I never get a peaceful plane trip," a psychologist colleague of mine lamented. "Once people hear what I do, they proceed to tell me their theories of human behavior. Chemists must have relaxing plane rides—no one has his own theory of stereoscopic chemistry, but everyone thinks he is an expert in psychology." I imagine this observation is doubly true for some branches of psychology. Who, for instance, doesn’t have ideas about why we fall in and out of love? That’s one reason David Buss’s The Dangerous Passion is bound to ruffle feathers. Like other evolutionary psychologists, Buss believes that science can unravel many of love’s mysteries. Evolutionary psychology holds that human impulses and behaviors—everything from why we cooperate or cheat to why we find others attractive—are best understood as the products of natural selection. Using the theory, several new books seek to explain what sparks love, passion, sexuality, and all that is associated with them, including Bobbi Low’s Why Sex Matters and Randy Thornhill and Craig Palmer’s A Natural History of Rape. Buss, a professor of psychology at the University of Texas at Austin, and the author of The Evolution of Desire (1994), focuses on jealousy. "Jealousy did not arise from capitalism, patriarchy, culture, socialization, media, character defects or neurosis," he contends. Rather, it is "an adaptive emotion, forged over millions of years," one inexorably connected with long-term love.

To chart the boundaries of jealousy, Buss surveyed hundreds of men and women in the United States, the Netherlands, Germany, Japan, South Korea, and Zimbabwe. He asked which scenario they would find more distressing: learning that one’s partner is involved in a passionate sexual relationship with someone of the opposite sex, or that the partner is involved in a deep emotional relationship. In line with previous research and common experience, most men in the survey chose sexual infidelity, and most women chose emotional infidelity.

Evolutionary psychologists explain the disparity this way: A woman can be certain that a child she bears carries her genes, but paternity is iffier. For a man, it is always possible that the partner is having someone else’s baby. Investing resources in such a child is an evolutionary dead-end for him, so he is particularly jealous about matters of sex. And he knows that the sexual competition is fierce: In surveys, three-quarters of men say they would have sex with an attractive member of the opposite sex who propositioned them on the street (a scenario that appeals to zero percent of women). Males’ fear of being cuckolded has even made it into our legal code. Until 1974, if a man killed his wife and her lover after catching them in the act, he was not guilty of any crime under Texas law.

Women’s jealousy, by contrast, tends to revolve around emotional issues. Buss and other evolutionary psychologists argue that women are almost always involved in raising children, and they want to ensure that the male will provide resources sufficient for the children to thrive. A one-night stand probably will not divert his resources, but an affair encompassing a strong emotional commitment may do so. Hence, females are particularly attuned to emotional infidelity, which may ultimately affect the well-being of their offspring.

Along with his own poll, Buss cites other opinion surveys that ask how people would feel or behave in particular situations. While it is refreshing to see data, and not just the imaginative theorizing that is so prevalent in this realm, one must view these surveys with caution. For one thing, the respondents are often university students who receive academic credit for participating, and their attitudes may not match the general population’s. Even where a survey reaches a more diverse group, as with Buss’s own study, people’s answers to hypothetical questions may not reflect how they would actually behave. That is especially the case when some of the scenarios border on soft porn. (With some judicious editing, Buss could have advanced his arguments without quite so many tawdry details.)

Not all of the author’s notions rest on data. He provides, for instance, an evolutionary explanation for the fact that people often falsely suspect a partner of infidelity based on a new tie, new perfume, or other flimsy evidence. Groundless suspicion, he notes, may land you on the couch for a night; groundless complacency may cost you your partner. Natural selection favors making an error in the less costly direction, hence the false positives.

Buss also notes that infidelity is easily explained for the male—the more sexual partners he has, the greater his reproductive potential—but not for the female, who can get pregnant only every nine months. So why do women have affairs, risking abandonment and sometimes violence? Buss suggests some combination of five factors, each of which will probably offend some portion of his readership: A woman may seek the superior genes associated with men who have affairs; she may hope to produce a sexually alluring son; she may want to establish a partnership with a male higher on the social hierarchy than her current partner; she may seek a "back-up" mate should something happen to her partner; or she may hope for sexual gratification. Here again, the theories are provocative and plausible, but, as Buss acknowledges, the data are weak or nonexistent. In his view, jealousy—unlike some adaptive behaviors rooted in the huntergatherer era—remains a positive force in the modern age. "Properly used," he writes, "jealousy can enrich relationships, spark passion, and amplify commitment.... The total absence of jealousy, rather than its presence, is a more ominous sign for romantic partners. It portends emotional bankruptcy." He urges therapists to recognize jealousy as potentially healthy and important rather than seeing it as a "negative emotion" that invariably portends a relationship in distress. Like much of this fascinating book, it is a prescription guaranteed to tantalize some readers and infuriate others.

> Lee Alan Dugatkin is a professor of biology at the University of Louisville. He is the author of Cheating Monkeys and Citizen Bees: The Nature of Cooperation in Humans and Animals (1999).