Sex at Dawn

Little’s concern is not misplaced. In 1995, Swiss biological researcher Claus Wedekind published the results of what is now known as the “Sweaty T-shirt Experiment.” He asked women to sniff T-shirts men had been wearing for a few days, with no perfumes, soaps, or showers. Wedekind found, and subsequent research has confirmed, that most of the women were attracted to the scent of men whose major histocompatibility complex (MHC) differed from her own.⁹ This preference makes genetic sense in that the MHC indicates the range of immunity to various pathogens. Children born of parents with different immunities are likely to benefit from a broader, more robust immune response themselves.

The problem is that women taking birth control pills don’t seem to show the same responsiveness to these male scent cues. Women who were using birth control pills chose men’s T-shirts randomly or, even worse, showed a preference for men with similar immunity to their own.¹⁰

Consider the implications. Many couples meet when the woman is on the pill. They go out for a while, like each other a lot, and then decide to get together and have a family. She goes off the pill, gets pregnant, and has a baby. But her response to him changes. There’s something about him she finds irritating—something she hadn’t noticed before. Maybe she finds him sexually unattractive, and the distance between them grows. But her libido is fine. She gets flushed every time she gets close enough to smell her tennis coach. Her body, no longer silenced by the effects of the pill, may now be telling her that her husband (still the great guy she married) isn’t a good genetic match for her. But it’s too late. They blame it on the work pressure, the stress of parenthood, each other..

Because this couple inadvertently short-circuited an important test of biological compatibility, their children may face significant health risks ranging from reduced birth weight to impaired immune function.² How many couples in this situation blame themselves for having “failed” somehow? How many families are fractured by this common, tragic, undetected sequence of events?¹²

Psychologist Richard Lippa teamed up with the BBC to survey over 200,000 people of all ages from all over the world concerning the strength of their sex drive and how it affects their desires.¹³ He found the same inversion of male and female sexuality: for men, both gay and straight, higher sex drive increases the specificity of their sexual desire. In other words, a straight guy with a higher sex drive tends to be more focused on women, while higher sex drive in a gay guy makes him more intent on men. But with women—at least nominally straight women—the opposite occurs: the higher her sex drive, the more likely she’ll be attracted to men and women. Lesbians showed the same pattern as men: a higher sex drive means more women-only focus. Perhaps this explains why nearly twice as many women as men consider themselves bisexual, while only half as many consider themselves to be exclusively gay.