I love you! I hate you!

As the playwright, poet and marriage counselor all know, the emotional boundary between love and hate can be surprisingly porous. For many, partners can inspire intense tenderness and devotion one week and glassy-eyed loathing the next.

For others, the feelings for a loved one are much more constant and less apt to flip between extremes.

Such flips between loving and loathing can make for a wilder life -- witness the sizzling combat and sex scenes between Brad Pitt and Angelina Jolie in “Mr. & Mrs. Smith” -- but not necessarily a happier one.

Until now, there have been few explanations for why some people exist in states of emotional extremes while others are able to accept the fact that nobody is perfect.

Now social psychologists have come up with a possible explanation for the emotional roller coaster some people ride. A recent study exploring love-hate relationships suggests that the fuel behind it is that old workhorse of the self-help movement: self-esteem.

Margaret Clark, professor of psychology at Yale University and lead author of the study, wanted to see if self-esteem was the trait enabling some people to integrate positive and negative information about others in a healthy way, and whether its absence prevented a realistic picture of partners as that inevitable mix of positive and negative characteristics.

Researchers found that people with low self-esteem -- as measured by standard psychological scales -- seem to have “separate stores of positive and negative partner information.” In other words, their partner, relative or friend is either idealized or vilified.

The report, published in May in the Journal of Personality and Social Psychology, suggests that healthy self-esteem -- and not superhuman powers of forgiveness -- is what enables a wife to say, “Even though my husband completely forgot our anniversary, he was so sweet and apologetic when he realized his mistake we had a glorious evening out two days later.”

Clark and colleagues also found that self-esteem is important in all kinds of relationships, not just romantic ones.

Self-esteem has been studied in many psychological contexts over the years -- at the workplace, between cultures or in the case of a serious illness such as cancer -- and psychologists say a pattern is emerging. “It’s now becoming clear that its purpose may be to help us behave in acceptable ways, in order to be accepted, form relationships, receive social support and so on,” says Mark Leary, professor of psychology at Duke University.

The report consisted of seven separate studies involving a total of approximately 2,200 participants. It examined the relationships between not just romantic partners and spouses but also best friends, roommates, mothers and offspring and an individual and the mother of a friend.

In two of the studies, participants sat at computers and were timed for how quickly they responded to positive and negative words about a person they knew. Participants were also given self-esteem evaluations well in advance of the tests.

When asked to indicate as quickly as possible whether 10 adjectives (five positive and five negative, presented in jumbled order) applied to their roommate, mother or friend’s mother, people with low esteem responded more slowly than those whose self-esteem was high, the researchers found. But both groups were equally fast when asked to apply the words to an inanimate object, implying that the task was only trickier for the low-esteem group when they were thinking about people.

Also, both groups responded equally as fast when all five positive, or negative, traits were grouped together, implying that those with low self-esteem had trouble with the task only when they were asked to view the person as a mix of good and bad.

Clark and her colleagues hypothesized that people with low self-esteem were more likely to judge their partners as being all good, or all bad, and also more likely to experience a tremendous range in what they felt about their partners.

To test their idea, the researchers came up with statements that captured this Jekyll and Hyde thinking, such as, “When I’m mad at my partner, I can’t think of anything good about him/her.” They also devised questions that illustrated the converse, such as, “Even when my partner does something to hurt me, it is easy to remind myself of his or her positive attributes.” Then they asked participants to rate how much they agreed or disagreed with each statement. They found that the higher the self-esteem, the greater the capacity to see the good and bad in a partner.

In the final study, 173 adults filled out questionnaires about their relationships and themselves a few weeks before their weddings. Seven months later, they filled out a diary about their relationship for five days running. The lower the self-esteem of the spouse, the more anxious they appeared in their diary entries, and the more widely swinging were their feelings about their relationships.

“We do know that people with low self-esteem do worse in relationships,” says Art Aron, a social psychologist at Stony Brook University in New York. “We haven’t known why. And this research is part of the reason why.”

But this doesn’t mean that self-esteem is easy to spot in another person. Researchers point out there is a big difference between self-confidence -- which is typically associated with a particular task -- and self-esteem, which goes to the heart of someone’s sense of himself or herself. A person can have a great deal of self-confidence in athletic abilities, for example, yet still lack this core sense of self-worth.

“I think that people with high self-esteem probably do make better mates,” Clark says. “However, I’m not sure it is easy to judge self-esteem upon first meeting a person, or even after quite a few meetings.”

Unless, of course, that person is named Dr. Jekyll or Mr. Hyde.