But it also marks a broader shift in psychology, away from social explanations for human behavior. Burned by their postmillennial status as "soft" scientists unable to prescribe antidepressants, psychologists are turning from their field's humanistic roots and toward biochemical and genetic research. The unwillingness of insurance companies to pay for unlimited psychotherapy is also spurring the change.

"Neural ailments, chemical imbalances and the legitimate and illegitimate use of drugs is occupying the time of more psychologists," says Ronald B. Evans, Ph.D., professor of psychology at East Carolina University in North Carolina. "We are leaving the field as a social science."

Until the oxytocin study, most evolutionary and cognitive psychology focused on sexual behavior. MIT's Steven Pinker, for example, defended President Clinton's philandering by explaining that he, like all males, is cognitively hard-wired to impregnate as many females as possible, thus ensuring species survival. David Buss, author of "The Evolution of Desire," cites genetic evidence for the male impulse to pair with younger, more fertile women, while women seek older males because of their greater earning power. The oxytocin hypothesis of women as nurturing caregivers and men as emotionless warriors suggests that evolutionary theorists are moving beyond a carnal focus to find chemical bases for all behavior, from why men pout to why even employed women take on all the child care.

This is distressing to anyone who believes that human behavior is governed by something higher, more mysterious, more and peculiarly human, than genes and hormones. Without social explanations, or even a social context into which to fit their findings, evolutionary psychologists promote friction between the sexes. Even more troubling, their research proposes no hope of change.

The study, published last month in the journal Psychological Review, argues that women's higher levels of oxytocin, a mood regulator released by the pituitary gland, causes them to seek social interaction to relieve stress. This newly coined "tend and befriend" response evolved, they say, from female primates genetically coded to protect their young. To this day it contrasts with the solitary, wound-licking behavior familiar to anyone who has ever shared a home or office with a man.

Low levels of oxytocin, according to the study, explains why my boyfriend spends hours in his "music room" strumming the guitar when we have a fight, forgoing food and even the bathroom with his torturous pouting. I, on the other hand, will make a round robin of phone calls to my girlfriends, seeking an ear, advice, anything to break the isolation. Because oxytocin operates synergistically with other sex hormones, his behavior is supported by testosterone, while mine is supported by estrogen. Theoretically, when my estrogen levels dip after menopause, we'll both spend more time sulking and our phone bills will be lower.

"We began this study in 1998 with the hypothesis that men and women responded differently to stress," says Laura Cousino Klein, a professor of bio-behavioral health at Pennsylvania State University and one of the study's authors. "That was quite controversial." It was previously assumed that everyone displayed a "fight or flight" response to any kind of stress, from a nasty day at the office to being held up at gunpoint.

Women have traditionally been left out of human stress studies because researchers assumed that menstrual mood swings would muck up any results. But since 1995, when the federal government mandated broad representation of both sexes in research grants -- and made more money available for research on women -- psychologists have shifted their focus. The oxytocin study, one of the largest of stress, examined thousands of studies on male and female humans, apes and rats -- even deer and moles.

"There will now be a surge of new research in this field," says Brian Lewis, an assistant professor of psychology at Syracuse University who specializes in stress.