What makes you jealous? That depends on whether you're a man or woman... or so researchers have long thought.

Traditionally, theories of evolutionary psychology assert that a man should care more about the physical act of a partner's infidelity than the emotional aspect, because raising and protecting another man's child does not perpetuate his own genes. Women, on the other hand, are believed to be more upset by emotional betrayal, as their evolutionary interests are better served when their mate is around to help raise the children. But Christine R. Harris, Ph.D., a psychology professor at the University of California, San Diego, challenged the theories in three related studies, all published in the Journal of Personality and Social Psychology.

In the first study, Harris measured the blood pressure and heart rate of 43 women and 36 men as they imagined scenarios of their mates committing either sexual or emotional infidelity. She found that men showed a greater physical reaction to sexual infidelity, while women reacted similarly to both scenarios -- with a slight tendency to react more like the men to the sexual infidelity scenario.

Wondering whether the men were simply more aroused by images in the sexual scenario, Harris removed the infidelity aspect and measured men's responses to imagined scenarios, in which they had sex and also fell in love with their partners. In each case, men responded more to the sexual images -- confirming Harris' suspicions. A similar follow-up study of women found that 80 percent believed emotional fidelity would bother them more; however, their physical responses to both scenarios were the same. Surprisingly, those women with experience in committed relationships showed a significantly greater reaction to the sexual infidelity scenario than the emotional one.

"The results raise serious doubts about the veracity of the claim that men are innately wired to be upset over sexual infidelity, and that women are innately more concerned with emotional infidelity," Harris said. Instead, her findings suggest that regardless of gender, our individual experiences play a major role in determining our desires and reactions.