Having had several late night conversations about the ‘rightness / wrongness’ of having multiple partners at once (I mean polyamory not threesomes), I decided to look into the research. There is very little research on polyamory (being in multiple consensual intimate relationships) per se but much more on sexual non-monogamy so I’ll use that as a proxy and adjust appropriately e.g. ignoring the effect on offspring. Most of the research is pretty poor quality / biased but I’ll try my best to highlight major flaws.
Firstly, some definitions (thank you Wikipedia):
- Polyamory: the practice of, or desire for, intimate relationships involving more than two people, with the knowledge and consent of everyone involved
- Polygamy: marriage with more than one spouse
- Polygyny: practice where the man has more than one wife
- Polyandry: practice where the woman has more than one husband
- Monogamy: Social monogamy refers to two partners living together, having sex with each other, and cooperating in acquiring basic resources such as shelter, food, and money. Sexual monogamy refers to two partners remaining sexually exclusive with each other and having no outside sex partners. Genetic monogamy refers to sexually monogamous relationships with genetic evidence of paternity. Serial monogamy refers to having a series of monogamous relationships one after the other.
[All can be appropriately adapted to ‘pair-bonding’ for cross-species comparisons.]
On average, humans seem to have moderate amounts of sexual non-monogamy:
- Historic breeding ratios: geneticists have measured the ratio of population recombination rates between the X chromosome and the autosomes. Since the X chromosome recombines only in female meiosis but autosomes recombine in both sexes, this allows the calculation of historical human female-to-male breeding ratios. Estimates vary from 1.4 : 1 to 1.1 : 1, consistent with a high prevalence of monogamy and low pravalence of polygyny.
- Testicular size: in species where the females are promiscuous, the males compete for their sperm to ‘win the race’ inside the female. This means the guys have got to have a lot of sperm and larger testicles to carry it! Comparing testicle size relative to body size between primates gives an insight into how human mating patterns compare. Female chimpanzees are pretty promiscuous and the males have relatively large testicles whilst female gorillas are the kept in the harem of a single male and males have relatively small testicles. Humans are somewhere in between but closer to the gorillas, suggesting that human females are relatively monogamous.
- Sexual dimorphism: males in polygynous species tend to be substantially larger than female (1.5–2.0 x). This is because there is much more intra-sexual competition among the males for the females and there is a selection advantage to being a big male. Men are indeed on average bigger than women but only slightly.
Although ‘on average’ is almost always problematic. There does appear to be significant variation:
- Genetic: estimates of paternity being different from the social father range from 0.03–30%.
- Surveys: Estimates range from 47–53% of men and 18–36% of women engaging in extramarital sex in Nigeria to 20–25% of men and 10–15% of women in America. Although the sampling methods are often poor, and social desirability is likely to inflate or deflate such estimates substantially.
- Ethnographic surveys: suggest a much higher proportion of polygynous societies: of 1,231 societies from around the world noted, 186 were monogamous; 453 had occasional polygyny; 588 had more frequent polygyny; and 4 had polyandry
- Two strategies: recent survey evidence from the UK indicated that both men and women split into two strategies: monogamous / more promiscuous. (Again this is survey evidence so should be taken with a pinch of salt but see the graphs for the bimodal distribution.)
It’s worth keeping in mind that even socially monogamous societies have relatively high rates of divorce, allowing serial monogamy, and prior to relatively easy divorce high rates of widowhood.
It’s also worth keeping in mind observed correlations between polygynous arrangements and violence. Since the ‘big men’ get nearly the girls, there is left over a proportion of bachelors fighting for very few girls. The most convincing causal evidence for this is the increase in crime rates following changing sex ratios in China due to the one child policy. The different timing of the implementation of the policy in different provinces provides an exogenous variable to evaluate the direction of causality.
Conclusion: considerable variation characterised perhaps by different strategies and moderate amounts of sexual non-monogamy in socially monogamous societies.