In a survey of people on and around college campuses, women were generally unaffected by separation, according to the study led by Todd Shackelford, an assistant professor of psychology at Florida Atlantic University.
The researchers proposed in a paper presented at the annual meeting of the American Psychological Association that men are unwittingly responding to an urge from way back in human evolution.
Basically, the researchers suggest, the urge that drives a man to want sex after separation is the thought that another man might have gotten there first and beat out his sperm in the race to fertilize his partner's egg. After all, in the view of evolutionary biology, passing on genes to the next generation is what sex is all about.
Shackelford acknowledged the measured effect of time apart on men's attitudes about sex is small. Many other factors could effect how interested a man is in sex on a given day. The results of the study are preliminary and can suggest only that time apart affected men's attitudes, because they come from a one-time survey.
The researchers gave anonymous one-time questionnaires to 388 women and 304 men who said they were in committed sexual relationships. The participants, in their mid-20s on average, were recruited at universities and public areas in Texas, Florida and Germany.
Participants rated their current interest in their partner on a 10-point scale. Results suggest that for every 100 hours of time apart since the last time they had sex with their partner, men's interest in doing it again rose an average of about one point, and their rating of their partner's attractiveness rose about a half-point.
Similarly, the less time men had spent with their partners since last intercourse, the higher they rated the partner's sexual interest in themselves and her attractiveness to other men. It didn't matter how much total time had elapsed since the last copulation.
Women, in contrast, appeared unaffected by the amount of time spent apart.
Timothy Perper, an independent sex researcher in Philadelphia and author of Sex Signals: The Biology of Love, said he doubted that Shackelford had really detected a psychological signal of sperm competition.
Perper said the researchers didn't directly test whether the male-female differences were big enough to be considered real, rather than just a fluke. If there's no real difference, there's no evidence for sperm competition, he said.
By Malcolm Ritter