It's been likened to a green-eyed monster -- a complex, consuming, bestial set of emotions that can lead to depression, guilt, anger and even murder.

"Jealousy," says psychologist Gregory L. White, "and the way a couple deals with it, may be the single most crucial issue in an intimate relationship."

From the moment Madeira headmistress Jean Harris was charged with the murder of "Scarsdale Diet" doctor Herman Tarnower, jealousy has been a hot topic and White a sought-after speaker. The 30-year-old University of Maryland assistant professor has completed several studies of the dynamics of jealousy and is considered by some to be the nation's foremost expert on the subject.

"Jealous Jean" has captured the country's sympathy, says White, "because people feel close to understanding her motive. Most people have been jealous at one time or another and realize how explosive those feelings can be."

Jealousy, he says, is not a basic human emotion like sadness or joy, but "a complex constellation of emotions, behaviors and coping strategies that vary from person to person and culture to culture.

"In some cultures husband-sharing or wife-sharing is legitimate, and jealousy isn't appropriate. In other cultures it is acceptable, even expected, to be jealous and kill your wife if you find her with another man." But in all societies, "jealousy is a raction to a perceived threat to oneself and/or one's relationship. In jealousy you see the raw edges of how a couple deals with issues or power, dependence and intimacy."

At least three people -- two lovers and a potential (sometimes imaginary) rival -- are always involved in the jealousy triangle. "Even in business jealousy -- when you're jealous of the person who got promoted instead of you -- there's a third party who's given away your promotion."

In envy, which is often confused with jealousy, he says just two people are involved. "The envious person wants something similar to what you have. If I'm envious of your relationship with your wife I want one of my own just like it. If I'm jealous of your relationship with your wife I probably want your wife."

There are four basic theories, he says, of why jealousy occurs.

"The biological view is that jealousy is triggered innately because pair-bonding has been evolutionally advantageous. So whatever kept the couple together survived.The social-anthropological view says our culture dictates pair-bonding, so our society has constructed jealousy to counter any threat to the couple."

The psycho-dynamic view has two parts. "One is that jealousy is a way of projecting internal anxiety. Say I'm married and attracted to another woman -- or to a man. That makes me anxious. So I project my feelings and decide my wife is attracted to someone else, and I become jealous."

The other part of that theory, he says, considers jealousy a replay of the Oedipal conflict. Just as a child thinks its desire for the opposite-sex parent is frustrated by an outside rival, an adult may feel there's a rival for his or her mate.

Social psychologists like White consider all these viewpoints viable, and add their own theory focusing "on the nature of the relationship and the immediate social surroundings.

"One way of exploring the relationship's dynamics is to look at the power structure and its link with jealousy. A person in a high-power position is less likely to be jealous and more likely to evoke jealousy. If their position is threatened they are more likely to feel angry, while a person in a low-power position who is threatened is likely to feel depressed."

Since men are traditionally in high-power roles and women in low, White says, "this may explain why women are perceived to be more jealous. But that perception doesn't appear to be true. Only one study out of nine found any sex difference in how jealous people rate themselves to be."

When one mate tries to make the other jealous, White notes, "it's a power tactic. Usually it's done to get a specific goodie, like going out to the movies more. Sometimes it's done to test the relationship and can backfire."

White lists four basic "causes" or "predispositions" of jealousy:

Being relatively more dependent than your partner.

Buying into the idea of sexual exclusivity.

Feeling inadequate as a partner in this particular relationship.

Measuring your self-esteem largely by how your partner feels about you.

The first reaction, when feeling threatened, is usually emotional. The two most common emotions, he says, "are anger and depression. Severe anger may lead to feelings of revenge -- perhaps by retaliating with an affair of your own or by taking revenge on your rival. Depression can lead to guilt. Some people, most often women, feel that it's their fault that their mate is interested in someone else."

Next, the person tries to intellectually figure out what's going on. "They start asking themselves questions," says White. "They wonder what their rival is like and why their partner is interested in someone else. They think about what would happen if their relationship were to change or end, and they start listing alternatives."

People usually attribute their partner's outside interest, says White, to one of four basic motives: sex, escape from a bad relationship, the desire for a stronger (or different) commitment, and different (or better) fulfillment of needs.

"Violence occurs," says White, "when people find themselves locked into their jealousy and don't see any other way to handle it. It may also involve a status orientation -- 'No one's going to run off why my wife.'"

Roughly one in five murders involves jealously, claims White, who is writing a grand proposal to study this theory. "We know so little about jealousy's violence response. Studies like this could come up with helpful counseling strategies."

The way a couple deals with jealousy, says White, "has implications for their whole relationship. In intimacy people really open themselves up and are extremely vulnerable. Their self-esteem is on the line, and they become dependent -- to a certain extent -- on their mate. This can be very dangerous."