Does exposure to semen make women happy? Apparently so, according to the remarkable conclusion of a study comparing women whose partners wear condoms with those whose partners don't.

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The study 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," said Gordon Gallup, a 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."

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, women who never or sometimes used condoms got more depressed the longer the interval since they last had sex. 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And Gallup told New Scientist that his team already has unpublished data from a larger group of 700 women confirming these findings.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 said.

But is it really the semen that affects women's mood?

The researchers said 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 said.

But the results aren't a complete surprise because semen does contain several mood-altering hormones, some of which have been detected in a women's blood within hours of exposure to semen.

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.

"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 said.

Previous studies published in New Scientist have suggested that exposure to semen -- either vaginally or orally -- may help prepare a women's immune system for pregnancy.

But why should semen have such a mood-altering effect?

"It makes no sense to me for this phenomenon to have evolved," said Satoshi Kanazawa, an evolutionary psychologist at the Indiana University of Pennsylvania.