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Reciprocal altruism and kin selection are not clearly separable phenomena
J. theor. Biol. (1980)87,255-261
Reciprocal
Altruism and Kin Selection Are Not Clearly Separable Phenomena STEPHEN I. ROTHSTEIN
Department (Received
of Biological Sciences, University of California, Barbara, California 93 106, U.S.A. 29 January
Santa
1980, and in revised form 12 May 1980)
Reciprocal altruism (RA) is usually thought of as occurring between non-relatives and is considered to be distinct from kin selection (KS). But because similar traits expressed by conspecifics are usually due in part to identical genetic determinants, individuals that engage in RA are likely to be doing so because of shared genes. This is true so long as there is any genetic determination of RA. Thus, RA is not clearly distinct from KS unless it occurs between different species. A new definition of RA presented here stresses that RA is likely to involve gains in inclusive fitness (as in KS) and does not have the requirement that future benefits outweigh the costs of an altruist’s aid. Given the arguments presented here, it seems that no certain cases of true intraspecific RA are known. RA can be viewed as a genetic recognition system with the tendency to return aid being the marker by which reciprocal altruists recognize one another. Lastly, speculations are presented concerning the potential origin of true RA within an original context of KS. 1. Introduction Altruism provides what is perhaps the most important prima facie evidence against traditional viewpoints concerning natural selection and explaining its origin and maintenance has thus been a major objective of evolutionary biology and sociobiology. Theorists have formulated three basic conceptual frameworks to explain altruism-group selection, kin selection and reciprocal altruism. Group selection appears to have little general importance; the conditions that it requires are either rarely fulfilled in nature or result in situations that actually represent special cases of kin selection (Alexander & Borgia, 1978). The major point of this paper is to show that the two remaining explanations for altruism are not clearly separable phenomena, at least as currently defined. Reciprocal altruism (RA) is defined as exchanges of aid between non-relatives in which the dispensations of aid occur at different times and in which the costs of providing aid are lower than the benefits of receiving it (Trivers, 1971). Kin selection 0022-5193/80/220255+07$02.00/0
@ 1980 Academic
Press Inc. (London)
Ltd.
256
S. I. ROTHSTEIN
(KS) is the process by which individuals increase their inclusive fitness by aiding other individuals (usually close relatives) who share the genes responsible for their aid-giving behavior. It is possible that most examples of RA are special cases of KS. 2. The Genetic Basis of Reciprocal
Altruism
and a New Definition
The most potent unit of selection is the gene. If RA is to have relevancy to biological evolution, its genetic basis must be considered. There must be alleles for dispensing aid. Further, if the altruism is to represent an evolutionary stable strategy (Maynard Smith & Price, 1973), and if cheaters (i.e. non-reciprocators) are not to enjoy an advantage, there must be alleles for recognizing individuals who return aid. I am not suggesting that there are special RA alleles, only that there must be pleiotropic alleles that impart to an individual the ability and willingness to engage in RA. Such alleles or gene complexes might have generalized effects that make RA possible, e.g. increased social tendencies or powers of memory. Within a species it is likely that some or possibly all of the alleles at the gene loci enabling individuals to dispense aid and to recognize altruists and cheaters will be identical in each member of a pair of reciprocating altruists. It is highly improbable that identical traits-engaging in RA-are coded by completely different alleles or even gene loci within different members of the same species. Thus, when conspecific reciprocal altruists exchange aid they are almost certainly doing so because they share at least some of the genes responsible for their altruism. Also, individuals who are similar genetically at the loci coding for RA are more likely to benefit one another than individuals who differ from them at these loci. Unless it can be shown that individuals who engage in RA have totally different alleles at gene loci that cause a predilection for reciprocation, there is no clear separation between KS and RA. In both, individuals aid others who share their genes. Of course, KS is usually thought of in terms of aid dispensed to individuals that have a high overall genetic similarity to the altruist, with genetic similarity dependent on familial relationships and quantified by the coefficient of relationship. But the important measure of genie similarity is the proportion that is shared of the specific genes that code for aid-giving behavior; even as originally formulated the concept of KS included the notion of aid dispensed to individuals who share genes coding for the aid-giving behavior even if these individuals are not closely related (Hamilton, 1964). Dawkins (1976, p. 96) has similarly stressed the importance of the precise genes responsible for social behavior and even speculated on a hypothetical system in which altruists recognize one another
RECIPROCAL
ALTRUISM
AND
KIN
SELECTION
257
by their behavior. But in Dawkins’ model, individuals aid anyone they see aiding anyone else. Such a system is not RA, nor is it evolutionarily stable unless made more complex because altruists will end up aiding non-relatives who aid only their own kin. Unlike Dawkins’ model, RA as usually envisioned, implies that individuals aid repeatedly only those who have aided them. To reiterate my basic argument, individuals who remember aid-recipients they have helped and who help them again only if the aid has been reciprocated-i.e. individuals who engage in RA-are more likely to be similar genetically for these behaviors than individuals who do not engage in RA. This argument is true almost to the point of being a tautology, so long as there is any genetic determination of RA. In light of these considerations, I suggest that the only type of RA that is manifestly distinguishable from KS is reciprocity between members of different species, such as the cleaner fish phenomenon reviewed by Trivers (1971). The reciprocity involved in interspecific RA is unlikely to be determined by identical genes; furthermore, the fact that different gene pools are involved makes questionable any connection with KS. The remainder of this paper deals only with intraspecific RA and assumes that individuals who engage in intraspecific RA are always more similar genetically for the genes coding for RA than conspecifics who do not engage in RA, i.e. cheaters. My discussion does not invalidate the possibility that in some cases the predilection to engage in RA is due to completely different genetic determinants in individuals of the same species; but I do not consider this idea further because in most cases conspecifics who exhibit similar traits have similar genetic determinants for these traits, especially if they are members of the same population. Although RA is not clearly separable from KS because it necessitates interactions between individuals genetically similar or identical for the trait in question it is worth exploring the extent to which RA is a distinct phenomenon. Both KS and RA result in increases in inclusive fitness as regards the genes for aid-dispensing behavior. Thus, if the usual considerations of shared genes and of the benefits and costs of an altruistic act that apply in KS can alone account for the act, there is no justification to identify it as something other than KS, even if it is reciprocated in the future and even if it involves non-relatives. However, the following new definition does characterize RA as a phenomenon that is distinct from KS. RA is reciprocity that is not maintainable (i.e. selected for) solely on the basis of the increases in inclusive fitness that are likely to occur in reciprocity and whose maintenance depends on the fact that the altruist will at a later time receive benefits from the individual he has aided. Under this definition an individual who practices RA need not receive future direct benefits that outweigh the
258
S. I. ROTHSTEIN
costs of the aid he has dispensed (cf. Trivers, 1971) because reciprocated aid is not the only benefit the individual receives. Aiding another reciprocal altruist aids the individual indirectly via an increase in inclusive fitness. This definition is difficult to apply because the costs, benefits and relatedness needed to adequately assess inclusive fitness cannot be determined easily but it is more meaningful than defining RA as the exchange of aid between nonrelatives in which each participant gains in terms of his own costs and benefits.
3. The Adaptive
Value
and Evolutionary Altruism
Consequences
of Reciprocal
Because RA is almost certain to involve at least slight gains in inclusive fitness, past discussions of this phenomenon have generally underestimated its adaptive value. However, West Eberhard (1975, p. 20) also discussed a relationship between RA and inclusive fitness and contrasted “pure RA” (i.e. between non-relatives) and “RA among kin”. For the latter, she developed a formal expression for the relationship between benefits, costs and relatedness needed to maintain the altruism. The point I am making here is that it is probable that pure RA rarely or never exists; individuals who engage in RA are to some extent kin, at least as far as the genes for RA go. It is necessary to stress that there are no clear examples of true RA. Even when aid is exchanged between individuals that definitely are not close relatives as in some non-human primates (Packer, 1977) there is no way of determining whether the aid requires reciprocity for its maintenance or whether the genetic similarity of these individuals alone, and hence inclusive fitness, can account for the aid. Although my revised definition of RA differs significantly from Trivers’ original one it does not alter Trivers’ (197 1) discussion of some of the major evolutionary consequences of RA. As did Trivers, I would still predict the evolution of subtle cheating and of abilities enabling individuals to detect such anti-social behavior. Even if, as I have argued here, individuals are similar in those gene complexes predisposing them to engage in RA, selection on any specific locus will still initially favor a mutation that causes an individual to accept aid but to not reciprocate. However, such a “cheating” mutation will be favored only so long as other genes or alternative alleles at the cheating locus fail to produce the discriminatory ability needed to detect cheating. Once this discriminatory ability is evolved only a more subtle form of cheating will succeed and the spiral of ever-more subtle
RECIPROCAL
cheating and heightened ensue. 4. Reciprocal
ALTRUISM
discrimination
Altruism
AND
KIN
259
SELECTION
envisioned by Trivers should still
as a Genetic Recognition
System
The concept of RA is germane to that of genetic recognition systems once it is recognized that reciprocal altruists exchange aid because of shared alleles. This is true whether RA is defined rigidly as in this paper or broadly as reciprocity between non-relatives. Hamilton (1964) formulated the concept of genetic recognition systems in his early discussions of KS when he pointed out that an individual can direct altruistic acts towards a nonrelative so long as it is able to recognize that the aided individual shares the allele(s) coding for the altruism. Hamilton was equivocal about the reality of genetic recognition systems. By contrast, Alexander & Borgia (1978) stated that recognition systems are unlikely to evolve, arguing that it is improbable that any gene or tightly linked complex of genes could code for all the conditions needed to maintain a recognition system. As discussed by Alexander & Borgia (p. 463), a genetic recognition system must do the following to be maintained by selection: (a) influence some perceptible feature of the phenotype, (b) cause perception of the feature, (c) bring about the appropriate social response. I suggest that RA is in fact a genetic recognition system and that it requires all of the conditions listed by Alexander and Borgia. Thus, for RA to be maintained between individuals who do not know one another’s familial relationships there must be [followingconditions (a), (b) and (c) listed above] a genetic unit: (i) causes an individual, A, to return aid to another individual, B, who has aided him, i.e. has a perceptible influence on A’s phenotype, (ii) causes B to learn that A is a reciprocater, i.e. causes perception of a phenotypic feature, (iii) causes B to again aid A when A needs help, i.e. brings about the appropriate social response. Essentially the tendency to return aid is the marker by which reciprocal altruists recognize one another. When RA is viewed in this light, it becomes evident that its maintenance is dependent on a genetic recognition system. A reciprocator must be able to recognize individuals who preferentially dispense aid to those who have aided them. Since a genetic recognition system is required for RA and since RA seems to occur, at least in humans and other primates (Packer, 1977) and possibly in birds (Ligon & Ligon, 1978), it is probable that recognition systems have
260
S. I. ROTHSTEIN
evolved. This suggests that such systems may have more importance generally realized. 5. Origin of Reciprocal
Altruism
than is
and Conclusions
Recognition of the fact that all types of RA probably contain an element of KS and that most or all apparent cases of RA may be maintainable solely on the basis of increases in inclusive fitness makes it easier to understand how reciprocation between non-relatives evolved. Although the current theory behind RA can account for the maintenance of this phenomenon it cannot easily explain its origin. A reciprocal altruist that appears in a population of non-altruists will obviously be selected against since none of the individuals it aids will reciprocate. But if RA had its basis in phenomena that first evolved in the context of KS its origin is more easily explained. A possible scenario is as follows: (1) non-reciprocated aid to a relative which is maintained by KS, (2) frequent reciprocation of the aid, (3) recognition of those individuals, within family lines, who are the most likely to reciprocate, (4) recognition of individuals outside family lines who are also likely to reciprocate. I have not attempted to invalidate the concept of intraspecific RA by arguing that it is often or always a special case of KS. Rather, my goal has been to stress that current concepts of RA and KS are not clearly separable. Recognition of the similarity between RA and KS will, I believe, lead to a fuller understanding of RA and to an identification of those types of reciprocity that constitute phenomena that are distinct from KS, i.e. reciprocity that cannot be maintained solely on the basis of gains in inclusive fitness that are realized by the help dispensed to the aided individuals. It remains to be seen whether such altruism ever occurs outside of the contexts of interspecific situations and possibly that of the complex interactions that occur among humans. I have qualified my reference to RA in humans because it is possible that personality traits that promote RA have some degree of genetic determination. Thus even within our species, individuals who form reciprocating relationships may be more similar at those gene loci contributing to personality traits that promote RA than individuals who do not form such relationships. I thank E. Arnason, T. R. Carlisle, C. Janson, R. Panza, R. L. Trivers and R. R. Warner for their comments on the manuscript and G. H. Orians and M. Slatkin for their discussions of the problem. This paper waswritten while I spent a sabbatical
RECIPROCAL
ALTRUISM
AND
KIN
SELECTION
261
leave with the Department of Zoology at the University of Washington and I express my appreciation to that department for its hospitality. REFERENCES ALEXANDER, R. D. & BORGIA, G. (1978). Ann. Rec. Ecol. Syst. 9,449. DAWKINS, R. (1976). The Selfish Gene. New York: Oxford University Press. HAMILTON, W. D. (1964). J. theor. Biol. 7, 1. LIGON, J. D. & LIGON, S. H. (1978). Nature 276,496. MAYNARD SMITH, J. & PRICE, G. R. (1973). Nature 246, 15. PACKER, C. (1977). Nature 265, 441. TRIVERS, R. L. (1971). Q. rer. Biol. 46,35. WEST EBERHARD, M. J. (1975). Q. ret’. Biol. 50, 1.
Reciprocal
Altruism and Kin Selection Are Not Clearly Separable Phenomena STEPHEN I. ROTHSTEIN
Department (Received
of Biological Sciences, University of California, Barbara, California 93 106, U.S.A. 29 January
Santa
1980, and in revised form 12 May 1980)
Reciprocal altruism (RA) is usually thought of as occurring between non-relatives and is considered to be distinct from kin selection (KS). But because similar traits expressed by conspecifics are usually due in part to identical genetic determinants, individuals that engage in RA are likely to be doing so because of shared genes. This is true so long as there is any genetic determination of RA. Thus, RA is not clearly distinct from KS unless it occurs between different species. A new definition of RA presented here stresses that RA is likely to involve gains in inclusive fitness (as in KS) and does not have the requirement that future benefits outweigh the costs of an altruist’s aid. Given the arguments presented here, it seems that no certain cases of true intraspecific RA are known. RA can be viewed as a genetic recognition system with the tendency to return aid being the marker by which reciprocal altruists recognize one another. Lastly, speculations are presented concerning the potential origin of true RA within an original context of KS. 1. Introduction Altruism provides what is perhaps the most important prima facie evidence against traditional viewpoints concerning natural selection and explaining its origin and maintenance has thus been a major objective of evolutionary biology and sociobiology. Theorists have formulated three basic conceptual frameworks to explain altruism-group selection, kin selection and reciprocal altruism. Group selection appears to have little general importance; the conditions that it requires are either rarely fulfilled in nature or result in situations that actually represent special cases of kin selection (Alexander & Borgia, 1978). The major point of this paper is to show that the two remaining explanations for altruism are not clearly separable phenomena, at least as currently defined. Reciprocal altruism (RA) is defined as exchanges of aid between non-relatives in which the dispensations of aid occur at different times and in which the costs of providing aid are lower than the benefits of receiving it (Trivers, 1971). Kin selection 0022-5193/80/220255+07$02.00/0
@ 1980 Academic
Press Inc. (London)
Ltd.
256
S. I. ROTHSTEIN
(KS) is the process by which individuals increase their inclusive fitness by aiding other individuals (usually close relatives) who share the genes responsible for their aid-giving behavior. It is possible that most examples of RA are special cases of KS. 2. The Genetic Basis of Reciprocal
Altruism
and a New Definition
The most potent unit of selection is the gene. If RA is to have relevancy to biological evolution, its genetic basis must be considered. There must be alleles for dispensing aid. Further, if the altruism is to represent an evolutionary stable strategy (Maynard Smith & Price, 1973), and if cheaters (i.e. non-reciprocators) are not to enjoy an advantage, there must be alleles for recognizing individuals who return aid. I am not suggesting that there are special RA alleles, only that there must be pleiotropic alleles that impart to an individual the ability and willingness to engage in RA. Such alleles or gene complexes might have generalized effects that make RA possible, e.g. increased social tendencies or powers of memory. Within a species it is likely that some or possibly all of the alleles at the gene loci enabling individuals to dispense aid and to recognize altruists and cheaters will be identical in each member of a pair of reciprocating altruists. It is highly improbable that identical traits-engaging in RA-are coded by completely different alleles or even gene loci within different members of the same species. Thus, when conspecific reciprocal altruists exchange aid they are almost certainly doing so because they share at least some of the genes responsible for their altruism. Also, individuals who are similar genetically at the loci coding for RA are more likely to benefit one another than individuals who differ from them at these loci. Unless it can be shown that individuals who engage in RA have totally different alleles at gene loci that cause a predilection for reciprocation, there is no clear separation between KS and RA. In both, individuals aid others who share their genes. Of course, KS is usually thought of in terms of aid dispensed to individuals that have a high overall genetic similarity to the altruist, with genetic similarity dependent on familial relationships and quantified by the coefficient of relationship. But the important measure of genie similarity is the proportion that is shared of the specific genes that code for aid-giving behavior; even as originally formulated the concept of KS included the notion of aid dispensed to individuals who share genes coding for the aid-giving behavior even if these individuals are not closely related (Hamilton, 1964). Dawkins (1976, p. 96) has similarly stressed the importance of the precise genes responsible for social behavior and even speculated on a hypothetical system in which altruists recognize one another
RECIPROCAL
ALTRUISM
AND
KIN
SELECTION
257
by their behavior. But in Dawkins’ model, individuals aid anyone they see aiding anyone else. Such a system is not RA, nor is it evolutionarily stable unless made more complex because altruists will end up aiding non-relatives who aid only their own kin. Unlike Dawkins’ model, RA as usually envisioned, implies that individuals aid repeatedly only those who have aided them. To reiterate my basic argument, individuals who remember aid-recipients they have helped and who help them again only if the aid has been reciprocated-i.e. individuals who engage in RA-are more likely to be similar genetically for these behaviors than individuals who do not engage in RA. This argument is true almost to the point of being a tautology, so long as there is any genetic determination of RA. In light of these considerations, I suggest that the only type of RA that is manifestly distinguishable from KS is reciprocity between members of different species, such as the cleaner fish phenomenon reviewed by Trivers (1971). The reciprocity involved in interspecific RA is unlikely to be determined by identical genes; furthermore, the fact that different gene pools are involved makes questionable any connection with KS. The remainder of this paper deals only with intraspecific RA and assumes that individuals who engage in intraspecific RA are always more similar genetically for the genes coding for RA than conspecifics who do not engage in RA, i.e. cheaters. My discussion does not invalidate the possibility that in some cases the predilection to engage in RA is due to completely different genetic determinants in individuals of the same species; but I do not consider this idea further because in most cases conspecifics who exhibit similar traits have similar genetic determinants for these traits, especially if they are members of the same population. Although RA is not clearly separable from KS because it necessitates interactions between individuals genetically similar or identical for the trait in question it is worth exploring the extent to which RA is a distinct phenomenon. Both KS and RA result in increases in inclusive fitness as regards the genes for aid-dispensing behavior. Thus, if the usual considerations of shared genes and of the benefits and costs of an altruistic act that apply in KS can alone account for the act, there is no justification to identify it as something other than KS, even if it is reciprocated in the future and even if it involves non-relatives. However, the following new definition does characterize RA as a phenomenon that is distinct from KS. RA is reciprocity that is not maintainable (i.e. selected for) solely on the basis of the increases in inclusive fitness that are likely to occur in reciprocity and whose maintenance depends on the fact that the altruist will at a later time receive benefits from the individual he has aided. Under this definition an individual who practices RA need not receive future direct benefits that outweigh the
258
S. I. ROTHSTEIN
costs of the aid he has dispensed (cf. Trivers, 1971) because reciprocated aid is not the only benefit the individual receives. Aiding another reciprocal altruist aids the individual indirectly via an increase in inclusive fitness. This definition is difficult to apply because the costs, benefits and relatedness needed to adequately assess inclusive fitness cannot be determined easily but it is more meaningful than defining RA as the exchange of aid between nonrelatives in which each participant gains in terms of his own costs and benefits.
3. The Adaptive
Value
and Evolutionary Altruism
Consequences
of Reciprocal
Because RA is almost certain to involve at least slight gains in inclusive fitness, past discussions of this phenomenon have generally underestimated its adaptive value. However, West Eberhard (1975, p. 20) also discussed a relationship between RA and inclusive fitness and contrasted “pure RA” (i.e. between non-relatives) and “RA among kin”. For the latter, she developed a formal expression for the relationship between benefits, costs and relatedness needed to maintain the altruism. The point I am making here is that it is probable that pure RA rarely or never exists; individuals who engage in RA are to some extent kin, at least as far as the genes for RA go. It is necessary to stress that there are no clear examples of true RA. Even when aid is exchanged between individuals that definitely are not close relatives as in some non-human primates (Packer, 1977) there is no way of determining whether the aid requires reciprocity for its maintenance or whether the genetic similarity of these individuals alone, and hence inclusive fitness, can account for the aid. Although my revised definition of RA differs significantly from Trivers’ original one it does not alter Trivers’ (197 1) discussion of some of the major evolutionary consequences of RA. As did Trivers, I would still predict the evolution of subtle cheating and of abilities enabling individuals to detect such anti-social behavior. Even if, as I have argued here, individuals are similar in those gene complexes predisposing them to engage in RA, selection on any specific locus will still initially favor a mutation that causes an individual to accept aid but to not reciprocate. However, such a “cheating” mutation will be favored only so long as other genes or alternative alleles at the cheating locus fail to produce the discriminatory ability needed to detect cheating. Once this discriminatory ability is evolved only a more subtle form of cheating will succeed and the spiral of ever-more subtle
RECIPROCAL
cheating and heightened ensue. 4. Reciprocal
ALTRUISM
discrimination
Altruism
AND
KIN
259
SELECTION
envisioned by Trivers should still
as a Genetic Recognition
System
The concept of RA is germane to that of genetic recognition systems once it is recognized that reciprocal altruists exchange aid because of shared alleles. This is true whether RA is defined rigidly as in this paper or broadly as reciprocity between non-relatives. Hamilton (1964) formulated the concept of genetic recognition systems in his early discussions of KS when he pointed out that an individual can direct altruistic acts towards a nonrelative so long as it is able to recognize that the aided individual shares the allele(s) coding for the altruism. Hamilton was equivocal about the reality of genetic recognition systems. By contrast, Alexander & Borgia (1978) stated that recognition systems are unlikely to evolve, arguing that it is improbable that any gene or tightly linked complex of genes could code for all the conditions needed to maintain a recognition system. As discussed by Alexander & Borgia (p. 463), a genetic recognition system must do the following to be maintained by selection: (a) influence some perceptible feature of the phenotype, (b) cause perception of the feature, (c) bring about the appropriate social response. I suggest that RA is in fact a genetic recognition system and that it requires all of the conditions listed by Alexander and Borgia. Thus, for RA to be maintained between individuals who do not know one another’s familial relationships there must be [followingconditions (a), (b) and (c) listed above] a genetic unit: (i) causes an individual, A, to return aid to another individual, B, who has aided him, i.e. has a perceptible influence on A’s phenotype, (ii) causes B to learn that A is a reciprocater, i.e. causes perception of a phenotypic feature, (iii) causes B to again aid A when A needs help, i.e. brings about the appropriate social response. Essentially the tendency to return aid is the marker by which reciprocal altruists recognize one another. When RA is viewed in this light, it becomes evident that its maintenance is dependent on a genetic recognition system. A reciprocator must be able to recognize individuals who preferentially dispense aid to those who have aided them. Since a genetic recognition system is required for RA and since RA seems to occur, at least in humans and other primates (Packer, 1977) and possibly in birds (Ligon & Ligon, 1978), it is probable that recognition systems have
260
S. I. ROTHSTEIN
evolved. This suggests that such systems may have more importance generally realized. 5. Origin of Reciprocal
Altruism
than is
and Conclusions
Recognition of the fact that all types of RA probably contain an element of KS and that most or all apparent cases of RA may be maintainable solely on the basis of increases in inclusive fitness makes it easier to understand how reciprocation between non-relatives evolved. Although the current theory behind RA can account for the maintenance of this phenomenon it cannot easily explain its origin. A reciprocal altruist that appears in a population of non-altruists will obviously be selected against since none of the individuals it aids will reciprocate. But if RA had its basis in phenomena that first evolved in the context of KS its origin is more easily explained. A possible scenario is as follows: (1) non-reciprocated aid to a relative which is maintained by KS, (2) frequent reciprocation of the aid, (3) recognition of those individuals, within family lines, who are the most likely to reciprocate, (4) recognition of individuals outside family lines who are also likely to reciprocate. I have not attempted to invalidate the concept of intraspecific RA by arguing that it is often or always a special case of KS. Rather, my goal has been to stress that current concepts of RA and KS are not clearly separable. Recognition of the similarity between RA and KS will, I believe, lead to a fuller understanding of RA and to an identification of those types of reciprocity that constitute phenomena that are distinct from KS, i.e. reciprocity that cannot be maintained solely on the basis of gains in inclusive fitness that are realized by the help dispensed to the aided individuals. It remains to be seen whether such altruism ever occurs outside of the contexts of interspecific situations and possibly that of the complex interactions that occur among humans. I have qualified my reference to RA in humans because it is possible that personality traits that promote RA have some degree of genetic determination. Thus even within our species, individuals who form reciprocating relationships may be more similar at those gene loci contributing to personality traits that promote RA than individuals who do not form such relationships. I thank E. Arnason, T. R. Carlisle, C. Janson, R. Panza, R. L. Trivers and R. R. Warner for their comments on the manuscript and G. H. Orians and M. Slatkin for their discussions of the problem. This paper waswritten while I spent a sabbatical
RECIPROCAL
ALTRUISM
AND
KIN
SELECTION
261
leave with the Department of Zoology at the University of Washington and I express my appreciation to that department for its hospitality. REFERENCES ALEXANDER, R. D. & BORGIA, G. (1978). Ann. Rec. Ecol. Syst. 9,449. DAWKINS, R. (1976). The Selfish Gene. New York: Oxford University Press. HAMILTON, W. D. (1964). J. theor. Biol. 7, 1. LIGON, J. D. & LIGON, S. H. (1978). Nature 276,496. MAYNARD SMITH, J. & PRICE, G. R. (1973). Nature 246, 15. PACKER, C. (1977). Nature 265, 441. TRIVERS, R. L. (1971). Q. rer. Biol. 46,35. WEST EBERHARD, M. J. (1975). Q. ret’. Biol. 50, 1.