Samuel Bowles and Herbert Gintis have given us a superb synthesis of economics, evolution, psychology and anthropology, all with an eye toward the design of modern society. I find little to criticize and therefore will review developments in evolutionary biology that lend support to their thesis.

Multilevel Selection. Economics and evolutionary biology have helped to construct a conceptual universe in which individuals stand at the center and groups are marginalized as byproducts of self-interest. Many evolutionary biologists would heartily agree with Margaret Thatcher's infamous claim that there is no such thing as society, only individuals and families. Bowles and Gintis depart from this tradition by according a central role to cooperative social groups above the level of the family. Individuals appear willing not only to help unrelated others, but to punish others with such zeal that punishment itself becomes a form of altruism. It might seem that these traits cannot be explained by an evolutionary theory based on selfishness, but a closer look reveals developments in evolutionary theory that parallel what Bowles and Gintis are trying to accomplish in economics. In particular, the very mechanisms they discuss can turn large human groups into potent units of selection.

One way to see this is to focus on the fundamental ingredients of natural selection–phenotypic variation, heritability, and fitness consequences. Almost all evolutionary models assume a direct connection between genes and behaviors, not because this assumption is realistic but because it simplifies the models, hopefully without invalidating their real-world relevance. Nevertheless, a direct connection between genes and behaviors creates a tight linkage between genetic variation and phenotypic variation. For example, the only way to get a behaviorally uniform group is to have a genetically uniform group. This conclusion is patently false for humans (and probably many other species), in which near behavioral uniformity within groups and extreme differences among groups can arise despite genetic diversity. Social norms enforced by punishment are one of the chief mechanisms that concentrate phenotypic variation at the between-group level, allowing between-group selection to be a much more powerful force than simple genetic models would suggest. My recent book with Elliot Sober, Unto Others: The Evolution and Psychology of Unselfish Behavior, reviews the developments that put Bowles and Gintis's thesis on a firm evolutionary foundation.

Individual Differences. Evolutionary models routinely predict that natural selection will evolve a mixture of behavioral strategies in a single population. The proximate mechanisms that evolve to cause phenotypic polymorphisms can include genetic polymorphisms, various forms of developmental plasticity, or a mixture of both. Economic models also frequently predict mixed outcomes, but the image of human populations as a community of interacting behavioral strategies has not emerged as strongly from economic theory as from evolutionary theory. It is therefore gratifying that Bowles and Gintis emphasize the possibility of more than one human nature; human populations may consist of a spectrum from extreme altruists to extreme sociopaths. In addition to its theoretical plausibility, there is growing empirical evidence that a propensity to cooperate or exploit forms an important axis of human behavioral variation. Seeing human groups as both communities of interacting strategies and (partially) adaptive units deserves to become major theme in the future.

Modularity. Evolutionary psychologists such as Leda Tooby and John Cosmides have portrayed the human mind as a collection of adaptations that evolved to solve the most important adaptive problems faced in ancestral environments. Modularity can be interpreted as a claim about the environment and the phenotype (fast and adaptive behavioral response to certain environmental challenges but not others) or as a claim about how the brain is organized (each adaptive behavioral response is caused by a separate circuit), which must be evaluated separately. A. P. Fiske has proposed four modules for human social behavior: communal sharing, authority ranking, equality matching, and market pricing.1 Each form of social behavior is adaptive under certain situations and is invoked by the appropriate cues, much as records are selected by pushing the buttons of a juke box. Fiske's proposal may sound simplistic, but it is backed by intriguing empirical evidence and is relevant to the social policies discussed by Bowles and Gintis. For example, debates over a contemporary issue such as homelessness often appear to be a battle over which module to evoke. If the homeless are merely unfortunate ("there but for the grace of God go I") the communal sharing module becomes appropriate, but if the homeless brought their fate upon themselves, they have violated equality matching rules and do not deserve charity. In the future it will be interesting to relate Fiske's proposal in particular and the modularity thesis in general to the themes raised by Bowles and Gintis.

For many who felt threatened by E. O. Wilson's Sociobiology, evolution could only restrict the human potential to reach cherished goals such as equality. This dismal conclusion has failed to materialize, and Bowles and Gintis show how evolutionary thinking can be used as a tool to achieve those very goals. I hope their views come to occupy a central position in economic and political thought.