities, may provide a mechanism by which individuals regulate their decisions to cooperate or compete with others. This process may be part of a broader class of mechanisms by which individuals develop and maintain close social relations (Segal et al., 2008).
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Significant multivariate effects were found for Condition (Coordination higher) and Questions (Part I higher), as in the 2008 study. Apparently, both MZ and DZ twins put forth greater effort to match their co-twin when instructed to do so, yielding a higher number of matches in the Coordination condition than in the Individual condition. Part I included two more questions (1–12) than Part II (13–22), most likely explaining the higher number of Part I matches. The unanticipated significant Condition Questions interaction may reflect both the higher number of Part I questions and/or the additional efforts made by participants. The item analysis revealed some informative findings. First, a larger number of items distinguished between MZ and DZ adolescent and adult twins (7), as compared with the young twins (5), in the Individual condition, but not in the Coordination condition. In the absence of pressure to conform, MZ twins appear to match more easily than DZ twins, possibly accounting for the discrepancy. The fewer distinguishing items found for the younger sample may reflect their relatively reduced range of experience. Distinguishing items common to both twin samples were the boy’s name and a place to meet in the park. It appears that these items have many possible answers on which MZ twins, especially, concur. Four items distinguished between MZ and DZ twins in both studies during Coordination, two of which (car and musical group) were common to both. These items showed higher agreement among the MZ twins, compared to the DZ twins. Somewhat surprisingly, interests and preferences in these domains appear well established even in childhood. A much higher percentage of young DZ twins were correctly classified in both the Individual (89.5% vs. 47.2%) and Coordination conditions (90.7% vs. 34.0%). Perhaps this is because the distinguishing items showed such low percentages of agreement among the DZ twin children, relative to the MZ twin children, making DZ twins easier to classify. Young MZ twins, while more successful in TC than DZ twins, may be somewhat variable in their coordination, relative to older MZ twins. It is possible that as twins age, MZ twins are better able to coordinate their responses as their interests and preferences increasingly converge. 4.1. Limitations Findings from the present study provide additional support for kin selection theories of social interaction, yet the Zygosity Condition interaction was non-significant. This may be partly due to the small sample size (n = 77 pairs). Alternatively, it is likely that as individuals mature they spend less time in decision-making situations with their families and more time with their colleagues and friends. These professional and social experiences could conceivably affect decision-making processes with family members. Regardless, future researchers are encouraged to include larger and more comparable twin group sizes to address these possibilities, and to arrange repeated experimental sessions. 4.2. Future research Future research is needed to explore possible associations between selected behavioral and physical similarities and TC, more specifically the role that perceived similarities play. However, physical similarity and shared genes are confounded in MZ twin pairs. Therefore, MZ pairs would have to be organized according to both perceived and measured physical and behavioral similarities to obtain a more comprehensive understanding of TC processes and outcomes. Behavioral similarities would be expected to play a larger role in TC, given the lack of personality similarity and lack of close social relatedness found in unrelated look-alike pairs (Segal, Graham, & Ettinger, 2013). Future analyses should also consider comparing TC between each twin and their close friends
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and colleagues to investigate the nature and effect of other relationships in twins’ expanding social networks. Overall, the present study of tacit coordination provides an informative view of human social interaction. Successful TC and its associated factors enhance our understanding of why some individuals work better together than others. The use of genetically sensitive designs provides an informative approach to understanding the nature and origin of this behavior. Acknowledgments The twins are gratefully acknowledged for their time and interest in this project. Jorge L. Torres, Jamie L. Graham, Ammar Altowaji and Jaime M. Velazquez provided research assistance. This work was partly supported by a CSUF Faculty Research Award to Dr. Segal and a CSUF Graduate Equity Fellowship to Jaimee Munson. Appendix Tacit Coordination Questionnaire. Write/name: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22
Year (past, present or future) Flower Car manufacturer Day of the year/week Year Month/date US town/city Positive number Color Boy’s name A coin was tossed. It came down_______________ The doctor asked for the patient’s records. The nurse gave them to__________ Favorite thing to do Ice cream flavor Movie Sport Clothing article You are meeting your twin at a big park. Where in the park will you meet? You are looking for your twin at a big department store. Where will you look for him/her? Musical group Country Book
Items 4–6 presented as one three-part item
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