Amanda is a dark-haired stunner with a zesty disposition, and Elliott took a fast liking to her. Within days of their meeting, bouquets began arriving at the office for her. As if it were the NASDAQ index of her appeal flashed across Times Square, Amanda relished the surprise deliveries—and the admiring notice of officemates.

Elliott himself made less of an impression; he was ordinary looking and wiry, with a slight nervous edge. But his retro style of dressing did stand out: Plaid jackets and saddle shoes made him look as if he were always sneaking a quick break from a low-budget comedy act.

Over weeks and then months, Elliott showed up regularly to take Amanda to dinner or out with friends. He called frequently, too—to make sure she got the flowers, to find out who ogled them, or just to hear her voice. If Amanda wasn't at her desk, the calls often bounced to her increasingly annoyed colleagues.

"Elliott had a goofy side that appealed to me, and at first, I thought he was just kind of love-struck. I was charmed and amused," Amanda recalls. But, she realized gradually, the flowers were a kind of camouflage. "He needed to know where I was every minute, and if he didn't hear what he liked, his voice would crack with rage. That really creeped me out. I woke up one day and thought: Why does this romance feel like it's becoming a prison? In that instant, I knew I had to get out."

The flowers notwithstanding, Elliott exhibited many of the classic signs of jealousy—fear of losing his lover, lack of trust, anger at real or imagined attention to others, the need to control a loved one. Even the flowers were a time-honored mate-retention strategy of the kind kicked off by jealousy, although we're more inclined to associate jealousy with negative tactics, from vigilance to violence.

More often than not, feelings of jealousy flare with such intensity that they burn a hole in the brain, obliterating rational thought and setting off behaviors that create a self-fulfilling prophecy by pushing away the very person one desires, or needs, the most. Think of astronaut-in-training Lisa Nowak, who in 2007, at the age of 44, drove a thousand miles nonstop from Houston, Texas, to Orlando, Florida, with a diaper on, the quicker to kidnap the new girlfriend of a fellow astronaut with whom she had had an affair. Ironic that an impulse that arises from love can so easily destroy it.

Yet jealousy, experts agree, is a survival mechanism, although what is most at stake is a matter of debate. The most destructive of passions—it is a leading cause of homicide—and the least studied, it is, like all emotions, born of necessity, with roots deep in our evolutionary past. Its purpose: to help maintain intimate relationships.

Jealousy is not envy, although the words are often used interchangeably. "Jealousy arises when a relationship is infringed on by a rival who threatens to take away something that is in a sense rightfully yours," explains Richard Smith, professor of psychology at the University of Kentucky. The rival may or may not have features that also incite envy. "But to feel jealous you need not have any sense of what that third party is like," notes Smith. Envy, on the other hand, derives from the basic fact that so much of the spoils of life come from how we compare to others. It arises when another person possesses some trait or object that you want, and includes a mix of discontent, a sense of inferiority, and a frustration that may be tinged with resentment.

Here's the shocker: Jealousy may be losing its utility in contemporary life, more useful to our ancestors than to us, given our penchant for changing partners. As our high divorce rate attests, sometimes, we're just not all that interested in saving our closest relationships. It may also be that jealousy is on a shifting course in our emotional repertoire, moving from coercive social emotion, a socially sanctioned response to infidelity, to sign of personal pathology.

A signal to look within

Jealousy is an extremely painful emotion; social exclusion, whether real or imagined, always hurts. It throws the mind into turmoil and is difficult to dislodge. Those in its grip typically blame the discomfort on a partner for bestowing attention on others. But there are huge individual differences in the propensity for jealousy, and there is emerging evidence that elements of personality influence some of them. Those who are most insecure, in fact, may be most unrealistic in perceiving threats and making accusations. But this same view of jealousy also suggests that the emotion need not be unleashed on a destructive path; it can instead serve a highly constructive purpose—as a valuable signal to look within and repair one's own sense of self. That, in turn, can only improve relationships. Jealousy, it seems, says more about the bearer than about the deeds or misdeeds of a mate.

No one can say for sure what jealousy is; attempts to define it are elusive for a reason. As a complex emotion it involves, at a minimum, such distressing feelings as fear, abandonment, loss, sorrow, anger, betrayal, envy, and humiliation. And it recruits a host of cognitive processes gone awry, from doubt to preoccupation with a partner's faithlessness. It may take much of its primal force from activating the attachment system of the brain, a genetically ingrained circuit that is the foundation of our social bonds and that prompts widespread distress when they are threatened.

According to University of Texas psychologist David Buss, jealousy is a necessary emotion, a potential deterrent to infidelity that arises in both men and women when a threat materializes to intimate relationships. Boyfriend talks to beautiful woman at party and smiles admiringly at her; to girlfriend, a rival is born, a flesh-and-blood warning that what she thought was hers might now be endangered. Or wife suddenly embarks on series of brief out-of-town trips with co-head of her team. What is at stake is survival of our most valued relationships and thus the future of our children—which is to say, the species.

The "crystalline logic" of evolutionary psychology, argues Buss in The Dangerous Passion: Why Jealousy Is as Necessary as Love and Sex, holds that men and women experience jealousy differently, and that it's the threat of sexual infidelity that most stirs jealousy in men. The burden of manhood is uncertainty of paternity; jealousy serves to keep a mate from straying, upping a man's confidence that he is the genetic father of his partner's children. Jealousy arose to keep him from the reproductive dead-end of investing his finite resources in raising some other man's children. Women respond most to the possible loss of love to a rival female, a way of protecting a partner's needed commitment to home and kids. And perhaps in the small bands in which humans lived for most of evolutionary history, jealousy was effective in keeping a mate from straying.

Yes, says Buss, jealousy can cause men, especially, to "explode violently," although that's just a way "to reduce the odds that their partners will stray." Jealousy is not just the main motivation for spouse battering. Sexual jealousy is the leading cause of spousal murder worldwide. Even then, it's not really jealousy that's to blame, contends Buss. "It is the delusion that a loved one has committed an infidelity when none has occurred." But "this double-edged defense mechanism" wouldn't exist if long-term love hadn't emerged among primates. Jealousy is love's necessary protector—even if, given the cognitive biases built into the brain, it errs on the side of seeing betrayal where it does not exist. Delusion is jealousy's yes-man.

Not inevitable

However much Buss sees jealousy as a necessary evil, his newest work suggests that it isn't quite as inevitable as it's been made to appear. In a not-yet-published study of nearly 1,000 people in various stages of commitment—married, engaged, dating, or single—he and a colleague in Spain find that the individual inclination to jealousy is strongly influenced by two of the so-called big five personality factors. It is positively associated with neuroticism, or emotional instability, the liability to such unpleasant emotions as anger, anxiety, and depression. The higher the level of instability, the more one is prone to jealousy.

And it is negatively related to agreeableness; the tendency to be cooperative and compassionate rather than suspicious and antagonistic. Like all the major personality factors, neuroticism and agreeableness are both influenced by heredity and environment, including early experience, in roughly equal proportions.

"What we find," says Buss, "is that neuroticism is positively correlated with a whole slew of mate-retention tactics"— defense maneuvers, like increased vigilance, intended to guard a partner from straying. "Agreeableness is negatively correlated, and low-agreeable people tend to use cost-inflicting mate-retention tactics," like yelling at a partner for talking to someone else, cutting a partner off from friends and family, derogating the partner, undermining a mate's self-esteem, or threatening violence against a partner or perceived rivals. "We view these as abhorrent," says Buss, "but sometimes they work to keep a mate in a relationship."

As Exhibit A in the neuroticism category, Buss cites his own experience long ago with a former partner. "She thought that every female graduate student was coming on to me, and after I had a meeting with a student, she would tear into my office and interrogate me. Who knows; maybe her neuroticism or hyper-jealousy in fact deterred other women."

Not all jealousy is activated by immediate threats of sexual infidelity or loss of a partner to a rival—so-called mate poaching. It also responds, says Buss, to such factors as subtle indicators of discrepancy between the "mate value" of two partners—one partner is more attractive than the other.

Or, by sheer genetic chance, one partner (think: John Edwards) stays far more youthful-looking than the other. But it works in both directions. "Some men luck into a woman who is higher in mate value than they are," reports Buss. "And on some level such a man has some realization that he is not going to be able to replace her with someone of equivalent value. So he is on jealousy hyperalert."

The thing is, neuroticism itself is not a very appealing attribute in a mate; on its own it lowers an individual's mate value. But, as with Elliott and Buss' former partner, it's not a trait that's necessarily on full display when one enters into a relationship; it tends to reveal itself only over time. The more emotionally stable you are, says Buss, the higher your mate value.

"The formula for jealousy," says psychologist Steven Stosny, "is an insecure person times an insecure relationship." But it's insecure people who tend to destabilize relationships and make them insecure. And a person who is very insecure is not just sexually jealous but jealous of any kind of friendship or even of a child—"anything that takes attention off them," he adds.

Because jealousy is accompanied by a sense of inadequacy, it is hard to bear, says Stosny, and most people convert the discomfort into anger, which they regulate by trying to control a partner—distrusting them, going through their belongings and cell phone call logs, making accusations, behaviors more likely to drive a partner away. "The trick is you have to control jealousy within yourself. You have to do something that will make you feel more lovable, because basically you feel unlovable when you're jealous."

That, observes French psychiatrist Marcianne Blevis, is the heart of jealousy. For Blevis, who has spent years in the trenches with patients, jealousy is not the guardian of love but more typically its destroyer. It arises in relationships whenever we feel "erased" by a partner's lack of attention. She insists it is a signal—not to blame the partner for attention to someone else, which is what we usually do, but to look inside oneself. There, she contends, we will find the source of insecurity that instantly makes the rival seem so superior to us. What's at stake in jealousy, she argues, is nothing less than survival of the sense of self.

Just what are we doubting?

"We assume that jealousy is a necessary evil, the collateral damage of love," says Blevis, author of Jealousy: True Stories of Love's Favorite Decoy. "'Jealousy lives upon doubts,' said the 18th century moralist Francois de la Rochefoucauld. But what exactly are we doubting? All human emotions exist to help us figure out who we are in the world, and jealousy is no exception. It is a resource we call on when we feel at risk, when our sense of self is put in jeopardy. When we are jealous, we are in fact in the grip of an identity crisis."

Invariably, however, we misdirect our attention. Blevis explains: The target of jealousy, the one who has provoked our jealousy, shatters our self-regard. From the rival emanates an aura of magical attributes that he or she possesses and we don't. Yet we are the ones who assign those attributes to the rival; what they really represent is something unrealized in ourselves. "It signals our wish to be better than ourselves, to reconfigure who we are." Blevis cites the case of a patient who had not dared to pursue the career of her dreams—becoming a doctor. But her boyfriend left her for a woman more accomplished than she. "She came to realize that her jealousy of her rival masked her craving for the part of herself she had ignored. She lost the boyfriend, but she went back to school."

It's a mistake to assume that jealousy always involves love, argues Aaron Ben-Ze'ev, a philosopher who heads Israel's University of Haifa. "A man who despises his wife may nevertheless become jealous when someone else looks covetously at her. Here the central feature is losing to a rival." In this case, he insists, jealousy is more germane to the personal characteristic of selfishness than to love.

People indeed use jealousy as a signal—to try to control their partner. But that only makes the relationship worse. "They substitute power for value," says Stosny. "They feel a loss of personal value and rather than do something that will make them feel more valuable, they do something that will make them feel more powerful." But the more that people try to soothe their own emotions by controlling a partner, the more powerless they are destined to feel—because, observes Stosny, "you're dependent on your partner's whims to feel OK." And that's a setup for anger.

Our brains don't titrate jealousy, releasing just enough to penetrate our self-awareness. Jealousy lands with brutal force, lending itself far too readily to obsession and delusion. "It makes you think the same thing over and over," says Stosny, "and the more you do that, the less reality-testing you do. Emotions all have an illusion of certainty, and jealousy makes you certain of your perception of the world." Buss contends that the tendency to deceptionthat is a hallmark of jealousy is built into the brain. "We have evolved cognitive biases designed to overestimate the likelihood of costly things happening. So we make all kinds of errors in judgment—erroneous inferences, erroneous suspicions. But our brains are designed that way to avoid the more costly error of being exposed to a mate poacher."

A measure of commitment

However much jealousy encourages an outsize response, a little bit is good for relationships, especially in the early stages, before trust has a chance to develop. "The paradox of jealousy is that we all want some of it," says Stosny. It's a measure of commitment. "In small doses it's an expression of caring. Jealousy is like a way of testing whether it's safe to invest more emotion. It's safe when a person cares enough to be uncomfortable. Jealousy is a fear of losing something you perceive you have—the affection, the fidelity of another person. The threat of losing it is a test of how much you value it."

Although the perceptions that accompany jealousy may be distorted, the pain it gives rise to is real. The neural circuitry that underlies our psychological response to such complex social events as being accepted or rejected is the same circuitry that underlies the simplest physical pains and pleasures. Experiencing envy, which is a first cousin to jealousy and similarly gives rise to feelings of inferiority and resentment, activates pain-related neural circuits in the brain, neuroimaging studies show.

According to Japanese neuroscientist Hidehiko Takahashi, envy activates the anterior cingulate cortex, the site where cognitive conflicts of social pain are processed, and the stronger the emotion, the greater the activation. "We usually have a positive self-concept, and we experience discomfort when we perform in a way that violates this self-concept," Takahashi reports in Science. "The anterior cingulate cortex is activated when this positive self-concept conflicts with external information."

Neuroimaging is only at the earliest stages of investigation of jealousy. In a 2006 study, reported in NeuroImage, Takahashi found some significant sex differences in the neural response to statements depicting sexual and emotional infidelity. In men, jealousy activates the amygdala and hypothalamus, regions rich in testosterone receptors and involved in sexual and aggressive behavior. In women, by contrast, especially in response to thoughts of emotional infidelity, activation was greater in the posterior superior temporal sulcus, a region implicated in detection of intention, deception, and trustworthiness as well as violation of social norms. The greater activation elicited by emotional infidelity in females, Takahashi reported, suggests that they are particularly sensitive to changes of a partner's mind.

The emotional sensitivity of women may explain why their jealousy is not limited to romantic relationships. Women experience the upsetting emotion in female friendships as well, feeling anger, loss, and betrayal when a friend defects or pays more attention to a female interloper. "They're in competition for alliances with each other a lot more than males are," says Stosny.

Relationships end. Divorce is a reality. And situations that give rise to mate poaching are not going away anytime soon. Over 50 percent of males and females report having tried to steal a friend's partner. "You can't eliminate loss," says Stosny. But secure people can handle disappointment without feeling like a total loser. The only way to eliminate the pain of loss is to eliminate value, and you can't do that because then life would not be worth living."—Hara Estroff Marano

Jealousy as a test of affection

A little pang can go a long way.

Once jealousy took up residence in the repertoire of human emotions, who was to say it couldn't be used strategically? In fact, reports University of Texas psychologist David Buss, 40 percent of women deliberately provoke a bit of jealousy in a partner to get a reading on the strength of the bond. (Men do it too, but not nearly as often as women.) It can also up one's desirability in a mate's eyes.

Because jealousy, especially in the early stages of a relationship, correlates with caring, deliberately provoking it can be a way of testing whether it's safe to invest more emotion. "If your partner doesn't respond," says Steven Stosny, "then it's not safe."

Common tactics include talking about an ex or another guy in the presence of a partner, talking to another man at a party or actually even dating another guy, and talking about attractions to other men. There are sins of commission: Women are not above dressing highly provocatively when going out with friends or lying about being attracted to another man. And there are sins of omission: deliberately failing to answer a phone call from a boyfriend to make him think she's out with someone else.

The surprise is how easy it is to trip the male jealousy switch. "A lot of times men aren't even aware of the emotional manipulation," says Buss.

And nothing is more effective than flirting with another man, as simple as flashing a smile, a signal men invariably read as sexual while women do not. There's very little risk. It's hard to blame a woman for "just being polite" or "friendly."

Women are also likely to provoke a little jealousy when they perceive a partner is less committed to the relationship than they are. The aim is to alter their partners' perception of how desirable they are.

"No one wants to be with someone whom no one else wants," says Buss. "If no one else wants them, people start taking each other for granted."

What to say when suspicion strikes

There are steps either partner can take to contain jealousy and keep it from wrecking a relationship. Marriage and family therapist Lori Gordon offers several suggestions.

•  As a general insurance against jealousy, nurture your relationship. Take time to be together, and spend time talking even when apart. It's important to share your inner worlds with each other.

• Make a decision whether you want to confront your suspicions or not.

• If you feel suspicious, worried, possessive, threatened, or unsure regarding your partner—or, alternatively, you feel crowded, controlled, restricted, or blamed by your partner about friendships or activities—don't let your assumptions run away with themselves. Check them out with your partner. But first, reflect on how to put your thoughts into words.

• The goal is to start a conversation non-belligerently, to be constructive and non-blaming. To avoid setting off defensiveness in a partner, use statements that begin with "I," not "you."

• Identify a specific behavior of your partner's that is upsetting to you ("when you let that guy pour you a drink") and explain how it makes you feel.

• Knowing how to give voice to things that disturb you is crucial. A good structure to follow has three parts:

"I notice..." (you seemed quiet last night, or you were unusually friendly with that woman at the party).

"I assume that it means..." (you were upset about something, or you were just being friendly).

"I wonder..." (what it is and if you would tell me, or whether there is more to it).

• Pause to be sure your partner is listening and to give your partner time to respond.