The question of cognitive differences between human populations is one of the most contentious issues in the study of human diversity. After reviewing the worldwide patterns of cognitive test performance, this article evaluates alternative causal hypotheses and evolutionary mechanisms. Racial affiliation and latitude correlate with IQ test performance, as does economic development. Religion, a history of colonialism, and a history of Communist rule are important in some cases. This article proposes mechanisms of gene-culture co-evolution that can explain the worldwide patterns. The genetic component of these mechanisms is likely to become testable with further advances in molecular genetics.
Key Words: Evolution; Intelligence; Population genetics; Brain size; Race differences; Natural selection; Polymorphism; Mutational load; Geneculture coevolution.
Population genetics is the study of genetic variation in human populations, and IQ population genetics is the population-level study of those genetic variations that influence mental ability. Admittedly, this field does not yet exist as an area of established scientific inquiry because hardly any of the genetic variations that influence human cognition in the nonpathological range are known at the present time. However, the first reports about intelligence-related genetic traits have appeared in the literature (Egan et al., 2001, 2003), and more information is likely to become available in the not too distant future.
At this point in time we can only chart the worldwide variations in people's performance on standardized cognitive tests, and evaluate the plausibility of causal hypotheses. In this article I review the available evidence, not to advance one or another explanation but to provide a road map for future studies that will increasingly rely on the methods of molecular genetics.
The Nature of Human Genetic Diversity
When the genomes of two unrelated people are compared, about one in every 1200 nucleotides in their DNA is different: 2.5 million differences altogether. A disproportionate amount of this variation is in non-coding "junk DNA" and is presumably irrelevant for any phenotypic trait (Venter et ai, 2001). This level of genetic diversity is low when compared with other animal species. The only sensible explanation is that modern humans evolved from a relatively small population only recently. Over time, all small populations tend toward homogeneity because many of the less common genetic variants get lost by natural selection or by chance.
Reconstructions of the history of mitochondrial DNA (Ingman et al., 2000), the Ychromosome (Underbill et al., 2001) and autosomal nuclear DNA (Marth et al., 2003) all converge on the conclusion that our ancestors emerged from a moderately severe population bottleneck sometime between 50,000 and 150,000 years ago, at roughly the time when physically modern Homo sapiens began spreading over the world.
Both genetic evidence (Irigmari et al., 2000; Underbill et al, 2001; Zhivotovsky et al., 2003) and the fossil record (White et al., 2003) point to Africa as the likely homeland of our species. According to the most widely accepted scenario, one or more subgroups of early modern humans left Africa between 120,000 and 100,000 years ago to become the ancestors of the non-African populations.
This implies that most of the "racial" variation we see today evolved in this short time period. For example, population-level differences in climate-selected traits such as skin color can have evolved only after the spread of modern humans out of the tropical and subtropical regions of Africa and South Asia. Cavalli-Sforza, L.L., Menozzi, P and Piazza, A. (1994) The History and Geography of Human Genes., Princeton, NJ: Princeton University Press (p. 156) state that Homo Sapiens migrated into the Near East about 100,000 bp. and were in China by 60,000 bp.
A similar time scale applies to the evolution of possible cognitive differences between human populations. …