The study, which is bound to provoke controversy, showed that the women who were directly exposed to semen were less depressed. The researchers think this is because mood-altering hormones in semen are absorbed through the vagina. They say they have ruled out other explanations.

“I want to make it clear that we are not advocating that people abstain from using condoms,” says Gordon Gallup, the psychologist at the State University of New York who led the team. “Clearly an unwanted pregnancy or a sexually transmitted disease would more than offset any advantageous psychological effects of semen.”

Suicide attempts

His team divided 293 female students into groups depending on how often their partners wore condoms, and assessed their happiness using the Beck Depression Inventory, a standard questionnaire for assessing mood. People who score over 17 are considered moderately depressed.

The team found that women whose partners never used condoms scored 8 on average, those who sometimes used them scored 10.5, those who usually used them scored 15 and those who always used them scored 11.3. Women who weren’t having sex at all scored 13.5.

What’s more, the longer the interval since they last had sex, the more depressed the women who never or sometimes used condoms got. But the time since the last sexual encounter made no difference to the mood of women who usually or always used condoms.

The team also found that depressive symptoms and suicide attempts were more common among women who used condoms regularly compared with those who didn’t. The results will appear in the journal Archives of Sexual Behavior.

And Gallup told New Scientist that his team already has unpublished data from a larger group of 700 women confirming these findings. In this study, the always-use-condoms group were more depressed than the usually-use-condoms group, suggesting the discrepancy in the smaller study was a sampling error, he says.

Alternative explanations

But is it really the semen that affects women’s mood? The researchers say they looked at alternative explanations such as whether women who seldom use condoms took oral contraceptives, how often they had sex, the strength of relationships, and the possibility that having a certain type of personality influenced the decision to use condoms. But none of these factors can explain their findings, they say.

In fact, the results aren’t a complete surprise because semen does contain several mood-altering hormones, including testosterone, oestrogen, follicle-stimulating hormone, luteinising hormone, prolactin and several different prostaglandins. Some of these have been detected in a women’s blood within hours of exposure to semen.

The question many people will ask is whether oral sex could have the same mood-enhancing effects. “Since the steroids in birth control pills survive the digestion process, I would assume that the same holds true for at least some of the chemicals in semen,” Gallup says.

“I understand that among some gay males who have anal intercourse, it is not uncommon to attempt to retain the semen for extended periods of time,” he adds. “Suggesting, of course, that there may be psychological effects.” But further research will be needed to confirm whether exposure to semen through oral or anal sex really does affect mood in heterosexual or homosexual partners.

But why should semen have such an effect? “It makes no sense to me for this phenomenon to have evolved,” says Satoshi Kanazawa, an evolutionary psychologist at the Indiana University of Pennsylvania. But Gallup counters that men whose semen promotes long-term mood enhancement might have more chances to indulge in sexual activity.