The Joy of Sex emphasizes the importance of male scent in female sexual selection. “When it's a question of bed or not-bed, they let their noses lead them,” it says. It turns out that science supports this claim.
All else being equal, women decide with their noses, according to Swiss biologist Claus Wedekind, who explored the question in 1995. The human female, he learned, evolved over the eons to do an amazing thing; she can sniff out a superior genetic match specifically for her, a personalized partner selection mechanism that results in offspring more likely to survive.
The mechanism is her nose, and the active agent in infant survival is a bolstering of the major histocompatibility complex, a group of genes that determine the ability of the child’s immune system to identify invading bodies. As with so many aspects of human biology, the more diversity, the better; the more different the MHC genes of the father and mother, the stronger the immune system of the child.
This amazing ability – improved mate selection for immune system robustness, by female olfaction – was discovered in a very innovative experiment, since replicated many times and in many variations.
Wedekind gathered a group of male college students and gave them new t-shirts, having them wear them to sleep two nights in a row, without a shower, without any cologne, so the shirts would absorb their natural scent. He then had a group of female students smell the shirts, rating the scent according to preference. The women tended to prefer the scents of men with MHC genes that were the most dissimilar to their own. “This smells like my boyfriend,” said several of the women.
As fascinating as this finding is, its long-term implication is even more so: for women to have this capability, it had to evolve – it had to be a driver in mate selection that took hold in the general human population, a key feature in mating, a significant factor that achieved its position in the genome, to be handed down to present-day females.
And for this to have happened, the female must have been the sexual choosers in prehistory – not the male, as is commonly assumed. Since women are this choosy as a result of evolution, given the freedom to be, then the myth of the male alpha clubbing them over the head can’t be correct. The Paleolithic woman was clearly not subordinate.
How did this female trait evolve, and what were the real-world effects?
Among chimpanzees – our closest relatives, along with the bonobo – males out-migrate from their birth tribes into neighboring tribes, increasing genetic diversity. But among the bonobo, it is the female, not the male, who out-migrates to other groups, likewise increasing genetic diversity.
A look at the mitochondrial DNA distribution of human beings gives us the answer: like the bonobo, human females were the cross-tribe migrators in Paleolithic times. And this is a fit with the Stinky T-shirt phenomenon – males from neighboring tribes, being more genetically different, would be better mating choices than males in one’s own tribe. The female olfactory response would establish a familial male as less attractive and thus a poor genetic choice, while responding sexually to the scent of a completely unrelated male would yield stronger offspring – and explain how this trait took hold in the genes of human females.
And, finally, the Stinky T-Shirt Test explains why women aren’t nearly as enthusiastic about male cologne and body spray as men might hope: it sort of blurs their sexual vision, and they can’t be blamed for not being crazy about that…
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