Max M. Krasnow

Evolution of direct reciprocity under uncertainty can explain human generosity in one-shot encounters

Are humans too generous? The discovery that subjects choose to incur costs to allocate benefits to others in anonymous, one-shot economic games has posed an unsolved challenge to models of economic and evolutionary rationality. Using agent-based simulations, we show that such generosity is the necessary byproduct of selection on decision systems for regulating dyadic reciprocity under conditions of uncertainty. In deciding whether to engage in dyadic reciprocity, these systems must balance (i) the costs of mistaking a one-shot interaction for a repeated interaction (hence, risking a single chance of being exploited) with (ii) the far greater costs of mistaking a repeated interaction for a one-shot interaction (thereby precluding benefits from multiple future cooperative interactions). This asymmetry builds organisms naturally selected to cooperate even when exposed to cues that they are in one-shot interactions.

Spatial adaptations for plant foraging: women excel and calories count

We present evidence for an evolved sexually dimorphic adaptation that activates spatial memory and navigation skills in response to fruits, vegetables and other traditionally gatherable sessile food resources. In spite of extensive evidence for a male advantage on a wide variety of navigational tasks, we demonstrate that a simple but ecologically important shift in content can reverse this sex difference. This effect is predicted by and consistent with the theory that a sexual division in ancestral foraging labour selected for gathering-specific spatial mechanisms, some of which are sexually differentiated. The hypothesis that gathering-specific spatial adaptations exist in the human mind is further supported by our finding that spatial memory is preferentially engaged for resources with higher nutritional quality (e.g. caloric density). This result strongly suggests that the underlying mechanisms evolved in part as adaptations for efficient foraging. Together, these results demonstrate that human spatial cognition is content sensitive, domain specific and designed by natural selection to mesh with important regularities of the ancestral world.

What Are Punishment and Reputation for

Why did punishment and the use of reputation evolve in humans? According to one family of theories, they evolved to support the maintenance of cooperative group norms; according to another, they evolved to enhance personal gains from cooperation. Current behavioral data are consistent with both hypotheses (and both selection pressures could have shaped human cooperative psychology). However, these hypotheses lead to sharply divergent behavioral predictions in circumstances that have not yet been tested. Here we report results testing these rival predictions. In every test where social exchange theory and group norm maintenance theory made different predictions, subject behavior violated the predictions of group norm maintenance theory and matched those of social exchange theory. Subjects do not direct punishment toward those with reputations for norm violation per se; instead, they use reputation self-beneficially, as a cue to lower the risk that they personally will experience losses from defection. More tellingly, subjects direct their cooperative efforts preferentially towards defectors they have punished and away from those they haven’t punished; they avoid expending punitive effort on reforming defectors who only pose a risk to others. These results are not consistent with the hypothesis that the psychology of punishment evolved to uphold group norms. The circumstances in which punishment is deployed and withheld–its circuit logic–support the hypothesis that it is generated by psychological mechanisms that evolved to benefit the punisher, by allowing him to bargain for better treatment.

Meeting now suggests we will meet again: Implications for debates on the evolution of cooperation

Humans are often generous, even towards strangers encountered by chance and even in the absence of any explicit information suggesting they will meet again. Because game theoretic analyses typically conclude that a psychology designed for direct reciprocity should defect in such situations, many have concluded that alternative explanations for human generosity—explanations beyond direct reciprocity—are necessary. However, human cooperation evolved within a material and informational ecology: Simply adding consideration of one minimal ecological relationship to the analysis of reciprocity brings theory and observation closer together, indicating that ecology-free analyses of cooperation can be fragile. Using simulations, we show that the autocorrelation of an individuals location over time means that even a chance encounter with an individual predicts an increased probability of a future encounter with that same individual. We discuss how a psychology designed for such an ecology may be expected to often cooperate even in apparently one-shot situations.

Conditions under which function information attenuates name extension via shape.

Children often extend names to novel artifacts on the basis of overall shape rather than core properties (e.g., function). This bias is claimed to reflect the fact that nonrandom structure is a reliable cue to an object having a specific designed function. In this article, we show that information about an objects design (i.e., about its creators intentions) is neither necessary nor sufficient for children to override the shape bias. Children extend names on the basis of any information specifying the artifacts function (e.g., information about design, current use, or possible use), especially when this information is made salient when candidate objects for extension are introduced. Possible mechanisms via which children come to rely less on easily observable cues (e.g., shape) and more on core properties (e.g., function) are discussed.

Evolution of fairness: rereading the data.

J. Henrich et al. s Research Article (“Markets, religion, community size, and the evolution of fairness and punishment,” 19 March, p. [1480][1]) is a valuable addition to the growing literature testing behavioral hypotheses through careful cross-cultural measurement. However, the data they report falsify their theory. The authors propose that increases in third-party punishment of unfairness drove an increase in fairness norms, enabling the emergence of large-scale market economies. Critical to this theory is their hypothesis that exposure to third-party punishment actually elicits an increase in fairness. Relevant to evaluating this hypothesis, the authors conducted two near-parallel economic games: a Dictator Game (in which a “dictator” unilaterally divides a windfall gain with another person—a measure of fairness) and a Third-Party Punishment Game (the Dictator Game with the addition of exposure to possible third-party punishment). Their central hypothesis requires that adding punishment to the Dictator Game will increase fairness, but their data show that the addition of punishment decreases fairness (p. 1483). This finding unambiguously refutes their central hypothesis. Henrich et al. also assert that their data—showing patterned cultural variability in cooperation—can determine whether modern levels of generosity and altruism are driven by an evolved social psychology or by cultural processes. The authors claim that their data decisively favor the cultural processes hypothesis. Yet nothing in their data can test (even in principle) whether it is psychological or cultural processes (or both) that cause these cross-cultural differences. Only long-abandoned instinct-as-reflex theories expect invariant responses in the face of different social inputs. By contrast, modern adaptationist theories predict that our evolved social psychology will be calibrated by relevant environmental inputs. Many of these inputs—such as the local value of long-term cooperative relationships and the fidelity of reputations—are likely to covary with market integration, making it at least as likely that an evolved, context-sensitive social psychology is driving the results that the authors observe. That is, psychological and cultural theories both predict cross-cultural variation. [1]: /lookup/doi/10.1126/science.1182238

Group Cooperation without Group Selection: Modest Punishment Can Recruit Much Cooperation

Humans everywhere cooperate in groups to achieve benefits not attainable by individuals. Individual effort is often not automatically tied to a proportionate share of group benefits. This decoupling allows for free-riding, a strategy that (absent countermeasures) outcompetes cooperation. Empirically and formally, punishment potentially solves the evolutionary puzzle of group cooperation. Nevertheless, standard analyses appear to show that punishment alone is insufficient, because second-order free riders (those who cooperate but do not punish) can be shown to outcompete punishers. Consequently, many have concluded that other processes, such as cultural or genetic group selection, are required. Here, we present a series of agent-based simulations that show that group cooperation sustained by punishment easily evolves by individual selection when you introduce into standard models more biologically plausible assumptions about the social ecology and psychology of ancestral humans. We relax three unrealistic assumptions of past models. First, past models assume all punishers must punish every act of free riding in their group. We instead allow punishment to be probabilistic, meaning punishers can evolve to only punish some free riders some of the time. This drastically lowers the cost of punishment as group size increases. Second, most models unrealistically do not allow punishment to recruit labor; punishment merely reduces the punished agent’s fitness. We instead realistically allow punished free riders to cooperate in the future to avoid punishment. Third, past models usually restrict agents to interact in a single group their entire lives. We instead introduce realistic social ecologies in which agents participate in multiple, partially overlapping groups. Because of this, punitive tendencies are more expressed and therefore more exposed to natural selection. These three moves toward greater model realism reveal that punishment and cooperation easily evolve by direct selection—even in sizeable groups.

Looking Under the Hood of Third-Party Punishment Reveals Design for Personal Benefit

Third-party intervention, such as when a crowd stops a mugger, is common. Yet it seems irrational because it has real costs but may provide no personal benefits. In a laboratory analogue, the third-party-punishment game, third parties (“punishers”) will often spend real money to anonymously punish bad behavior directed at other people. A common explanation is that third-party punishment exists to maintain a cooperative society. We tested a different explanation: Third-party punishment results from a deterrence psychology for defending personal interests. Because humans evolved in small-scale, face-to-face social worlds, the mind infers that mistreatment of a third party predicts later mistreatment of oneself. We showed that when punishers do not have information about how they personally will be treated, they infer that mistreatment of other people predicts mistreatment of themselves, and these inferences predict punishment. But when information about personal mistreatment is available, it drives punishment. This suggests that humans’ punitive psychology evolved to defend personal interests.

Reply to McNally and Tanner: Generosity evolves when cooperative decisions must be made under uncertainty

We thank McNally and Tanner (1) for their considered critique of our article (2). Our article addressed the puzzle of why humans, in one-shot interactions, often choose to incur costs to allocate benefits to others, with no possibility of recouping these losses (i.e., “irrational” generosity) (2). This empirical pattern challenges standard models of economic and evolutionary rationality, and has prompted the development of ever-more-cumbersome explanations (e.g., group selection, gene–culture coevolution, cultural group selection)—models fragilely dependent on many hard-to-verify assumptions. In our article, we demonstrated that the well-documented selective regime of direct reciprocity produces agents willing to cooperate in apparently one-shot encounters, when the cognitive problem of discriminating one-shot from repeated encounters under uncertainty has been incorporated into the analysis (as, realistically, it must be). This is a parsimonious solution to the puzzle that flows naturally out of well-understood selection pressures.

Are Humans Too Generous and Too Punitive? Using Psychological Principles to Further Debates about Human Social Evolution.

Are humans too generous and too punitive? Many researchers have concluded that classic theories of social evolution (e.g., direct reciprocity, reputation) are not sufficient to explain human cooperation; instead, group selection theories are needed. We think such a move is premature. The leap to these models has been made by moving directly from thinking about selection pressures to predicting patterns of behavior and ignoring the intervening layer of evolved psychology that must mediate this connection. In real world environments, information processing is a non-trivial problem and details of the ecology can dramatically constrain potential solutions, often enabling particular heuristics to be efficient and effective. We argue that making the intervening layer of psychology explicit resolves decades-old mysteries in the evolution of cooperation and punishment.