The 7-repeat variant of the DRD4 gene, traditionally linked with ADHD, is, according to a 2010 paper, also strongly associated with cheating and promiscuity. This finding indicates genetic roots in sociosexual orientation, and the possibility that environmental and cultural rewards may result in statistical genetic differences in population hardwiring:
These results are the first evidence (to our knowledge) of a significant association between a specific genetic locus and both promiscuous sexual behavior and infidelity. These findings show that genetic variation in the brain′s dopaminergic reward pathway appears to be an influential factor in individual differences in motivation to engage in sexual behavior of a risky and uncommitted nature. Further, this potentially suggests an evolutionary mechanisms contributing to the substantial global allelic variation of the DRD4 VNTR genotype. Individuals genotyped as 7R+ were significantly more likely to reported having ever engaged in promiscuous sex (i.e., a one-night stand). Of those reporting infidelity, 7R+ individuals were cheating on romantic partners more often, which under certain circumstances could result in higher genetic fitness via greater offspring diversity as well as increased total fecundity.
Because of this, an environment with strong monogamy will result in a population genetically oriented towards fidelity and “dad” characteristics. Conversely, an environment with free sex will result in the sexual selection of “cads”.
This suggests that in local environments where monogamy and sexual fidelity are advantageous, the 7R- genotype would be subject to positive selective pressure. In contrast, in environments where monogamy and fidelity are disadvantageous, the 7R+ genotype would be subject to positive selective pressure. This may be additionally elucidated from the perspective of r/K selection . In r-selected environments (i.e., unpredictable and unstable environments, where the ability to mate more and produce more offspring is favored), 7R+ genotype would be expected to rise in frequency. That is, in environments where “cad” behavior is adaptive, selective pressure for 7R+ would be positive; but in environments where “dad” behavior is adaptive, selective pressure for 7R+ would be negative. This is consistent with the dramatic differences in DRD4 VNTR allele frequencies and behavioral patterns found globally such as in the generally polygamous and agonistic Yanomamö Indians of South America (high 7R+ frequencies) and the generally egalitarian !Kung of the Kalahari (low 7R+ frequencies). Evidence that DRD4 VNTR status is related to social and sexual behavioral strategies provides a plausible mechanism for varying selective pressure and observed racial, ethnic, and regional differences in allele frequencies.
An earlier (2002) paper provides insights into the origins and positive sexual-selection of this 7R DRD4 variant. According to it, the 7R variant has existed for hundreds of thousands of years, but only became frequent in the last thousands or few tens of thousands of years:
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.
Thus, a recent shift in selection pressures caused the preponderance of the 7R variant. This has implications for evolutionary psychology, which traditionally tries to link all behavior to the paleolithic:
Evidence of adaptive genetic variation affecting human psychology should be of interest to evolutionary psychologists, particularly because they have argued that it cannot exist. For example Tooby and Cosmides (7) claim that there are only two kinds of human nature, male and female, and that apparent variation in personality is either facultative response to environmental cues or nonadaptive. They argue that complex adaptations require the coordinated action of many genes, and that if individuals of a sexually reproducing species differed in the genes required to build these adaptations, sexual reproduction would disrupt the necessary gene complexes. They also argue that there has been insufficient time since the advent of modern humanity for the development of significant novel mental adaptations.
As has been pointed out by D. Wilson (8), their arguments are unconvincing. They imply that no sexual species should have heritable adaptive morphs, whereas in fact many examples are known, such as the male morphs of the side-blotched lizard Uta stansburiana that act out a scissors–paper–rock game, in which morph A beats morph B, morph B beats morph C, and morph C beats morph A (9). The “insufficient time” argument is probably just incorrect; considering the measured heritability of psychological traits and the expected response to mild selection over hundreds of generations, it is instead surprising that we seem to have changed little over historical time.
Even if 40 or 50 thousand years were too short a time for the evolutionary development of a truly new and highly complex mental adaptation, which is by no means certain, it is certainly long enough for some groups to lose such an adaptation, for some groups to develop a highly exaggerated version of an adaptation, or for changes in the triggers or timing of that adaptation to evolve. That is what we see in domesticated dogs, for example, who have entirely lost certain key behavioral adaptations of wolves such as paternal investment. Other wolf behaviors have been exaggerated or distorted. A border collie’s herding is recognizably derived from wolf behaviors, as is a terrier’s aggressiveness, but this hardly means that collies, wolves, and terriers are all the same. Paternal investment may be particularly fragile and easily lost in mammals, because parental investment via internal gestation and lactation is engineered into females but not males.
The authors point out the fact that the 7R and default 4R variants coexist suggests that one doesn’t have an absolute evolutionary advantage over the other; rather, the evolutionary advantage is dependent on the relative frequency of the two, and the two variants will reach an equilibrium distribution (with the exact equilibrium being determined by the physical and cultural environment).
What we see today looks much more likely to be the consequence of some kind of frequency-dependent selection, in which the frequency of the new allele increases up to a certain point and then stabilizes. These selective forces must not be the same in all populations, because the 7R allele is quite common in some populations (South American Indians), exists at intermediate frequencies in others (Europeans and Africans), and is rare to nonexistent in yet others (East Asia, !Kung Bushmen) (2).
The evolutionary payoff of an individual’s behavior in a complex society will depend strongly on the reaction of others to that behavior.
When the advantage of an allele is frequency-dependent, two or more different alleles can persist indefinitely at polymorphic frequencies. In evolutionary game theory, such a stable mix is known as an evolutionary stable strategy or ESS (10).
Thus, there has likely been a recent shift that gave a reproductive advantage to males (who exhibit the greatest effects of having the 7R gene). This shift was probably the rise of agriculture.
Because the prominent phenotypic effects of 7R are in males, we need to ask what is the niche in human societies for males who are energetic, impulsive (i.e., unpredictable), and noncompliant? Whereas tests of hypotheses ought to be careful and conservative, generation of hypotheses ought to be speculative and free-ranging. There is a tradition of caution approaching self-censorship in discussions of human biological diversity, but we will break that tradition in what follows.
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.
I quote the authors’ description of cad and dad societies in full here, as a summary would not do it adequate justice. It is eery how modern American society matches the described cad society paradigm of thug and hipster cads, and single mothers. Contrast with the dad societies, which are allegedly filled not with real nice guys, but with nice guys ™, although the end result is the same – men are beta providers.
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.
Contrary to claims that monogamy is a recent invention, relatively monogamous dad societies are the norm amongst primitive hunter-gatherers. It was only through agriculture that cads were able to thrive. In modern agriculture and industrial states, it is the presence of law enforcement and moral norm enforcement that put the cads at a disadvantage and allow dads to thrive.
Among low density gardeners, on the other hand, the typical pattern is that most of the gardening work is done by women, freeing men from subsistence responsibilities. Boserup (16) calls these “female farming systems,” a euphemism for societies where men live off women. Freed from domestic responsibility, men can occupy their time decorating themselves and planning the next raid. Widespread systems of such societies, as in highland New Guinea or lowland South America, seem to be stable as the chronic raiding and warfare suppress population growth. But powerful polities can break the stability, suppressing local male coalitional violence, leading to population growth, agricultural intensification, and ultimately to males again working at farming to provision their families. Thus there are drab males working at subsistence among foragers and, at the other end of the density continuum, among intensive agriculturalists and peasants. In between we see decorative competitive males engaged in local male coalitional violence. There is an unsettling parallel with the dad males of the working class in contemporary industrial societies and the cad males of the underclass (17).
It is interesting to note that China, perhaps the only place in the world that has had essentially continuous political civilization (no dark ages) for the past few thousand years, has an almost complete absence of the 7R allele.
Our hypothesis suggests that the absence of 7R in East Asia is recent, consequent to the establishment of powerful polities that allowed population growth and forced agricultural intensification. It is of interest in this context that 2R alleles in China are probably derived from 7R alleles by recombination, suggesting that the loss of 7R is indeed recent.
Counterintuitively, it is perhaps the “patriarchal” norms of monogamy, chastity, and fidelity that allow the existence of a dad society in which men and women work together to provide for and raise children in a relatively egalitarian way. By fighting for liberation from “oppressive” sexual norms, feminists are creating a cad society today – one in which the women have to do all the work and in which men must aggressively compete each other through violence and peacocking to successfully mate with women.