There is an astonishing amount of variation in sexual behavior around the globe and over time. Many of these differences can be explained as adaptive responses to the environment – a possibility that is largely overlooked.
Most of us are well aware of how much change there has been in the past century. For instance, British common law used to criminalize premarital sex that is now normative. Women have also gone from being legally considered possessions of their husbands to having nearly equal rights with men, consistent with their increased economic role in developed countries.
Human sexual variation around the globe is affected by a broad range of environmental factors ranging from subsistence ecology and marriage markets, to economic development and female labor force participation, which brings female and male sexual psychology much closer.
Such variation is more adaptive than it might seem. The local ecology affects polygamy for humans in much the same way as it affects polygynous mating for other vertebrates (1).
Interest in casual sex that varies so widely from the prudish Middle East to the more relaxed approach of developed countries is another good example of ecological influences at work (2).
The relevance of ecology is strongly suggested by the fact that polygamous societies cluster close to the equator (3). This reflects a combination of food availability and prevalence of tropical diseases. At low latitudes, food plants produce plentifully throughout the year. In contrast, high latitudes have long cold winters when it is a huge energetic challenge to stay warm and well nourished.
Such challenging conditions require the intense cooperation of two parents to raise small children. High latitudes have a low risk of infectious diseases that are more common in the tropics where there are more vectors such as mosquitoes, tsetse flies, and waterborne parasites.
Where there is a heavy disease load, birds adopt polygynous mating systems and large, healthy, brightly-colored males sire a large share of the offspring.
In human societies having a heavy disease load, women are more swayed by physical attractiveness in their choice of a mate and are more interested in extramarital relationships, suggesting that they seek disease-resistant males to father their offspring (4).
Ecological factors such as climate and disease prevalence influence human marriage and sexuality. The supply of males relative to females (the mate market) is another feature that varies with location and it affects the level of sexual freedom in a society (5).
The Mate Market
In the Middle East, sexual behavior is still very restricted. In countries like Iran or Saudi Arabia, young people are harassed by moral police who want to clamp down on romantic touching, or kissing, in public. Police also prevent young women from wearing clothes that are considered revealing or provocative.
In such so-called societies of honor, premarital sexuality can have tragic consequences. Even the suspicion that single people are sexually active is enough to precipitate their slaughter by irate family members. So single Saudi Women are prohibited from going out alone without a chaperone.
Even there, women acquire greater freedom as they become more educated and take a greater role in the economy. So by next year, they will be allowed to drive legally for the first time.
A less restrictive approach to sexuality obtains in developing countries of sub-Saharan Africa, and this is highlighted by much higher levels of extramarital sexuality that elevates the incidence of sexually-transmitted diseases.
What determines such wide variation in sexual restrictiveness? One important influence is the proportion of females relative to males. In the Middle East, there is an extreme scarcity of females whereas adult women outnumber men in sub-Saharan Africa.
How the Mate Market Affects Sexuality
If young women are scarce, their optimal strategy is to be highly selective and marry attractive men who are kind and have high social status – these being the main attributes that determine male mate value.
Men in search of brides look for sexual fidelity, the primary evidence for which is current sexual restraint. That is why chastity is so important to a woman's marriageability in societies of honor.
Economic development is another major influence in liberating female sexual behavior. Developed economies provide unprecedented economic opportunities for women and they exploit these opportunities through higher education.
As more women complete college degrees and enter careers, marriage and childbearing get postponed. During the long period between sexual maturity and marriage, the majority of women become sexually active and use contraception to prevent unwanted pregnancies.
The fact that so many single women are competing for masculine attention means that men are less interested in marriage because single men can enjoy a satisfactory sex life without long-term commitment.
There is a corresponding shift in sensibility as women become more competitive, risk-taking, and hedonistic. Their sexual psychology is converging with men with greater openness to short-term relationships, voluntary non-marriage, and choice of non-traditional living arrangements.
Whether it is the impact of latitude and climate, endemic diseases, the mate market, or economic development, societal variation in human sexuality is surprisingly predictable as a response to environmental differences.
1 Barber, N. (2008a). Explaining cross-national differences in polygyny intensity: Resource-defense, sex ratio, and infectious diseases. Cross-Cultural Research, 42, 103-117.
2 Barber, N. (2008b). Cross-national variation in the motivation for uncommitted sex: The impact of disease and social risks. Evolutionary Psychology, 6, 217-228.
3 Marlowe, F. W. (2007). Hunting and gathering: The human sexual division of foraging labor. Cross-Cultural Research, 41, 170-195.
4 Gangestad, S. W., & Buss, D. M. (1993). Pathogen prevalence and human mate preferences. Ethology and Sociobiology, 14, 89-96.
5 Guttentag, M., & Secord, P. F. (1983). Too many women: The sex ratio question. Beverly Hills, CA: Sage.