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.