What forces have driven human evolution since the grand human diaspora? In this paper, I argue that the scale effects so central to endogenous growth theory in the field of economics (e.g., Kremer's widely-cited "Population Growth and Technological Change: 1,000,000 B.C. to 1990," Quarterly Journal of Economics, 1993) have been important drivers of human brain development since the diaspora. Scale effects have made prominent appearances in recent explanations of continent-level outcomes. For instance, in Kremer’s model, big continents create larger, denser, faster-growing populations. In Diamond’s Guns, Germs, and Steel model, wide continents raise the chance that an innovation will arise at a given latitude, an innovation which can then disperse across that latitude, enriching those who live on wider continents. In both models, the Malthusian nature of pre-Industrial Revolution existence imposes strong conditions on the general equilibrium outcome. My model takes those channels as given, and works out the theoretical implications for the divergent evolution of human brains on these continents. Brains are biologically costly, so evolution will only select for larger brains if there is a substantial payoff. And since larger brains tend to have higher levels of intelligence [corr(Brain Size, IQ)= 0.4 in recent in brain-scan studies], larger brains tend to have more processing and memory power. Under certain parameter values, Kremer’s and Diamond’s models both imply that the payoff to a big brain—a brain that can better adopt someone else’s ideas—will be higher on wider, larger continents. Thus, we would expect human populations living on larger, wider continents to develop larger, more powerful brains. I model this relationship formally. This result should only hold on average: intra-group diversity is central to evolutionary theory, and massive intra-group diversity is an important fact of quantitative human genetics. The main purpose of the paper is to set forth the model, but I include some tests of its implications. I discuss whether, as the model predicts, human brain size and average IQ correlate positively with continent size and continent width. Indeed, evidence generally supports this hypothesis. Further empirical testing of the model’s predictions will occur as future researchers employ genetic diversity databases. I plan to present the results in a manner intelligible to non-economists.

Labels: Evolution, human biodiversity