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Psychology
Cross-cultural evidence of cognitive adaptations for social exchange among the Shiwiar of Ecuadorian Amazonia
aDepartment of Anthropology, University of Oregon, Eugene, OR 97403; and bCenter for Evolutionary Psychology, University of California, Santa Barbara, CA 93106
Communicated by Roger N. Shepard, Stanford University, Stanford, CA and approved June 12, 2002 (received for review December 24, 2001)
On the basis of evolutionary game theory, it was hypothesized that humans have an evolved cognitive specialization for reasoning about social exchange, including a subroutine for detecting cheaters. This hypothesis led to a specific prediction: Although humans are known to be poor at detecting potential violations of conditional rules in general, they should nevertheless detect them easily when the rule involves social exchange and looking for violations corresponds to looking for cheaters. This prediction was subsequently confirmed by numerous tests. Evolutionary analyses further predict that: (i) in humans, complex adaptations will be distributed in a species-typical fashion; and (ii) aspects of cognitive organization relevant to performing the evolved function of an adaptation should be more buffered against environmental and cultural variation than function-irrelevant aspects. Here we report experiments testing whether social exchange reasoning exhibits these properties of adaptations. Existing tests of conditional reasoning were adapted for nonliterate experimental subjects and were administered to Shiwiar hunter–horticulturalists of the Ecuadorian Amazon. As predicted, Shiwiar subjects were as highly proficient at cheater detection as subjects from developed nations. Indeed, the frequency of cheater-relevant choices among Shiwiar hunter-horticulturalists was indistinguishable from that of Harvard undergraduates. Also as predicted, cultural variation was confined to those aspects of reasoning that are irrelevant to social exchange algorithms functioning as an evolutionarily stable strategy. Finally, Shiwiar subjects displayed the same low performance on descriptive conditionals as subjects from developed nations. Taken together, these findings support the hypotheses that social exchange algorithms are species-typical and that their evolutionarily stable strategy (ESS)-relevant subroutines are developmentally buffered against cultural variation.
reciprocation | cooperation | economics | game theory | evolutionary psychology
Abbreviations: ESS, evolutionarily stable strategy
d These features are necessary for the adaptations causing the behavior to be an evolutionarily stable strategy or ESS (14). For the application of the game-theoretic constraints to cognition, see ref. 9.
f Functionally specialized acquisition mechanisms buffer development by solving problems of combinatorial explosion in environments that do not uniquely determine an outcome when analyzed by more general inductive procedures (27, 28).
g For discussion of acquisition views, see refs. 29 and 30.
h That familiarity per se does not facilitate logical reasoning on this task is uncontroversial at this point (15–19).
i This condition holds if the adaptation is complex—that is, its genetic basis depends on the simultaneous presence of specific alleles at several different independent loci. The more loci involved, the more likely the adaptation in question is ancient and species-typical. Almost all cognitive adaptations will require, for their specification, more than a few loci, and so will be complex in this sense. (Also, adaptations for cheater detection are not predicted to be expressed facultatively.)
j Although counting systems vary widely, there are adaptations that make them possible and place certain constraints on them (35, 36). The same is true for color terms (37–39).
k Experimentation under field conditions injects higher levels of error variance into results than are obtainable under well-controlled laboratory conditions. More significant than factors such as added distractions, interruptions, and language difficulties is the extreme cultural strangeness of experimental testing itself, with its unfamiliar necessity of adhering to formal, abstract, and seemingly arbitrary behavioral and communicative constraints. Shiwiar subjects had no prior experience with experimental test-taking situations. This situation introduces confusion into the communicative pragmatics inherent in the task situation, and error variance into results. Restricting one's responses to the question explicitly asked, and ignoring information (such as who may be exhibiting generosity to whom) that is relevant to real life but not to a test problem, is a skill one learns in classrooms and courtrooms. Presumably, this is why schooling affects how people reason about problems involving hypotheticals (40) (such as those posed by Wason tasks). In virtually every other social context, when a question is asked, the pragmatic implication is that the asker does not already know the answer, and would like to be told whatever information might be relevant to solving his problem (41). Thus we predicted that Shiwiar performance would reflect two factors: (i) their lack of familiarity with the culturally specific pragmatics of Western testing situations (such as the task demand to ignore interesting information), which would cause cheater-irrelevant cards to sometimes be selected, and (ii) the presence of a species-typical cheater detection mechanism, which would cause a strong propensity to select cheater-relevant cards.
l The Harvard data (15, 16) were chosen for purposes of comparison because (i) these problems most closely paralleled the ones given to the Shiwiar, (ii) it was the most complete parallel data set, and (iii) this comparison placed the hypothesis in greatest jeopardy. Shiwiar look even more similar to subjects from developed nations when other data sets are used.
m Paired t tests: For P: t* = 4.16, r = 0.68, p = 0.00024; for not-P: t* = 3.21, r = 0.58, p = 0.0022; for Q: t* = 2.89, r = 0.54, p = 0.0045; for not-Q: t* = 4.28, r = 0.69, p = 0.00018.
n The frequency of choosing the benefit not accepted card was significantly below chance (p = 0.02) for the switched social contract, and for all other irrelevant card choices in the predicted direction, but not significantly so (i.e., p > 0.05). For the descriptive problem, only the P card was chosen more often than chance.
o A yes/no judgment for each of four cards results in 24 = 16 combinatorial possibilities; hence the probability of choosing all and only the correct cards by chance is 1/16 = 6.25%. The probability of getting at least one of two social contracts right by chance is [1 – (15/16)2] = 12%.
p This result is very robust to changes in assumptions about what counts as chance. Suppose, for example, that people everywhere think that one particular card needs to be chosen for any conditional rule, regardless of content. If it happened to be one of the cards that a person looking for cheaters would choose, then 8 of the 16 combinatorial possibilities would be eliminated, and 12.5% of subjects would answer correctly by chance. Nevertheless, such an arbitrary doubling of the value of chance does not affect the conclusions: cheater detection among Shiwiar would still be higher than chance at the p < 0.01 level.
q Given n = 21 and a medium effect size of 0.38, the probability of finding a difference that is significant at the 0.05 level is only 34% (42). Increasing sample size to increase the power of the test was not an option: we tested everyone in the village who was willing (to get 80% power, n = 80).
r Significance test for differences in effect size uses Fisher's z transformation of r (43).
s Q and not-P is the correct response if one is looking for cheaters on a switched social contract, but for a descriptive conditional, it is both logically incorrect and almost never produced by subjects from developed nations.
e For an opposite dissociation, where social exchange reasoning is preserved but more general cognitive abilities are impaired (in schizophrenia), see ref. 26.
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