Alfred Kinsey sent shockwaves though American society in 1948 when he reported that about half of all married men had sex with someone other than their wives during their married life. In 1953, Kinsey reported that about a quarter of women had extramarital sex by their 40s. Although Kinsey’s sampling methods have been questioned, other contemporary studies of marital adjustment indicated similar rates. More recently, the representative General Social Survey conducted by the National Opinion Research Center (NORC) has estimated that 21.5% of men and 12% of women have had extramarital sex, based on surveys in 1991, 1993, 1994, 1996, and 1998. These studies estimate that 5% of men and 2% of women have extramarital sex in a given year. The 1992 National Health and Social Life Survey, also produced by NORC, reported extramarital sexual experience by gender and age: 23% of males and 8% of females aged 50 to 59; 20% of males and 12% of females aged 40 to 49; 16% of males and 8% of females aged 30 to 39; and 10% of males and 6% of females aged 18 to 29.
Two of the most common reasons women report for having extramarital sex are retaliation toward an unfaithful husband and a lack of attention from or general dissatisfaction with a husband. The first reason is more closely associated with brief affairs or one night stands, the second with long-term affairs and an increased risk of divorce. Men are more likely to report opportunistic reasons for having extramarital sex, along with relief of tension and desire for sexual variety. These reports support the notion that men are motivated by a drive for sexual variety, whereas women are motivated by a search for sexual intimacy.
The more general term used for these behaviors in diverse human cultures and nonhuman species is extrapair copulation (EPC). The reproductive benefits of extramarital sex to men are more obvious. Men can enhance their reproductive success by fathering children with multiple women, and any genetically influenced tendency promoting this behavior would spread through the population as long as the costs (e.g., being seriously injured or killed by a woman’s partner or his kin) did not outweigh the benefits in terms of reproductive success. There are also multiple ways in which EPCs can benefit women’s reproductive success: acquisition of material resources; protection against injury or infanticide by the male; high quality genes that enhance offspring survival; “sexy son” genes that would make a son more attractive to females; genetic diversity of offspring as a hedge against changing or unpredictable environments; fertility backup if the woman’s primary partner is sterile; replacing a lost partner; and stimulating mate guarding by the woman’s partner to reduce chances for the man’s infidelity.
Women are more likely to choose men with higher social status and physiological cues of high genetic quality, such as high body symmetry and broad shoulders with narrow hips, for extramarital sex, and are more likely to desire EPCs during the fertile phase of their menstrual cycle. Men are also more likely to have extramarital affairs with women possessing physiological cues of high genetic quality, such as low waist-to-hip ratios.
References:
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- Ellis, B. J., & Symons, D. (1990). Sex differences in sexual fantasy: An evolutionary psychological Journal of Sex Research, 27, 490–521.
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- Gangstead, W., Thornhill, R., & Garver, C. E. (2001). Changes in women’s sexual interests and their partners’ mate-retention tactics across the menstrual cycle: Evidence for shifting conflicts of interest. Proceedings of the Royal Society of London, B, 269, 975–082.
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- Kinsey, C., Pomeroy, W. B., & Martin, C. E. (1948/1998). Sexual behavior in the human male. Philadelphia: W. B. Saunders; Bloomington: Indiana University Press.
- Kinsey, C., Pomeroy, W. B., Martin, C. E., & Gebhard, P. H. (1953/1998). Sexual behavior in the human female. Philadelphia: W. B. Saunders; Bloomington: Indiana University Press.
- Smith, L. (1984). Human sperm competition. In R. L. Smith (Ed.), Sperm competition and the evolution of animal mating systems (pp. 601–659). New York: Academic Press.