Part 2. Humans are (Blank) -ogamous: Promiscuity

This is the second part on the evolution of human mating behavior, comparing evidence for promiscuity and pair-bonding in our species. Please see the Introduction here.

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“I’ll be frank. True monogamy is rare. So rare that it is one of the most deviant behaviors in biology.” (Olivia Judson 2002: 153)

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In their best-selling book, Sex at Dawn, Chris Ryan and Cacilda Jethá suggest that there is a good amount of direct and circumstantial evidence that extended monogamy does not come easily for humans, and that this derives at least in part from our fairly promiscuous evolutionary history. (To clarify, they use the term ‘promiscuous’ not in a judgmental way, but merely to convey having multiple sex partners). Their main premise is that rigid monogamy became common only after our ancestors made the shift from hunting and gathering to agriculture. With agriculture came an emphasis on fixed settlements, private property that could be inherited, genetic paternity, and female sexual fidelity. They argue that this stands in contrast to our hunting-gathering past, when sexual relationships were more open and not confined to an exclusive pair-bond.

Bonobo sex. From Lola ya Bonobo

 

As a corollary, they note that agriculture is a very recent event, adopted at different times in different places (or not at all) only within the last 10,000 years. For perspective, our species has been around for 200,000 years (McDougall et al, 2005), and our genus (Homo) for about 2.5 million years. The implication is that, although culture and environment certainly impact behavior, old behaviors may carry over into the present. They are quick to note that they do not negate the importance of romantic love in our prehistory. Nor are they arguing for biological determinism. They are merely saying that we have overlooked the fact that we have a long history as hypersexual animals, and that this is salient to modern relationships.

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Certain signposts in our biology and behavior point in this direction. These include, among other things: “our extravagant sexual capacity, ubiquitous adultery in allcultures, rampant promiscuity in both our closest primate relatives [chimpanzees and bonobos], (and) the absence of any monogamous primate living in large social groups” (Ryan and Jethá, p. 142). The purpose here is to explore some of these points, as well as others made in their book and by various researchers, that suggest a human proclivity for promiscuity. It is impossible to recap the entire literature on this in a single blogpost, but below are a few points…

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Traits suggesting promiscuity

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1. The absence of any monogamous primate living in social groups. First,Fuentes (1998) noted that there is a ‘semantic haze’ around ethological definitions of monogamy. Helpfully, Reichard (2003) proposed distinguishing between social monogamy (close physical proximity of a pair), sexual monogamy (exclusive mating), and genetic monogamy (sex occurs outside the primary pair-bond, but not reproduction). With this in mind, Fuentes stated that we won’t have a full understanding of how common sexual/ genetic monogamy is in primates until genetic testing becomes more common. He concluded that only about 3% of the 200-plus primate species were monogamous (pair-bonded adults living exclusively in two-adult groups), which is the same rate found in mammals generally. These seven species included two genera of New World monkeys (Callicebus, Aotus) and two lemur species.

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Humans, of course, are highly social and do not live in exclusive two-adult groups. To Fuentes, “there is strong evidence that the average group in humans is multi-male/multi-female and that group size remained at or below the 90-220 individual range until the upper Paleolithic” (p. 897). Ryan and Jethá have a point here: if monogamy is the default human mating behavior, then we need to account for why we are bucking such a strong primate (indeed, animal) trend.

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Furthermore, Reichard also pointed out that monogamy in other species varies in terms of duration, and lifelong mating appears to be exceedingly rare (one of the best examples may be in schistosomes).¹ Even among monogamous gibbons, mating does not last for life, and in one study about 12% of copulations occurred outside the primary pair-bond (Fuentes, p. 896).

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2. Promiscuity in chimpanzees and bonobos. It is widely known that chimpanzees (Pan troglodytes) and bonobos (Pan paniscus) are our closest relatives. However, from these species’ point of view, humans – not gorillas or orangutans – are their closest relatives too. Together, the three species lie on a branch on the evolutionary family tree referred to as the tribe Hominini  (see Wood 2010 for a review). Chimpanzees and bonobos both live in multi-male/multi-female groups and are highly promiscuous, though there are substantive differences in sexual behavior between the two species. Chimpanzees generally confine sex to periods when females are in estrus. By contrast, bonobos have sex at any point of the ovulation cycle, in various positions and partner combinations (including same sex), and as young as infancy (Woods and Hare 2011).

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This is relevant, since “only two species can do it week in and week out for nonreproductive reasons: one human, the other very humanlike (bonobos)” (Ryan and Jethá, p. 85). That’s probably not a coincidence. For bonobos, sex is frequent and has a social function, particularly in relieving tensions and as a bonding mechanism. In fact, Sue Savage-Rumbaugh once said that bonobos are rarely found in zoos because their sexual behavior is too human-like. Maybe. As Ryan and Jethá suggest, it is tantalizing to think that since promiscuity is found in both of our closest cousin species, and seems to be the case for humans as well, that it could have been the way our last common ancestor behaved.

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Primate family tree. The most recent common ancestor of humans, chimps, bonobos dates to about 6 million y.a., circled in red (click to enlarge).

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3. Our extravagant sexual capacity. Ryan and Jethá highlight the rather basic observation that human sexual activity goes well beyond what is needed for reproduction, with up to thousands of copulations per child born, and surpasses that of most other primates. Eric M. Johnson put it this way: “there’s little denying that the human animal is one sexy beast.” They propose that, as in bonobos, sex in humans took on its current form not only in the pursuit of reproduction or physical pleasure confined to a pair-bond, but as an important means of social bonding within a tightly knit band of hunter-gatherers.

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My take is that a few milestones in evolution make this scenario plausible. (1) The origins of sex are unquestionably rooted in reproduction, likely as a means of creating variation in offspring. (2) At some point sex became pleasurable for animals, providing an extra incentive to pursue it, with fertilization then becoming a byproduct. An analogy I once heard – I forget where – is that this is akin to a farmer dangling a carrot (pleasure) in front of a mule to get it to work (reproduction). (3) For humans and bonobos, reproduction and sexual pleasure became sufficiently decoupled to the point where pleasure/ bonding became the goal and reproduction often took a back seat (mostly for the carrot, not the work). If that isn’t obvious, consider the many forms of sex where procreation is impossible (you’ll have to use your imagination here; my mom might read this). In fact, humans often go well out of their way to prevent sex from resulting in pregnancy.

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There might even be a 4th milestone, where sex became even farther removed from reproduction and entered some level of semi-rational conscious choice. Research suggests that people have sex for a variety of reasons other than reproduction. Apparently, there are 237 of them, roughly falling into four categories and thirteen subcategories (Meston and Buss, 2007). These include: Physical (stress reduction, pleasure, desirability, experience seeking), Goal Attainment (resources, social status, revenge, utilitarian), Emotional (love/ commitment and expression), and Insecurity (self-esteem boost, duty/pressure, mate guarding). Who knew?

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(As a side note, this post probably comes across as heteronormative, but I think findings such as those above, showing the many reasons people have sex other than for reproduction are obviously equally applicable to homosexual behavior. This is yet another reason that socially conservative arguments against homosexuality are hollow).

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4. Ubiquitous adultery in all cultures. Whatever we call it – adultery, infidelity, cheating – people have sex outside of marriage, and not at a trivial rate. Quoting Buss and Shackelford (1997: 194) “empirical estimates of affairs over the course of a marriage range from 30 to 60% for men and from 20 to 50% for women… Estimates of infidelity over the course of a single year … yield lower estimates such as 5%.” I’m skeptical of the higher estimates, but even if the lower end  is accurate these rates are substantial (and who knows how accurate they are since people are unlikely to reveal such intensely private information, even on anonymous surveys).

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What do we make of this? Buss and Shackelford found that traits correlating with infidelity include sexual dissatisfaction in the marriage, but also narcissism and low conscientiousness. Ryan and Jethá, however, feel that sexual novelty itself is a major reason people stray, in keeping with an ancestral pattern of promiscuity (p. 295). In some cultures this is permissible, or even encouraged and ritualized, while in others it is forbidden.

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5.   The Ethnographic Record. As commonly mentioned in most introductory cultural anthropology classes, monogamy (or serial monogamy) is the most common form of marriage for individuals in the world, but most cultures permit some degree of polygamy. Murdock’s (1967) Ethnographic Atlas categorized just 16% of 862 cultures as exclusively monogamous, with polygamy being found at some level in the rest. An updated version of the Atlas found a similar rate of about 15% (see Table 9 in the link). Culturally sanctioned marriage obviously does not represent the full range of sexual and mating behavior in any society (you probably knew that already), but it is shorthand for what practices are permissible.

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Looking for patterns among different types of human societies (i.e., hunter-gatherers, horticulturalists, pastoralists, agriculturalists), Marlowe (2000) noted that hunter-gatherer societies almost always allowed polygyny, and that “all else equal, the greater the level of paternal investment (in offspring), the more monogamous the mating system” (p.58). He also found that female extra-marital sexual relationships were fairly common in all types of societies except for agriculturalists, and that female affairs were negatively correlated with levels of male aggression in a given society.

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This helps corroborate Ryan and Jetha’s argument that concern with paternity and female sexual fidelity accompanied agriculture. They go further into the ethnographic literature, looking at populations like the matrilineal Mosuo in southern China, where adults of both sexes retain sexual autonomy throughout adulthood, and jealousy is culturally frowned upon. Other societies include the ‘partible paternity’ Native cultures in South America, where it is believed that many men can simultaneously be the father of a single child. The ethnographic literature is too vast to do it any justice here. However, culture truly is a powerful force.

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6. Multiple partners across cultures. At best, the closest humans come to monogamy would probably be serial monogamy. A 2011 study from the U.S. Centers for Disease Control found that just 23% of women and 14.7% of men aged 25-44 had one (or zero) opposite-sex lifetime partners (Chandra et al 2011). At the other end of the distribution, 30.5% of women and 49.4% of men reported having seven or more lifetime partners. A British study found a roughly comparable pattern (Johnson et al 2001). Similarly, the Kinsey Institute looked at 2,018 heterosexual, partnered adults (median age 50+) from the U.S., Germany, Japan, Brazil and Spain, and found that the mean number of lifetime sexual partners was 11.9 for men and 4.9 for women (Heiman et al 2011). However, there was lots of variation around the average for both sexes, ranging from 1 to 83 partners. Surveys from Zimbabwe, Uganda and South Africa found, consistently, that men aged 35+ years had an average of 6 or more lifetime sexual partners, while women averaged 1.3 to 3.6 partners (Todd et al 2009).

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Lifetime sex partners for U.S. men and women, aged 25 – 44 (data from Chandra et al.)

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The above studies have a few limitations: they did not specify whether sex partners were concurrent or sequential, casual or committed, and while there is some cultural variation in the samples, it is obviously not an exhaustive list. Nonetheless, exclusive lifetime mating does not seem to be the norm.

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Some have wondered whether the large gender gap between the number of reported partners is legitimate, or whether men and women adjust their survey answers to conform to cultural expectations (Wiederman 1997). According to classic parental investment theory, female mammals should be more discriminating in the number of partners because they have more to invest in pregnancy, gestation, and care of offspring (Trivers 1972). However, this is not the case at all times in all species. In fact, while some have argued that serial monogamy benefits male reproduction as a form of sequential polygyny, Monique Borgerhoff Mulder (2009) found that in the Pimbwe, a horticultural population in Tanzania, females were the main beneficiaries from serial monogamy in terms of the number of surviving children.

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Furthermore, parental investment theory implies that sex and reproduction are synonymous, but in humans the two obviously do not overlap completely, as seen above. In one study of mostly white American college students (admittedly, a limited sample), the mean number of desired sex partners over the next year, given no health or social risks, or opportunity limitations, was 12.9 for men and 4.9 for women (Fenigstein and Preston, 2007). When all risks were taken into account, the means were much closer, but men still desired more partners (2.4 vs. 1.5). However, the data also showed that a substantial percentage of both sexes desired more than one partner in the coming year (47% of women; 76% of men). Why? Who knows. It could be any number of 237 reasons.

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We’re scratching the surface here, but this post is already too long. Overall, I think Ryan and Jethá and others put together a strong case that promiscuity is built into us to some extent, though this is not the entire story. More to come…

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Footnote:

1.) In her wonderfully quirky book, Olivia Judson lists other examples of species that fit the definition of true genetic monogamy, including jackdaws, chinstrap penguins, Kirk’s dik-dik, the California mouse, and some termites (2002: 153).

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References:

Borgerhoff Mulder M. 2009. Serial monogamy as polygyny or polyandry? Marriage in the Tanzanian Pimbwe. Human Nature 20:130–150. (Link)

Buss DM, Shackelford T K. 1997. Susceptibility to infidelity in the first year of marriage. Journal of Research in Personality 31: 193-221. (Link)

Chandra A, Mosher WD, Copen C, Sionean C. 2011. Sexual behavior, sexual attraction, and sexual identity in the United States: data from the 2006-2008 National Survey of Family Growth. National Health Statistics Report 3:1-36. (Link)

Fenigstein A, Preston M. 2007. The desired number of sexual partners as a function of gender, sexual risks, and the meaning of “ideal.” Journal of Sex Research. 44(1):89-95.

Fuentes A. 1998. Re-evaluating primate monogamy. American Anthropologist.100 (4): 890–907.

Heiman JR, Long JS, Smith SN, Fisher WA, Sand MS, Rosen RC. 2011. Sexual satisfaction and relationship happiness in midlife and older couples in five countries. Archives of Sexual Behavior 40: 741-53. (Link)

Johnson AM, Mercer CH, Erens B, Copas AJ, et al. 2001. Sexual behaviour in Britain: partnerships, practices, and HIV risk behaviours. Lancet. 358(9296):1835-42. (Link)

Judson O. 2002. Dr. Tatiana’s Sex Advice to All Creation: The Definitive Guide to the Evolutionary Biology of Sex. Metropolitan Books. (Link)

Marlowe  F. 2000. Paternal investment and the human mating system. Behavioural Processes 51: 45–61. (Link)

McDougall I, Brown FH, Fleagle JG. 2005. Stratigraphic placement and age of modern humans from Kibish, Ethiopia Nature 433, 733-6. (Link)

Meston CM, Buss DM. 2007. Why humans have sex. Archives of Sexual Behavior 36:477-507. (Link)

Murdock GP. 1967. Ethnographic Atlas. University of Pittsburgh Press.

Reichard UH. 2003. Monogamy: past and present. In UH Reichard and C Boesch (eds): Monogamy: Mating Strategies and Partnerships in Birds, Humans and Other Mammals.  Pp. 3-25. Cambridge Univ Press. (Link)

Ryan C, Jethá C. 2010. Sex at Dawn: The Prehistoric Origins of Modern Sexuality. Harper. (Link)

Todd J, Cremin I, McGrath N, Bwanika JB, et al. 2009. Reported number of sexual partners: comparison of data from four African longitudinal studies. Sexually Transmitted Infections 85 Suppl 1:i72-80. (Link)

Trivers R. 1972. Parental investment and sexual selection. In B Campbell (ed): Sexual Selection and the Descent of Man 1871–1971. Pp. 139–179. New York: Aldine.

Wiederman MW.1997. The truth must be in here somewhere: examining the gender discrepancy in self-reported lifetime number of sex partners. The Journal of Sex Research 34: 375-86. (Link)

Wood B. 2010.  Reconstructing human evolution: Achievements, challenges, and opportunities. PNAS 107 (Suppl 2) 8902-9. (Link)

Woods V, Hare B. 2011. Bonobo but not chimpanzee infants use socio-sexual contact with peers. Primates 52: 111-6. (Link)

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