Editor’s note: A reader recently asked for our thoughts about the history of marriage across time. This is a topic that Dr. Lorne Campbell tackled in our book, The Science of Relationships: Answers to Your Questions about Dating, Marriage, and Family, so we’ve included an excerpt below.
The answer to this question is not straightforward. Research does suggest that although a lot of people are not monogamous, the majority of people do remain faithful to their partners. Any answer to the question, therefore, must address the conditions that make it more likely for some people to cheat on their partners but others to keep their zippers securely fastened. In fact, monogamy, or the practice of having a single mate during a period of time, seems to be a dying trend in modern American society. In many surveys, around 30% of both men and women in committed long-term relationships report that they have cheated on their partners at least once.1 Perhaps more surprisingly, between 2% and 10% of males are being cuckolded. That’s not nearly as fun or funny as it sounds. Being cuckolded basically means that these men are unknowingly raising children they believe to be their own genetic offspring, but are not.2 On a global scale, infidelity is the most frequently cited reason for divorce across cultures.3
Recent theorizing and research in evolutionary psychology— a field of psychology that believes that (1) our most basic instinct is to pass on our genes, and (2) much of the stuff we do today is a result of what made us best able to pass on our genes in our distant human past— provides one possible answer to whether monogamy is the best strategy to take. According to Robert Trivers’ parental investment theory,4 differences in how much time and effort men and women invest in the production of children can explain the types of mating strategies that men and women are more likely to use. When it comes to procreation, a male’s minimal contribution can take as little as a few minutes and a teaspoon of sperm. Women, however, at the very least must invest more than nine months of gestation and experience the pain and potential medical complications associated with childbirth. What’s more, while a woman is spending nine months carrying her child, the father still has the capacity to share his teaspoon and few quality minutes with other women. Based on these differences in minimal parental investment, Trivers suggested that women should prefer long-term committed relationships and be fairly choosy when selecting mates because of the high costs involved in becoming pregnant (and even higher costs if she raises the child herself ). According to the theory, men, however, should look for multiple mating opportunities and be less choosy when seeking mates (i.e., have lower standards), because mating opportunities usually have fewer costs associated with them.
This excerpt was written by Dr. Lorne Campbell for our book, The Science of Relationships: Answers to Your Questions about Dating, Marriage, and Family. You can get it here.
Dr. Justin Lehmiller also addressed the related question Why Do Some People Date Multiple Partners at the Same Time? here.
1Thompson, A. P. (1983). Extramarital sex: A review of literature. Journal of Sex Research, 19, 1–22.
2Baker, R. R., & Bellis, M. A. (1995). Human sperm competition. London: Chapman and Hall.
3Betzig, L. (1989). Causes of conjugal dissolution: A cross-cultural study. Current Anthropology, 30, 654–676.
4Trivers, R. L. (1972). Parental investment and sexual selection. In B. Campbell (ed.), Sexual selection and the descent of man (pp. 136–179). New York: Aldine de Gruyter.