The more a woman has sex with her partner, the more committed she becomes, and the less attracted she is to other men — at least in the short-term. I blogged about this study several weeks ago and offered explanations both hormonal and psychological. But another theory came to my attention today in the form of a study on fruit flies.

For decades, scientists have known that female Drosophila refuse sex for a period of time after contact with seminal fluid. This may give a reproductive edge to the first male with whom a female has sex. A study led by Geoffrey Findlay at the University of Washington has finally shed some light on the phenomenon. It turns out there are proteins in seminal fluid, many of which transfer to the female after sex. Some of these proteins are warrior-like — they fight the sperm of rival males. Other proteins are like hypnotists — entering the female’s circulatory system, they may hormonally dampen her interest in sex with other males. Somewhat disturbingly, the more semen a female is exposed to, the more influence the male has over her reproductive tract and her mating behavior. Semen, it seems, has mind control properties.

So what does this mean for humans? The same study hasn’t been done on men and women, but researchers speculate that many of the same proteins are in the semen of human males. “Does semen make you happier?” is one of the questions posed in BLONDES. In answering it I describe a fascinating study at SUNY Albany that found that, yes, women who are regularly exposed to their partner’s semen are less depressed than women who use condoms most of the time. Hormones and proteins are absorbed through the vagina, enter the bloodstream, and possibly breach the blood-brain barrier. Might these factors in men’s semen manipulate women’s minds (and bodies) in the way that fruit fly spunk affects female fruit flies? Perhaps, but it’s hard to prove. Women’s love lives (and psyches) are a little more complicated than those of Drosophila. After all, a woman in love with the man with whom she has unprotected sex would be happier and more faithful anyway. It’s a puzzle for all scientists who study human sexuality: Where to draw the line between chemical love and Eros? The seminal and the sacred?

n.b. Here is a list of the potential mind-altering “ingredients” in sperm.