Where does jealousy come from?

Imagine that a close friend came to you one day, weeping inconsolably. "What's wrong?" you ask. Your friend then drops a bombshell: "I just found out - Alex is cheating on me!"

How would you respond to this? One way you probably wouldn't respond is to say: "And that's upsetting… why?" Most of us "get" jealousy. Even before we experience it ourselves, we intuitively understand that most people would be deeply upset if a boyfriend, girlfriend, husband, or wife cheated on them.

To an alien scientist, however, poorly versed in the ways of human beings, jealousy would be something of a mystery. We don't usually mind our friends having other friends. Why, then, are most of us so worried about our sexual partners having other sexual partners? We don't usually mind our sexual partners having a nice meal without us. Why, then, are most of us so worried about our sexual partners having a nice sexual encounter without us? Things would function so much more smoothly if we didn't worry. So, why don't we just do that?

Bamboozled by our bizarre behaviour, the first thing the alien scientist would want to know is where this emotion called jealousy came from. Is it built into human nature, like the capacity for fear or hunger? Or is it an invention of culture, like money or a seven-day week? This is a question that many human scientists have asked themselves as well - and which we've recently made progress in answering.

Is jealousy a social construct?

On one side of the debate, are those who claim that jealousy is an invention of culture. Defenders of this view argue that, in many cultures, jealousy is just as much a stranger as it would be to our alien scientist. Among the Inuit, for instance, tribal chiefs sometimes offer their male guests one of their wives for the night. This shows, so the argument goes, that sexual exclusivity is a Western fetish and jealousy a Western neurosis.

Indeed, even in the West, some people refuse to tow the jealousy line. To take just one example, the actress Shirley MacLaine once said "I've never really had sexual jealousy." This is hard to square with the notion that jealousy is "in the genes." Or so some argue.

But are they right? No doubt, jealousy is shaped in part by learning and culture. But did culture really create jealousy out of nothing? Could we really just as easily learn to be overjoyed about our partners cheating on us as to have our hearts torn in two? And are there really cultures where people are indifferent to their partner's "extracurricular" sexual activities?

According to evolutionary psychologists, the answer to all these questions is no. Jealousy is part of our nature, found in people all over the world. Claims to the contrary, argue the evolutionary psychologists, tend to crumble on closer inspection.

Take Inuit wife-sharing. At first glance, this does look like a counterexample to the idea that sexual possessiveness is a human universal - but only if we assume that sharing one's wife is no big deal to the Inuit. That's not doing the custom justice, though; the whole point is that it's a hugely generous gesture. And it's generous because the Inuit, like human beings everywhere, are possessive of their spouses and lovers. How do we know? Because among the Inuit, male sexual jealousy is a common cause of spousal violence. Ditto other societies supposedly devoid of jealousy.

Certainly, there are individual exceptions. But they're few and far between. That's why, when Shirley MacLaine announces that she's never really experienced jealousy, it makes headlines around the world. Most people have experienced it. Like it or not, jealousy is a constant companion of love: an uninvited guest we can never quite banish, try though some of us might.

Born this way

But if that's true, why? Why would natural selection burden us with such a disruptive emotion? The answer from evolutionary psychology is that jealousy evolved to motivate "mate guarding," and that mate guarding is a solution to an ancient adaptive problem: infidelity.

Infidelity is not particularly common in our species - but it's not particularly rare either. And what's true of humans is true as well of many other animals. In the 1986 film Heartburn, Meryl Streep's character complains to her father that her new husband is having an affair. The father responds, rather heartlessly, "You want monogamy? Marry a swan." But around the same time, scientists discovered that swans - and indeed most pair-bonding species - are just as prone as humans to the occasional "extramarital" dalliance. And for humans and nonhumans alike, such dalliances are a serious threat to the partner's evolutionary success.

The reasons for this, however, differ for males and females. For males, the key issue is paternity. In species with internal fertilization, females are always more likely than males to end up caring for their own offspring, rather than someone else's. If a baby comes out of your body, that's a pretty good clue that it's yours. As far as we know, no woman in the history of the world has ever given birth and thought: "Wait a minute! How do I know that this baby is mine and not some other woman's?" In contrast, if a baby comes out of the body of a woman you slept with nine months ago, that's not nearly as reliable a clue. A man who cares for his partner's offspring is probably caring for his own - but there's always a chance that he's not.

So, here's what might have happened. Throughout the course of our evolution, any trait that increased the chances that a man would end up investing in his own offspring, rather than the offspring of his good-looking next-door-neighbour, had a good chance of being selected. One such trait was jealousy - the kind of jealousy that would lead a man to keep a wary eye on his partner and the good-looking neighbour, and to do what he could to keep them apart.

Men didn't need to have an actual, literal concern about paternity for their jealousy to do its job. They just needed to feel jealous. Any genes that inclined them in that direction automatically found themselves copied into more new bodies than genes that inclined them to think, "Hey, I'm an enlightened guy; I don't mind if my partner sleeps with other men."

Of course, it's not just men who get jealous; as any non-alien knows, women do too. But women's jealousy has a different adaptive logic. According to evolutionary psychologists, the primary issue is paternal care. Throughout most of our evolution, sex usually led to children, and children were a huge amount of work. Women in a robust relationship typically had more surviving offspring than women without one. As such, any trait that decreased the chances that a woman's partner would get involved with somebody else was likely to be selected. Jealousy again fits the bill.

In case this sounds like an evolutionary just-so story, bear in mind that analogues of human jealousy can be seen in many pair-bonding species. In gibbons, for instance, males chase away rival males and females chase away rival females. Coupled with the fact that, in our own species, jealousy appears to be a cross-cultural universal, the evolutionary explanation for jealousy is, at the very least, a hypothesis worth taking seriously.

The 10 per cent myth

This whole discussion raises another, rather awkward question. How common is infidelity in our species? One way to get a conservative fix on the numbers is to look at the human non-paternity rate. How many people's biological father is not the guy whose name is on their birth certificate?

The best-known estimate puts the figure at 10 per cent, and estimates range as high as 30 per cent. Most people are surprised by these numbers - and they're right to be surprised. More than likely, the estimates are vast overestimates. Most come from studies looking at people for whom the non-paternity rate is almost certainly higher than in the general population. Many, for instance, are based on data from professional paternity testing services. The problem with this is that the men who use these services usually already harbour doubts about the paternity of their ostensible offspring.

Taking this into account, we might want to argue that the human non-paternity rate is unexpectedly low. Even among the men who most strongly suspect that their offspring aren't their own, only a third are right at the most.

Sure enough, studies which look at more typical samples of people find much lower non-paternity rates: closer to 1 per cent than to 10 per cent or 30 per cent. Like swans, humans are no angels. But we're relatively faithful nonetheless.

If infidelity is so rare, though, why are people so prone to jealousy? Aren't most of us more jealous and suspicious than we need to be? The answer is probably yes. The irony, however, is that if people weren't more jealous than they needed to be, they'd probably do less mate guarding and their partners would be a little more likely to stray - in which case, they'd need to be excessively jealous after all.

People's mild paranoia regarding their mate's fidelity plausibly functions as a reverse self-fulfilling prophesy, helping to bring about its own falsity. Thus, the fact that infidelity is relatively rare in our species doesn't imply that jealousy isn't needed. On the contrary, part of the reason it's relatively rare is that people are so naturally prone to the torments of the green-eyed monster.