Houston, we have a monogamy problem. And it’s not just Kristen Stewart and Tiger Woods. Countless extramarital dating websites have cropped up (Ashley Madison, anyone?), and 19 percent of women and 23 percent of men admit to having cheated on their partners, according to a recent Archives of Sexual Behavior study. Roughly half of all marriages in the United States end in divorce, and many couples who do stay together hit dry spells — 12 percent of married individuals said they hadn’t gotten it on for at least three months before responding to the Austin Institute’s Relationships in America Survey.
There may be a scientific reason behind these bleak statistics: Monogamy might simply go against our nature. Social monogamy is the exclusive pairing of males and females who live and mate with each other, raising their offspring as a unit. But it’s extremely rare among mammals. According to a recent Science paper:
of mammal species are classified as monogamous
What’s more, a mere 17 percent of today’s human societies are strictly monogamous. The majority practice a mix of relationship types, such as polygyny — one man having sex with multiple women. (But most couples live monogamously, since some of the cultures counted have small populations.)
Monogamy has long puzzled biologists. According to evolutionary theory, males compete for females so that they can spread their genes more widely; mating with multiple partners makes this much easier. And although females may choose to live with a mate who is a good provider, why not seek other males in the meantime who have “better” genes that will produce better offspring?
“The name of the game is evolution,” said Cornell University animal behavior scientist Danielle Lee. “You have to survive, and you have to make babies. If you can get out and make more babies because you know your partner can handle raising your young, it’s in your best interest, evolutionarily speaking, to do so.”
We can trace our promiscuous lineage to other animal behaviors too.
As animals ourselves, let’s examine other members of our family tree for clues. It turns out that scientists have detected cracks in the monogamous images of many species. Genetic analyses have revealed that songbird couples — once considered extremely loyal — stay together for only a season. And when University of Memphis and University of Florida researchers released several single prairie voles into an enclosure, some formed pairs, some stayed single “and a lot of animals, relatively speaking, were cheating on each other” — about 25 percent, said Alexander Ophir, now a research faculty member at Cornell University’s psychology department.
We can trace our promiscuous lineage to other animal behaviors too, like how males that hold resource-rich territory attract more females. (Does the name Hugh Hefner ring a bell?) Or how many species defend their partners from others who want to mate with them. In 2009, Ophir and his team reported that philandering prairie voles were more likely to have partners that also strayed, no longer under their watchful eye. “We’re not as different as we sometimes wish we were,” Lee said.
But why would we evolve to be monogamous? Dieter Lukas, an evolutionary biologist at the University of Cambridge, concluded in 2013 that monogamy evolves when females spread out to establish their own territory, perhaps to secure food for themselves and their offspring. This makes it hard for males to wander and to fend off competing males. But a team led by Kit Opie, a biological anthropologist at University College London, inferred that males evolved to stay with one female to protect other males from killing her young.
Scientists aren’t entirely sure when humans first began practicing monogamy once they split off from the ancestors of chimpanzees 7 million years ago. Opie notes that hunter-gatherer populations — the way humans lived for most of their evolutionary history — are predominantly monogamous. Then the advent of agriculture in the last 10,000 years allowed resource-rich farmers to have multiple wives. Yet most developed nations today are monogamous, and monogamy makes it easier for breadwinners to pass on their wealth to the next generation.
“It’s difficult to say why in humans monogamy evolved,” Lukas said. “I don’t think there is a conclusive answer.” Case in point: In November 2014, Lukas and his colleagues published a study in Science that challenged Opie’s findings, concluding that male infanticide had little influence on the evolution of monogamy in mammals.
The fossil record has only further muddied matters. For example, male members of species that practice polygyny engage in fierce competition for females, with the biggest male often emerging as the victor. The males of those species tend to be much taller than the females. But in monogamous species, males and females are about the same height. Human skeletal remains fall somewhere in between, suggesting that humans didn’t evolve to be strictly monogamous.
So instead of wagging our fingers at people who stray from their partners, let’s first consider whether we’re really hardwired for monogamy. Maybe it’s simply a cultural ideal — a lofty, unattainable one at that.
Why you should care
Because there could be a scientific reason for the high divorce rate.