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The Rise and Fall of Sexual Promiscuity

Sleeping around once had its benefits, but turns out women may have given up better fertility for better security.

Long before and The Bachelor—even before yentas and oracles—there was the primal jungle, and finding a mate was messy business. Violence and hierarchy ruled the day. Alpha males got the girls; lesser competitors did not. Evolutionary biologists have long tried to trace the human path from combat to courtship: When did we crude animals trade polygamy and paternal absenteeism for “I do” and BabyBjörn?

A new study from Sergey Gavrilets, professor of ecology, biology, and mathematics at the University of Tennessee-Knoxville, reviews the current evidence and offers an intriguing hypothesis. The transition from “promiscuity to pair-bonding,” Gavrilets writes in the journal PNAS, occurred only when lesser male hominids, realizing their physical inferiority, adopted a “provider” role in partnerships, and female hominids, in turn, began to show fidelity to these partners. (Others have postulated that the rise of agriculture helped smooth the way for the transition.) The role of female choice is not often considered in evolutionary biology, but if Gavrilets’ models are correct, it may be integral to explaining our past.

Scientists have also long debated the origins of pair-bonding, not because its advantages are in dispute, but because, according to evolutionary logic, it ought never to have happened.

Many species are indeed better off, Gavrilets explains, when their males live cooperatively in societies, helping raise families, rather than warring over mates and leaving their offspring fatherless. But males face a “social dilemma” when it comes to spreading their genes: if they choose to spend their energy providing for—rather than fighting for—mates, other males may cheat and “free ride” on their largesse. (So-called “free rider” problems show up not just in biology, but in every corner of economics, psychology, and environmental policy.) Of all the theories that explain why cooperation (i.e. pair-bonding) replaced competition (i.e. promiscuity) among our ancestors, Gavrilets argues, none account for this social dilemma.

The study highlights several current theories of pair-bonding: instead of fighting, male hominids began to devote effort to caring for offspring, protecting their mate, or provisioning food for sex. But Gavrilets builds mathematical models to show how each of these hypotheticals leads to a “sub-optimal” outcome—how “investing more in offspring means that there is more paternity for other males to steal.” Run the model, and instead of choosing cooperation, the males will choose to fight. It’s not the outcome any one male desires, but the free-rider problem effectively “traps” the whole group in a benighted state.

Gavrilets proposes a modification to existing theory. What if we assume that males began to provide for one—and only one—female, and females, likewise, began to depend on a sole mate for food and help with childcare? First, not all men are created equal; there are few Ryan Goslings and many Kevin Redmons. The weaker among us quickly learned the futility of direct competition, and turned to “alternative reproductive strategies” to spread our genes. (I may not be a dreamboat, baby, but I’ll bring you coffee in bed.) Second, females make choices. Whereas previous models ascribe females a passive role, Gavrilets asserts that “because they receive direct benefits from provisioning males, females should be choosy, and they may become, to some extent, faithful” to their providers.

Promiscuity is a funny thing in nature. Despite what your mother and youth pastor spent so many years telling you, sleeping around has real (genetic) benefits. Polyandry—in which females take more than one mate—allows for better gene diversity, boots the likelihood of fertilization, decreases infanticide, and means more male providers. Gavrilets acknowledges that in switching from promiscuity to monogamy, females actually risked lower fertility. The tradeoff was security.

When he runs the model again, with these two assumptions in place, the outcome is different: instead of a spiral into violent competition, male provisioning and female faithfulness “co-evolve in a self-reinforcing manner.” Males escape the “social dilemma” and pair-bonding replaces promiscuity.

Critically, Gavrilets notes, this process begins with the weakest males—those who have the worst chances of beating out Ryan Gosling for a mate—because they stand to gain the most from an alternative strategy like provisioning. Slowly, the strategy works its way up the dominance hierarchy, as females begin to reward the weaker males with their fidelity. Out of this sexual revolution comes self-domestication. A new kind of society is born, one in which, “except for a very small proportion of the top-ranked individuals, males invest exclusively in provisioning for females, who have evolved very high fidelity to their mates.”

A few million years hence, here we are—imperfect creatures, a long way from the jungle, somewhat closer to commitment.