I love you to death

Jealousy is often seen as a neurosis that can lead to psychotic violence, but psychologist David Buss argues that it is a form of emotional wisdom that evolved as an early-warning system.

Jealousy poses a paradox. On the one hand, it is often tethered to long-term love. In one scientific study, 46 per cent of a community sample said jealousy was an inevitable consequence of love. Shakespeare's tormented Othello "dotes, yet doubts, suspects, yet strongly loves". People will often interpret a partner's absence of jealousy as an ominous sign of a lack of love.

On the other hand, jealousy can shatter the most harmonious relationships. It is a leading cause of violence against partners. Women at shelters for battered women invariably report that the beating they received from their partners stemmed from extreme jealousy. This paradox was reflected in O. J. Simpson's statement: "Let's say I committed this crime (the slaying of his ex-wife, Nicole Brown Simpson). Even if I did do this, it would have to have been because I loved her very much, right?" How can we understand an emotion simultaneously linked with violence and long-term love?

We live in an uncertain social world. Some problems, such as infidelity, are intentionally concealed. We have access only to a chaos of clues. A partner might suddenly come home smelling of strange perfume or possessing new sexual techniques. These changes may have innocent explanations, but when signals are muffled, accurate inferences about reality become difficult.

In the face of this uncertain world, I would argue that jealousy evolved as a hypersensitive defence against possible threats to loving relationships. Mainstream social scientists have historically regarded jealousy as a symptom of neurosis and a delusion of psychosis, a sign of immaturity and a defect of character, even as a byproduct of capitalism or a symbol of patriarchy. It has been seen as a neurotic embarrassment that we should expunge from the repertoire of human emotions.

But in evolutionary perspective, jealousy is an exquisitely designed component of human psychology that has a crystalline logic and precise purpose. It evolved to repel rivals, prevent infidelity, deter defection, protect paternity and preserve love. Jealousy is a smoke alarm designed to head off betrayals that are lurking on the horizon of a relationship. Individuals diagnosed as "pathologically jealous" often turn out to have partners who have strayed in the past, are straying or are contemplating straying.

Some people think men are more jealous, others think women are. The scientific truth is more complex. It turns out that both sexes are equally jealous in terms of how often they experience jealousy and the intensity of the emotion. But women and men differ in what triggers jealousy, with regard to the rivals that they find threatening and in the actions they perform when they get jealous.

Consider this dilemma, which I call the "Sophie's choice" of jealousy. Imagine that your partner became interested in someone else. What would upset you more? (a) Discovering your partner enjoying passionate sex with another person (b) Discovering that your partner is forming a deep emotional attachment and sharing confidences with another?

The majority of women we surveyed in Japan, Korea, the Netherlands, Germany and the United States said the emotional betrayal would be more upsetting. Most men, however, found the prospect of a partner's sexual infidelity more agonising.

The explanation for these profound differences lies deep in the evolutionary past of the human species. Because fertilisation occurs inside women, our ancestral fathers confronted a problem that no woman has ever had to face - certainty of parenthood. One African culture describes this asymmetry with the phrase: "Mama's baby; papa's maybe." Men's jealousy evolved in an uncertain social world to combat the potentially catastrophic reproductive costs linked with genetic cuckoldry.

Our ancestral mothers confronted a different problem - the loss of a partner's commitment to a rival woman and her children. Because emotional involvement is the most reliable signal of this disastrous loss, women home in on cues to a partner's feelings for other women. A husband's one-night stand is distressing, of course, but most women want to know: "Do you love her?" Most women find a single lapse in fidelity without emotional involvement easier to forgive than the nightmare of another woman capturing her partner's tenderness, affection and commitment to children.

These sex differences run deep, even to the very definition of infidelity. Men define infidelity narrowly in terms of sexual betrayal. Women include sexual betrayal, but define infidelity in broader terms that include emotional betrayal. These sex differences reflect emotional wisdom that modern humans have inherited from their successful ancestors - wisdom designed to deal with the different threats confronted by women and men in the context of a partner's sexual treachery.

Men and women also differ in the methods they use to cope with jealousy, although there are some similarities. Both sexes redouble their displays of love and affection. Both sexes derogate their rivals, impugning their intentions, appearance and intelligence. And both sexes, when jealous, strive to fulfil the desires of their partner.

It is a disturbing fact, however, that men are more likely to resort to violence. According to psychologists Margo Wilson and Martin Daly, men use violence against partners as a method of deterrence and control. B using violence, men convey an important signal to a partner: acts of infidelity come at a high price. By making the price of infidelity sufficiently high, men hope to deter their partners from sleeping with other men. A common refrain that jealous men utter is: "If I can't have her, no one can." Men's violence may represent a last-ditch effort to keep a mate who is on the verge of leaving.

The idea that partner violence is intended to deter defections is undoubtedly disturbing. But it should not be construed as condoning or justifying violence. Only by developing a deeper understanding of the dangerous passion can we take steps to reduce destructive manifestations.

Once jealousy evolved, however, it could be exploited by partners for purposes other than those for which it was designed. Have you ever intentionally brought up an old lover's name just to see how your partner would react? Have you ever deliberately ignored your partner at a party?If so, the chances are that you have engaged in a strategy of evoking the jealousy of your partner.

Why would anyone trifle with an emotion that is known to be the leading cause of violence against intimate partners?

Scientific studies suggest that people evoke jealousy for three reasons: to test the strength of the mating bond (by evoking jealousy, a person gains valuable information about the intensity of a partner's commitment); to demonstrate to a partner that other attractive mating alternatives are available, thereby increasing the commitment of the partner; and to ignite or rekindle sexual passion in a relationship that has dwindled over time.

The experience of jealousy can be psychologically painful, and for this reason it has been called a negative emotion. But jealousy alerts us to real threats by real rivals. It tells us when a partner's sexual indifference might not merely mean that he or she is distracted by work. It causes us to remember subtle signals that portend a real defection. We experience pain because pain motivates us to deal with the disturbing possibility of betrayal. Evolution forged jealousy as a hypersensitive defence system, designed to sound the alarm not just when infidelity has been discovered, but also when circumstances make it slightly more likely. This explains why we sometimes infer infidelity when none has occurred.

Jealousy, infidelity and love are co-evolved emotions. The knowledge that comes from a deeper understanding of our dangerous passions will not eliminate conflicts between lovers, between rivals, or between lovers who become rivals. But it may give us the emotional wisdom to deal with them.

David M. Buss is professor of psychology at the University of Texas at Austin. His most recent book, on which this article is based, is The Dangerous Passion: Why Jealousy is as Necessary as Love and Sex (Bloomsbury).