Research at PLEEP

What kind of research takes place at the PLEEP lab? The following gives information about Dr. Kurzban's current research. Note: citation information is omitted from the description below. For publication information, please see the References on the bottom of this page as well as the Publications page, which contains a number of downloadable files.


Humans have an array of specialized cognitive systems designed to navigate a complex social world. The PLEEP lab personnel have been exploring some of these, including the cognitive mechanisms designed for cooperation in group contexts and, more recently, close interpersonal relationships and issues surrounding modularity and the “self” . We draw on theory and methods from evolutionary psychology, experimental economics, and cognitive psychology.

Adaptations For Group Living

Multi-individual, non-kin coordinated, cooperative activity among humans remains a puzzle, and economists and psychologists, among others, have generated a wealth of experimental data documenting and quantifying cooperation in group contexts. There is, as yet, no consensus on either the correct evolutionary explanation for multi-individual, non-kin cooperation or for the proximate mechanisms that underpin this phenomenon.


Substantial numbers of researchers have argued that reciprocity is a key component of cooperation in groups. However, although reciprocity is well formalized for dyadic cooperation, the problem is more difficult to formalize for groups. My recent research has been aimed at using experiments to shed light on some of the details of reciprocal psychology. One debate, for example, is whether (and if) people in experimental games try to match their cooperation rate to those contributing the least to the public good, or those contributing the median amount.

Building on earlier work, we have recently completed a project designed to address this question by using an “information-seeking” method, in which participants are asked which group member’s contribution – the low, median, or high – they prefer to look at. Broadly, the results suggested that when participants displayed a preference for observing information – and, in particular, when they paid to do so – those who used reciprocal strategies preferred the median contributor.

Individual Differences

An important caveat, however, is that this research is a component of a parallel line of inquiry which investigates the robust individual differences which appear in these types of experiments. While most participants use some form of a reciprocal strategy, there seem to be, under a range of parameters of the game, relatively distinguishable morphs or “types” of players.. We have found that some players behave like revenue maximizers, free riding most or all of the time, while others seem to be of an altruistic type, cooperating independent of others’ cooperativeness. Distinguishing players by type has allowed us to make predictions about the dynamics of play in these game environments. A related set of experiments expands this agenda by investigating how differences among individuals assigned exogenously by the experimenter – in this case, asymmetries in “endowment,” or wealth – impact decisions to contribute to public goods. We have begun to replicate these studies cross-culturally. One such replication is complete, and additional replications are ongoing.


An additional avenue of research is driven by the flip side of reciprocity, punishment of non-cooperators in group contexts (also referred to as “negative reciprocity” by some researchers). Punishment has been receiving increasing theoretical and empirical attention, and represents an important research area because different models make different predictions about punishment, most of which have not been tested in the laboratory.

In particular, reputation-based models straightforwardly predict that decisions to engage in third party punishment will depend on the presence or absence of cues that one’s is being observed. In contrast, other models make no prediction regarding cues to the presence of others. Careful experiments have shown that under conditions of complete anonymity, people are very unwilling to endure a cost to punish someone who has “defected” in a two-party interaction in which they were not involved. However, when anonymity is reduced – when the experimenter is aware of an individuals’ decision to punish a “defector” – punishment increases substantially, lending weight to reputational models of third party punishment.

Ancillary Projects.

Three additional projects related to understanding adaptations surrounding cooperation in human groups are 1) a collaborative effort with a legal scholar that tries to integrate the literature on criminal law with evolutionary origins of the desire to punish defection, an issue closely connected with cooperation in groups, 2) a collaborative effort with Mark Van Vugt aimed at developing an evolutionary framework for understanding leadership and “followership”, and 3) agent-based computer simulations, led by Athena Aktipis, designed to link ultimate evolutionary explanations for cooperation in groups with empirical data from the laboratory.

Dyadic Relationships

A second set of research projects is aimed at understanding the adaptations that underlie dyadic relationships (as opposed to groups). One component of this research has been to address trust, including how it is built over time and its hormonal correlates. We have also begun to look at the formation and maintenance of relationships, both romantic and platonic. These investigations of romantic relationships are based in part on a data set obtained from HurryDate, a commercial dating service that runs events across the country that bring single men and women together to interact for a brief period of time. After these interactions, participants indicate which of the people they met they would like to see again. This data set represents a unique opportunity for analysis because the data are systematic and also have “real world” consequences. These data help avoid problems in the literature on dating and mating, which often relies on self-report data and the small sample sizes obtained in laboratories.

Briefly, our data allowed us to investigate a number of hypotheses, and yielded two interesting findings. First, in contrast to models that suggest mating is “assortative,” we found that there was broad consensus on who was preferred as potential future dates. This lends weight to a “market” construal of mating, rather than assortment. Second, and less surprising given the nature of the interaction, physically observable traits largely drove these preferences. Somewhat more surprisingly, this was true for men and for women, a slight contrast with some views in the literature, in which much has been made of sex differences in this arena. We are currently extending this work, comparing self-report of mate preferences with actual behavior. That is, we can compare what people say they want in a mate with the traits of the people they actually select to date again. Preliminary analyses indicates some discrepancies, in particular when people report preferences that run in a direction different from the common pattern of preference.

In collaboration with Sheen Levine, currently at the Singapore Management University, I have been investigating close non-romantic relationships. Extending the logic of the “Banker’s Paradox”, we have suggested that actors have evolved mechanisms that guide them in the choice of exchange partners towards clusters, groups of actors densely connected among themselves and only loosely connected to other groups. In particular, we have suggested that social clusters offer “network externalities,” the case in which third parties experiences a gain (or a loss) as a function of the number of users of a given product. In this case, the “product” is social relationships, and the application of this principle to the social world led us to suggest that humans might have evolved cognitive systems that guide them toward building social relationships with individuals who share multiple common ties. This idea might help to explain the omnipresence of social clustering, even in the context of (business) environments, in which such clustering has negative effects.. Empirical work is currently underway designed to explore these ideas.

Modularity and the Self

Finally, we have been developing some ideas about the relationships among evolution, cognitive modularity, and the (putative) "self".


Aktipis, C. A., & Kurzban, R. (2004). Is Homo economicus extinct?: Vernon Smith, Daniel Kahneman and the Evolutionary Perspective. In R. Koppl (Ed.), Advances in Austrian Economics (Vol. 7) (pp. 135-153). Elsevier: Amsterdam.
Aktipis, C. A., & Kurzban, R. (2004). Contingent movement and the evolution of cooperation in groups. Unpublished manuscript.
Barrett, H. C., & Kurzban, R. (2005). Modularity in cognition: Framing the debate. Unpublished manuscript.
Cosmides, L., Tooby, J. & Kurzban, R. (2003). Perceptions of race. Trends in Cognitive Sciences, 7(4), 173-179.
Goren, H., Kurzban, R., & Rapoport, A. (2003). Social loafing vs. social enhancement: Public goods provisioning in real-time with irrevocable commitments. Organizational Behavior and Human Decision Processes, 90, 277-290.
Goren, H., Rapoport, A., & Kurzban, R., (2003). Commitment in a real time step level public goods game with asymmetrical players and continuous contributions. The Journal of Behavioral Decision Making, 16, 1-21.
Houser, D., & Kurzban, R. (2002). Revisiting kindness and confusion in public goods games. The American Economic Review, 92(4), 1062-1069.
Ishii, K., & Kurzban, R. (2005) Real time public goods in Japan: Cultural and individual differences in trust and reciprocity. Unpublished Manuscript.
Kurzban, R. (2001). The social psychophysics of cooperation: Nonverbal communication in a public goods game. Journal of Nonverbal Behavior, 25(4), 241-259.
Kurzban, R. (2003). Biological foundations of reciprocity. E. Ostrom and J. Walker (Eds.), Trust, Reciprocity, and Gains from Association: Interdisciplinary Lessons from Experimental Research (pp. 105-127). New York: Sage.
Kurzban, R., & Aktipis, C. A. (in press). Modular minds, multiple motives. To appear in M. Schaller, J. Simpson, & D. Kenrick (Eds.) Evolution and Social Psychology.
Kurzban, R., & DeScioli, P. (2005). Reciprocal cooperation in groups: Information-seeking in a public goods game. Unpublished Manuscript.
Kurzban, R., DeScioli, P., & O’Brien, E. (in prep.). Audience effects on moralistic punishment. Unpublished Manuscript.
Kurzban, R., & Houser, D. (2001). Individual differences and cooperation in a circular public goods game. European Journal of Personality, 15, S37-S52.
Kurzban, R. & Houser, D. (2005) An experimental investigation of cooperative types in human groups: A complement to evolutionary theory and simulations. Proceedings of the National Academy of Sciences, 102(5), 1803-1807.
Kurzban, R., & Leary, M. R. (2001). Evolutionary origins of stigmatization: The functions of social exclusion. Psychological Bulletin, 127(2), 187-208.
Kurzban, R., & Neuberg, S. (2005). Stereotyping and discrimination: An evolutionary perspective. In D. Buss (Ed.) The Handbook of Evolutionary Psychology (pp. 653-675), Hoboken, NJ: Wiley.
Kurzban, R., & Weeden, J. (2005a). HurryDate: Mate preferences in action. Evolution and Human Behavior, 26(3), 227-244.
Kurzban, R., & Weeden, J. (2005b). HurryDate: Differences between reported and revealed mate preferences. Unpublished manuscript.
Kurzban, R., McCabe, K., Smith, V. L., & Wilson, B. J. (2001). Incremental commitment and reciprocity in a real time public goods game. Personality and Social Psychology Bulletin, 27(12), 1662-1673.
Kurzban, R., Rigdon, M., & Wilson, B. J. (2005). Incremental trust and reciprocity. Unpublished Manuscript.
Kurzban, R., Tooby, J., & Cosmides, L. (2001). Can race be erased? Coalitional computation and social categorization. Proceedings of the National Academy of Sciences, 98(26), 15387-15392.
Levine, S. S., & Kurzban, R. (in press). Explaining clustering within and between organizations: Towards an evolutionary theory of cascading benefits. Managerial and Decision Economics.
Navarrete, C., Kurzban, R., Fessler, D., & Kirkpatrick, L. (2004). Anxiety and worldview defense: Terror-management or coalitional psychology? Group Processes and Intergroup Relations, 7(4), 370-397.
Robinson, P., & Kurzban, R. (in prep.). Intuitions of justice. Unpublished Manuscript.
Sidanius, J., & Kurzban, R. (2003). Evolutionary approaches to political psychology. In D. O. Sears, L. Huddy, and R. Jervis (Eds.), Handbook of Political Psychology (pp. 146-181). Oxford: Oxford University Press.
Van Vugt, M. & Kurzban, R. (in prep.) Evolutionary Origins of Leadership and Followership: Managing the Social Mind. To appear in The evolution of the social mind: Evolutionary psychology and social cognition (The 9th Sydney Symposium of Social Psychology). J Forgas, M. G. Haselton & W. von Hippel (Eds).
Zak, P. J., Borja, K., Matzner, W. T., & Kurzban, R. (2005). The neuroeconomics of distrust: Sex differences in behavior and physiology. The American Economic Review, 95(2), 360-363.
Zak, P. J., Kurzban, R., & Matzner, W. T. (2004). The neurobiology of trust. Annals of the New York Academy of Sciences, 1032, 224-227.
Zak, P. J., Kurzban, R., Matzner, W. T. (2005). Oxytocin mediates interpersonal trust in humans. Hormones and Behavior, 48(5), 522-527.