ARTICLES The Social Brain Hypothesis

The concensus view has tradition-ally been that brains evolved to pro-cess information of ecological rel-evance. This view, however, ignores an important consideration: Brains are exceedingly expensive both to evolve and to maintain. The adult human brain weighs about 2 % of body weight but consumes about 20 % of total en-ergy intake.2 In the light of this, it is difficult to justify the claim that pri-mates, and especially humans, need larger brains than other species merely to do the same ecological job. Claims that primate ecological strategies in-volve more complex problem-solv-ing3,4 are plausible when applied to the behaviors of particular species, such as termite-extraction by chimpan-zees and nut-cracking by Cebus mon-keys, but fail to explain why all primates, including those that are con-ventional folivores, require larger brains than those of all other mammals. An alternative hypothesis offered during the late 1980s was that pri-mates ’ large brains reflect the compu-tational demands of the complex so-cial systems that characterize the order.5,6 Prima facie, this suggestion seems plausible: There is ample evi-dence that primate social systems are more complex than those of other species. These systems can be shown to involve processes such as tactical deception5 and coalition-formation,7,8 which are rare or occur only in sim-pler forms in other taxonomic groups. Because of this, the suggestion was rapidly dubbed the Machiavellian intel-ligence hypothesis, although there is a growing preference to call it the social brain hypothesis.9,10 Plausible as it seems, the social brain hypothesis faced a problem that was recognized at an early date. Specifi-cally, what quantitative empirical evi-dence there was tended to favor one or the other of the ecological hypoth-eses,1 whereas the evidence adduced in favor of the social brain hypothesis was, at best, anecdotal.6 In this article, I shall first show how we can test between the competing hypotheses more conclusively and then consider some of the implications of the social brain hypothesis for humans. Finally, I shall briefly consider some of the underlying cognitive mechanisms that might be involved