Polyamory, Sexual Jealousy, and Violence 

Maybe you’ve wondered when (and why) “throuple” entered our lexicon. Perhaps you’ve stumbled upon the bestseller, More: A Memoir of Open Marriage, or Peacock’s new reality show, “Couple to Throuple,” or the polyamory coverage in The New York Times, on BBC, or in The Wall Street Journal.

The topic of polyamory suddenly seems to be everywhere. And the practice may be more widespread than most realize: A majority of U.S. adults under 30 told Pew last year that open marriages are acceptable, while a 2023 YouGov poll found that roughly 1 in 8 Americans say they have engaged in sexual activity outside their relationship with their partner’s permission. 

Advocates of polyamory argue that monogamy is not natural but instead frustrates and restricts our pleasures and freedoms. That view has some basis in evolutionary psychology, but it doesn’t tell the whole human story. Evolution has left us conflicted, filled with competing desires for promiscuity but also fidelity.

A fulsome reading of the evolutionary, historic, and societal record suggests dark consequences —starting with more domestic violence—were polyamory to transition from a swinging ’20s fad to societal norm.

In an evolutionary framework, promiscuity is easy to explain. An evolving male who impregnated many females would reap significant benefits for extending their genetic line. But what about monogamy? It’s an unusual arrangement among mammals: Only about 3-5% are monogamous. So how did humans develop the capacity for monogamy?

Becoming bipedal five million years ago freed our ancestors to construct and manipulate tools with their hands. This increased the evolutionary importance of intelligence and adaptability because it allowed humans to use these tools in increasingly creative ways, enhancing their survival chances. But it required brains too big to fit through a biped’s birth canal. Nature’s solution? Brains which kept developing and growing after birth, allowing humans to produce remarkably intelligent and adaptable offspring.

This created another problem: Human infants require constant care and attention, leaving the mother insufficient time and energy to gather food or fend off predators. She needed help. Fathers had the most at stake in the child’s survival and so invested more time and energy in their children. This helped set the basis for long-term monogamy. Relatedly, fidelity became increasingly important because it was vital that a man had confidence that the child he was helping to raise was his. Marriage and monogamy increased a man’s confidence in his paternity. 

Hence, in a surprising way, both promiscuity and fidelity have deep evolutionary roots. Yale sociologist Nicholas Christakis has concluded that humans have fundamentally opposing desires in this respect: 

to possess one’s partner [fidelity] and to have multiple partners [promiscuity]. It’s hard to square those seemingly contradictory impulses the same group, and there really are only two options…. Societies cannot satisfy both, institutionally speaking.1

The long arc of human society toward monogamy through our recent evolutionary and cultural development has had substantial payoffs, including declines in rape, murder, robbery, assault, and abuse. One important way these social benefits have come about is through a reduction in jealousy and sexually-driven violence. 

The psychology professor David Buss, who wrote a book on sexual jealousy, has called it “possibly the most destructive emotion housed in the human brain.” As Buss and others have shown, sexual jealousy is the leading cause of murder between romantic partners worldwide. It’s also a top source of one-on-one homicide between “‘mate poachers’—interlopers who attempt to lure away our partners.”

Based on the way nature has shaped our psychology, it’s a near inevitability that infidelity triggers jealousy and anger in humans. Even imagining a romantic partner in another’s arms induces psychological distress, increased heart rate, muscular tension, and sweating, as experiments have demonstrated. Chimpanzees present a cautionary tale. While polyamory proponents may be quick to point out that chimps have indiscriminate sex, they might not mention the fact that 100% of female chimpanzees experience the equivalent of domestic violence. These facts are not unrelated. In one study of chimp violence, “most of the aggression … seemed to be directed toward females to prevent them from mating with other males.” Dr. Martin Muller, who conducted the study, added: “Males are basically trying to force females into exclusive mating relationships.” 

Normalizing polyamory for humans would predictably produce more domestic violence and even homicide. And, as with past cultural changes, those at the lowest rungs of the socioeconomic ladder would bear the brunt of the negative consequences. 

Human nature is a bit like chess. There are many ways it can play out, but there are hard and fast rules: Rooks can never move diagonally. So it is with human culture. It can vary widely, but there are absolutes. One rule seems to be that sexual promiscuity is almost inevitably linked with intense jealousy—and even violence. 

Dr. Samuel T. Wilkinson is Associate Professor of Psychiatry at the Yale School of Medicine and Associate Director of the Yale Depression Research Program. 

Editor's Note: This essay is adapted from Dr. Wilkinson’s new book, Purpose: What Evolution and Human Nature Imply About the Meaning of Our Existence. 

1. Quote from page 163.