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In our genes -- Harpending and Cochran 99 (1): 10 -- Proceedings of the National Academy of Sciences

In our genes -- Harpending and Cochran 99 (1): 10 -- Proceedings of the National Academy of Sciences

The D4 dopamine receptor (DRD4) locus may be a model system for understanding the relationship between genetic variation and human cultural diversity. It has been the subject of intense interest in psychiatry, because bearers of one variant are at increased risk for attention deficit hyperactivity disorder (ADHD) (1). A survey of world frequencies of DRD4 alleles has shown striking differences among populations (2), with population differences greater than those of most neutral markers. In this issue of PNAS Ding et al. (3) provide a detailed molecular portrait of world diversity at the DRD4 locus. They show that the allele associated with ADHD has increased a lot in frequency within the last few thousands to tens of thousands of years, although it has probably been present in our ancestors for hundreds of thousands or even millions of years.

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There are at least two hypotheses to explain the world distribution of 7R. The first, due to Chen et al. (2), is that it is a dispersal morph. They argue that the allele increases the likelihood that its bearers migrate. As modern humans colonized the earth, bearers of 7R were more likely to be movers so that populations far away from their ancient places of origin have, in effect, concentrated 7R. The world high frequencies in South America reflect the great distance of South America from the original human homeland: similarly migrations from China led to the presence of the allele in southeast Asian and Pacific populations, whereas none remained in China. This hypothesis does not account for the apparent long persistence at low frequency of 7R in human ancestors before the population movements occurred that were responsible for population frequency differences.

The second hypothesis is that 7R bearers enjoy a reproductive advantage in male-competitive societies, either in competition for food as children or in face-to-face and local group male competition. Societies in which this advantage would be present were rare before the spread of agriculture, but common after it. This hypothesis requires a brief review of human ecological history. We acknowledge our abuse of detail and of ethnographic diversity in the summary that follows.

Modern humans were successful colonizers of much of the Old World by 40,000 years ago. They lived by hunting and gathering and, later, by agriculture. The archaeological evidence suggests that agriculture appears soon after the end of the Pleistocene, but the antiquity of occasional tropical gardening is not known. Where human densities were low, agriculture was most often extensive, involving shifting cultivation of gardens that were then left fallow for years. Increasing human densities led to agricultural intensification with ever higher inputs of labor to ever scarcer land, resulting in plow agriculture and organized irrigation.

Among most hunting and gathering people both sexes work to provision offspring; in particular males allocate much of their reproductive effort to parental effort. These "dad" societies contrast with "cad" societies in which males allocate reproductive effort to mating effort, that is to competition with other males for access to females.

The general theory of the "war between the sexes" is described by Dawkins (11), whereas the human version of it is described in a landmark paper by Whiting and Whiting (12) and elaborated by others (13, 14). In general, in societies where males are dads, men and women live together with their offspring; they eat and sleep together; the males are not particularly gaudy; and they do not make fancy weapons and art. Pair bonds are durable, divorce rates are low; and nuclear families are the primary context for care of children.

In cad societies, the public relations between men and women are aloof; men and women often do not eat and sleep together; and males are involved in personal adornment, fancy and decorative weapons and art, and local raiding and warfare. In many such groups, for example, men eat and sleep in a men's house rather than with families. Marriages are not durable, and children from an early age are likely to be left to the care of siblings and other children. The latter societies are called "peer-rearing" societies in the literature, whereas dad societies are more often "parent-rearing" societies.

Most foraging people are dad societies, the exceptions being cases where there are periodic rich resource streams like salmon runs on the North American northwest coast. There is some controversy in the literature about whether apparently parental males in dad societies are really parental or whether they are instead engaged in many subtle forms of male competition and mate guarding (15). At any rate the end result is that men work and provide food to children.

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