Publications

In Press
Delton AW, Krasnow MM. The psychology of deterrence explains why group membership matters for third-party punishment. Evolution & Human Behavior. In Press.Abstract

Humans regularly intervene in others’ conflicts as third-parties. This has been studied using the third-party punishment game: A third-party can pay a cost to punish another player (the “dictator”) who treated someone else poorly. Because the game is anonymous and one-shot, punishers are thought to have no strategic reasons to intervene. Nonetheless, punishers often punish dictators who treat others poorly. This result is central to a controversy over human social evolution: Did third-party punishment evolve to maintain group norms or to deter others from acting against one’s interests? This paper provides a critical test. We manipulate the ingroup/outgroup composition of the players while simultaneously measuring the inferences punishers make about how the dictator would treat them personally. The group norm predictions were falsified, as outgroup defectors were punished most harshly, not ingroup defectors (as predicted by ingroup fairness norms) and not outgroup members generally (as predicted by norms of parochialism). The deterrence predictions were validated: Punishers punished the most when they inferred that they would be treated the worst by dictators, especially when better treatment would be expected given ingroup/outgroup composition.

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Mehr SA, Kotler J, Howard RM, Haig D, Krasnow MM. Genomic imprinting is implicated in the psychology of music. Psychological Science. In Press.
2016
Krasnow MM, Delton AW. The sketch is blank: No evidence for an explanatory role for cultural group selection. Behavioral and Brain Sciences. 2016;39 :43-44. PDF
Krasnow MM, Delton AW. Are humans too generous and too punitive? Using psychological principles to further debates about human social evolution. Frontiers in Psychology [Internet]. 2016. Publisher's VersionAbstract

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.

Krasnow MM, Delton AW, Cosmides L, Tooby J. Looking Under the Hood of Third-Party Punishment Reveals Design for Personal Benefit. Psychological Science. 2016. psychological_science-2016-krasnow-0956797615624469.pdf
2015
Krasnow MM, Delton AW, Cosmides L, Tooby J. Group cooperation without group selection: Modest punishment can recruit much cooperation. PLoS One [Internet]. 2015;10 (4) :e0124561. Web VersionAbstract

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. 

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Delton AW, Krasnow MM. Adaptationist Approaches to Moral Psychology. In: Decety J, Wheatley T The Moral Brain. Cambridge, MA: MIT Press ; 2015. pp. 19-34.Abstract

Keep your word. Love your neighbor. Help those in need. Only marry the child of your father’s sister. Stone those who have had sex out of wedlock. Kill outgroup members. Human moral communities have developed a variety of moral rules, injunctions, prescriptions, and suggestions. How are we to understand the origins and nature of human morality, both at the level of universal building blocks and the level of cultural variation and elaboration? As this volume illustrates, understanding human moral psychology is a truly interdisciplinary endeavor, drawing important contributions from psychologists, neuroscientists, philosophers, biologists, legal scholars, and many others. Our goal in this chapter is to illustrate the utility of taking an adaptationist approach from evolutionary biology to understand universal aspects of moral psychology. We first describe what it means to take an adaptationist approach. We next give several examples of how an adaptationist approach has informed the study of certain aspects of moral psychology. We then briefly conclude with what we see as the value of this approach to the study of moral psychology broadly.

2013
Delton AW, Nemirow J, Robertson TE, Cimino A, Cosmides L. Merely Opting Out of a Public Good Is Moralized: An Error Management Approach to Cooperation. Journal of Personality and Social Psychology. 2013;105 (4) :621-638. PDF
Krasnow MM, Delton AW, Tooby J, Cosmides L. Meeting now suggests we will meet again: Implications for debates on the evolution of cooperation. Scientific Reports. 2013;3.Abstract

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 individual's 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.

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2012
Krasnow MM, Delton AW. Is there evidence for special design of a group-selected psychology? Comment on Steven Pinker's The false allure of group selection. Edge [Internet]. 2012. Web Version PDF
Krasnow MM, Cosmides L, Pedersen EJ, Tooby J. What Are Punishment and Reputation for?. PLoS ONE. 2012;7 (9) :e45662.Abstract

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.

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2011
Delton AW, Krasnow MM, Cosmides L, Tooby J. Reply to McNally and Tanner: Generosity evolves when cooperative decisions must be made under uncertainty. Proc Natl Acad Sci U S A. 2011;108 (44) :E972. PDF
Krasnow MM, Truxaw D, Gaulin SJC, New J, Ozono H, Uono S, Ueno T, Minemoto K. Cognitive adaptations for gathering-related navigation in humans. Evolution and Human Behavior. 2011;32 :1 - 12.Abstract

Current research increasingly suggests that spatial cognition in humans is accomplished by many specialized mechanisms, each designed to solve a particular adaptive problem. A major adaptive problem for our hominin ancestors, particularly females, was the need to efficiently gather immobile foods which could vary greatly in quality, quantity, spatial location and temporal availability. We propose a cognitive model of a navigational gathering adaptation in humans and test its predictions in samples from the US and Japan. Our results are uniformly supportive: the human mind appears equipped with a navigational gathering adaptation that encodes the location of gatherable foods into spatial memory. This mechanism appears to be chronically active in women and activated under explicit motivation in men.

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Delton AW, Krasnow MM, Cosmides L, Tooby J. Evolution of direct reciprocity under uncertainty can explain human generosity in one-shot encounters. Proc Natl Acad Sci U S A. 2011;108 (32) :13335-40.Abstract

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.

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2010
Delton AW, Krasnow MM, Cosmides L, Tooby J. Evolution of fairness: Rereading the data. Science. 2010;329. PDF
2007
Krasnow MM, Truxaw D, New J, Gaulin SJC. Shopping for Explanations, Response. Science. 2007;318 (5851) :745. PDF
New J, Krasnow MM, Truxaw D, Gaulin SJC. Spatial adaptations for plant foraging: women excel and calories count. Proceedings of the Royal Society B: Biological Sciences. 2007;274 :2679 - 2684.Abstract

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.

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2006
Truxaw D, Krasnow MM, Woods C, German TP. Conditions Under Which Function Information Attenuates Name Extension via Shape. Psychological Science. 2006;17 :367 - 371.Abstract

ABSTRACT- 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 object's design (i.e., about its creator's intentions) is neither necessary nor sufficient for children to override the shape bias. Children extend names on the basis of any information specifying the artifact's 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.

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