For a PDF of the proofs for this article: www.psych.ucsb.edu/research/cep/papers/20finalbusssocexweb.pdf
Neurocognitive Adaptations Designed for Social Exchange
Leda Cosmides and John Tooby
Center for Evolutionary Psychology
University of California, Santa Barbara
Evolutionary Psychology Handbook
David M. Buss, Editor
Dept. of Psychology
University of California
Santa Barbara, CA 93106
Cosmides & Tooby—1
―If a person doesn‘t give something to me, I won‘t give anything to that person. If I‘m sitting
eating, and someone like that comes by, I say, ―Uhn, uhn. I‘m not going to give any of this to
you. When you have food, the things you do with it make me unhappy. If you even once in a
while gave me something nice, I would surely give some of this to you.‖
—Nisa from Nisa: The Life and Words of a !Kung Woman, Shostak, 1981, p. 89
―Instead of keeping things, [!Kung] use them as gifts to express generosity and friendly intent,
and to put people under obligation to make return tokens of friendship . . . In reciprocating, one
does not give the same object back again but something of comparable value.
Eland fat is a very highly valued gift . . . Toma said that when he had eland fat to give, he
took shrewd note of certain objects he might like to have and gave their owners especially
generous gifts of fat.‖
—Marshall, 1976, pp. 366–369
Nisa and Toma were hunter-gatherers, !Kung San people living in Botswana‘s inhospitable
Kalahari desert during the 1960s. Their way of life was as different from that in an industrialized, economically developed society as any on earth, yet their sentiments are as familiar and easy to comprehend as those of your neighbor next door. They involve social exchange, interactions in
which one party provides a benefit to the other conditional on the recipient‘s providing a benefit in return (Cosmides, 1985; Cosmides & Tooby, 1989; Tooby & Cosmides, 1996). Among humans, social exchange can be implicit or explicit, simultaneous or sequential, immediate or deferred, and may involve alternating actions by the two parties or follow more complex structures. In all these cases, however, it is a way people cooperate for mutual benefit. Explicitly agreed-to forms of social exchange are the focus of study in economics (and are known as exchange or trade), while biologists and anthropologists focus more on implicit, deferred cases of exchange, often called reciprocal altruism (Trivers, 1971), reciprocity, or reciprocation. We
will refer to the inclusive set of cases of the mutually conditioned provisioning of benefits as social exchange, regardless of subtype. Nisa and Toma are musing about social exchange interactions in which the expectation of reciprocity is implicit and the favor can be returned at a much later date. In their society, as in ours, the benefits given and received need not be physical objects for exchange to exist, but can be services (valued actions) as well. Aid in a fight, support in a political conflict, help with a sick child, permission to hunt and use water holes in your family‘s territory—all are ways of doing or repaying a favor. Social exchange behavior is both panhuman and ancient. What cognitive abilities make it possible?
For 25 years, we have been investigating the hypothesis that the enduring presence of social exchange interactions among our ancestors has selected for cognitive mechanisms that are specialized for reasoning about social exchange. Just as a lock and key are designed to fit together to function, our claim is that the proprietary procedures and conceptual elements of the social exchange reasoning specializations evolved to reflect the abstract, evolutionarily recurring relationships present in social exchange interactions (Cosmides & Tooby, 1989).
We picked social exchange reasoning as an initial test case for exploring the empirical power of evolutionary psychological analysis for a number of reasons. First, the topic is intrinsically important: Exchange is central to all human economic activity. If exchange in our species is made possible by evolved, neurocomputational programs specialized for exchange itself, this is surely worth knowing. Such evolved programs would constitute the foundation of
Cosmides & Tooby—2
economic behavior, and their specific properties would organize exchange interactions in all human societies; thus, if they exist, they deserve to be mapped. The discovery and mapping of such mechanisms would ground economics in the evolutionary and cognitive sciences, cross-connecting economics to the rest of the natural sciences. Social exchange specializations (if they exist) also underlie many aspects of a far broader category of implicit social interaction lying outside economics, involving favors, friendship, and self-organizing cooperation.
There was a second reason for investigating the computational procedures engaged by social exchange: The underlying counterhypothesis about social exchange reasoning that we have been testing against is the single most central assumption of the traditional social and behavioral sciences—the blank slate view of the mind that lies at the center of what we have called the standard social science model (Tooby & Cosmides, 1992). On this view, humans are
endowed with a powerful, general cognitive capacity (intelligence, rationality, learning, instrumental reasoning), which explains human thought and the great majority of human behavior. In this case, humans putatively engage in successful social exchange through exactly the same cognitive faculties that allow them to do everything else: Their general intelligence allows them to recognize, learn, or reason out intelligent, beneficial courses of action. Despite—
or perhaps because—this hypothesis has been central to how most neural, psychological, and social scientists conceptualize human behavior, it is almost never subjected to potential empirical falsification (unlike theories central to physics or biology). Investigating reasoning about social exchange provided an opportunity to test the blank slate hypothesis empirically in domains (economics and social behavior) where it had previously been uncritically accepted by almost all traditional researchers. Moreover, the results of these tests would be powerfully telling for the general issue of whether an evolutionary psychological program would lead to far-reaching and fundamental revisions across the human sciences. Why? If mechanisms of general rationality exist and are to genuinely explain anything of significance, they should surely explain social exchange reasoning as one easy application. After all, social exchange is absurdly simple compared to other cognitive activities such as language or vision, it is mutually beneficial and intrinsically rewarding, it is economically rational (Simon, 1990), and it should emerge spontaneously as the result of the ability to pursue goals; even artificially intelligent agents capable of pursuing goals through means-ends analysis should be able to manage it. An organism that was in fact equipped with a powerful, general intelligence would not need cognitive specializations for social exchange to be able to engage in it. If it turns out that humans nonetheless have adaptive specializations for social exchange, it would imply that mechanisms of general intelligence (if they exist) are relatively weak, and natural selection has specialized a far larger number of comparable cognitive competences than cognitive and behavioral scientists had anticipated.
Third, we chose reasoning because reasoning is widely considered to be the
quintessential case of a content-independent, general-purpose cognitive competence. Reasoning is also considered to be the most distinctively human cognitive ability—something that exists in
opposition to, and as a replacement for, instinct. If, against all expectation, even human reasoning turned out to fractionate into a diverse collection of evolved, content-specialized procedures, then adaptive specializations are far more likely to be widespread and typical in the human psychological architecture, rather than nonexistent or exceptional. Reasoning presents the most difficult test case, and hence the most useful case to leapfrog the evolutionary debate into genuinely new territory. In contrast, the eventual outcome of debates over the evolutionary origins and organization of motivation (e.g., sexual desire) and emotion (e.g., fear) are not in
Cosmides & Tooby—3
doubt (despite the persistence of intensely fought rearguard actions by traditional research communities). No blank slate process could, even in principle, acquire the human complement of motivational and emotional organization (Cosmides & Tooby, 1987; Tooby, Cosmides, & Barrett, 2005). Reasoning will be the last redoubt of those who adhere to a blank slate approach to the human psychological architecture.
Fourth, logical reasoning is subject to precise formal computational analysis, so it is possible to derive exact and contrasting predictions from domain-general and domain-specific theories, allowing critical tests to be devised and theories to be potentially or actually falsified.
Finally, we chose the domain of social exchange because it offered the opportunity to explore whether the evolutionary dynamics newly charted by evolutionary game theory (e.g., Maynard Smith, 1982) could be shown empirically to have sculpted the human brain and mind and, indeed, human moral reasoning. If it could be empirically shown that the kinds of selection pressures modeled in evolutionary game theory had real consequences on the human psychological architecture, then this would help lay the foundations of an evolutionary approach to social psychology, social behavior, and morality (Cosmides & Tooby, 2004). Morality was considered by most social scientists (then as now) to be a cultural product free of biological organization. We thought on theoretical grounds there should be an evolved set of domain-specific grammars of moral and social reasoning (Cosmides & Tooby, 1989) and wanted to see if we could clearly establish at least one rich empirical example—a grammar of social exchange.
One pleasing feature of the case of social exchange is that it can be clearly traced step by step as a causal chain from replicator dynamics and game theory to details of the computational architecture to specific patterns of reasoning performance to specific cultural phenomena, moral intuitions, and conceptual primitives in moral philosophy—showcasing the broad integrative
power of an evolutionary psychological approach. This research is one component of a larger project that includes mapping the evolutionary psychology of moral sentiments and moral emotions alongside moral reasoning (e.g., Cosmides & Tooby, 2004; Lieberman, Tooby, & Cosmides, 2003; Price, Cosmides, & Tooby, 2002).
What follows are some of the high points of this 25-year research program. We argue that social exchange is ubiquitously woven through the fabric of human life in all human cultures everywhere, and has been taking place among our ancestors for millions and possibly tens of millions of years. This means social exchange interactions are an important and recurrent human activity with sufficient time depth to have selected for specialized neural adaptations. Evolutionary game theory shows that social exchange can evolve and persist only if the cognitive programs that cause it conform to a narrow and complex set of design specifications. The complex pattern of functional and neural dissociations that we discovered during a 25-year research program reveal so close a fit between adaptive problem and computational solution that a neurocognitive specialization for reasoning about social exchange is implicated, including a subroutine for cheater detection. This subroutine develops precocially (by ages 3 to 4) and appears cross-culturally—hunter-horticulturalists in the Amazon detect cheaters as reliably as adults who live in advanced market economies. The detailed patterns of human reasoning performance elicited by situations involving social exchange correspond to the evolutionarily derived predictions of a specialized logic or grammar of social exchange and falsify content-independent, general-purpose reasoning mechanisms as a plausible explanation for reasoning in this domain. A developmental process that is itself specialized for social exchange appears to be responsible for building the neurocognitive specialization found in adults: As we show, the design, ontogenetic timetable, and cross-cultural distribution of social exchange are not
Cosmides & Tooby—4
consistent with any known domain-general learning process. Taken together, the data showing design specificity, precocious development, cross-cultural universality, and neural dissociability implicate the existence of an evolved, species-typical neurocomputational specialization.
In short, the neurocognitive system that causes reasoning about social exchange shows evidence of being what Pinker (1994) has called a cognitive instinct: It is complexly organized
for solving a well-defined adaptive problem our ancestors faced in the past, it reliably develops in all normal humans, it develops without any conscious effort and in the absence of explicit instruction, it is applied without any conscious awareness of its underlying logic, and it is functionally and neurally distinct from more general abilities to process information or behave intelligently. We briefly review the evidence that supports this conclusion, along with the evidence that eliminates the alternative byproduct hypotheses that have been proposed. (For more comprehensive treatments, see Cosmides, 1985, 1989; Cosmides & Tooby, 1989, 1992, 2005; Fiddick, Cosmides, & Tooby, 2000; Stone, Cosmides, Tooby, Kroll, & Knight, 2002; Sugiyama, Tooby, & Cosmides, 2002.)
Social Exchange in Zoological and Cultural Perspective
Living in daily contact affords many opportunities to see when someone needs help, to monitor when someone fails to help but could have, and, as Nisa explains, to withdraw future help when this happens. Under these conditions, reciprocity can be delayed, understanding of obligations and entitlements can remain tacit, and aid (in addition to objects) can be given and received (Shostak, 1981). But when people do not live side by side, social exchange arrangements typically involve explicit agreements, simultaneous transfer of benefits, and increased trade of objects (rather than intimate acts of aid). Agreements are explicit because neither side can know the other‘s needs based on daily interaction, objects are traded because neither side is present to provide aid when the opportunity arises, and trades are simultaneous because this reduces the risk of nonreciprocation—neither side needs to trust the other to provide help in the future. Accordingly, explicit or simultaneous trade is usually a sign of social distance (Tooby & Cosmides, 1996). !Kung, for example, will trade hides for knives and other goods with Bantu people but not with fellow band members (Marshall, 1976).
Explicit trades and delayed, implicit reciprocation differ in these superficial ways, but they share a deep structure: X provides a benefit to Y conditional on Y doing something that X
wants. As humans, we take it for granted that people can make each other better off than they were before by exchanging benefits—goods, services, acts of help and kindness. But when
placed in zoological perspective, social exchange stands out as an unusual phenomenon whose existence requires explanation. The magnitude, variety, and complexity of our social exchange relations are among the most distinctive features of human social life and differentiate us strongly from all other animal species (Tooby & DeVore, 1987). Indeed, uncontroversial examples of social exchange in other species are difficult to find, and despite widespread investigation, social exchange has been reported in only a tiny handful of other species, such as chimpanzees, certain monkeys, and vampire bats (see Dugatkin, 1997; Hauser, in press, for contrasting views of the nonhuman findings).
Practices can be widespread without being the specific product of evolved psychological adaptations. Is social exchange a recent cultural invention? Cultural inventions such as alphabetic writing systems, cereal cultivation, and Arabic numerals are widespread, but they have one or a few points of origin, spread by contact, and are highly elaborated in some cultures
Cosmides & Tooby—5
and absent in others. Social exchange does not fit this pattern. It is found in every documented culture past and present and is a feature of virtually every human life within each culture, taking on a multiplicity of elaborate forms, such as returning favors, sharing food, reciprocal gift givin