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In his fascinating book The Mating Mind, Geoffrey Miller looks at the issue of how sexual selection has shaped the human mind and the human body. Sexual selection, as the gentle reader will recall, was proposed by Darwin as a second force shaping evolution, alongside the better-known natural selection.

Natural selection increases traits that increase a creature's survival. Over generations, this can explain things like the giraffe's neck, or the beetle's camouflage.

Darwin explained that giraffes that were a bit taller or beetles that were a bit harder for predators to spot tended to live longer and produce more offspring. And those offspring carried these successful traits on to their offspring, and so over generations living things have gotten better and better adapted to their environments and the task of survival.

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Yet the mechanism of natural selection can't explain everything about the direction of evolution. Why have some kinds of elk and moose evolved such tremendous horns that they are seriously hampered in their ability to forage and to escape predators? Why do peacocks have such enormous tails that they can barely fly?

Darwin's answer was that a second force-- sexual selection-- was involved. We see evolution by sexual selection when some trait increases a creature's success in attracting sexual partners. So apparently, peacocks have evolved their enormous tails because peahens like them. And moose have enormous racks because moosettes find them enormously fetching.

Miller argues in his book that many of the features that distinguish humans from other primates may be the result of sexual selection rather than natural selection. A good example is the evolution of the penis. In gorillas and orangutans, the average erect penis is less than two inches long. In chimps, it is only three inches long. Why is the average human dick more than twice the size of a gorilla's? It doesn't seem that this difference could be related to better function-- a two-inch dick transfers sperm just as well as a six-inch dick.

Instead, Miller suggest that penises have gotten bigger over the course of evolution because women like bigger dicks. Especially before the invention of clothing, women could size up their prospective sexual partners easily. Their choice of males that had bigger muscles may have related to their partners' reproductive fitness, but females' preference for bigger dicks may have been essentially sexually aesthetic.

What of the fact that many modern women claim to not care about penis size? The theory of sexual selection doesn't claim that all females have the same preferences for their sexual partners. It would be sufficient for the theory if more than half of the females preferred a six-inch dick to a two-inch dick. Sexual selection would then tend to favor the reproductive chances of men with larger penises.

It seems obvious to Blanche that gay men show the preference for larger dicks that must have spurred the evolutionary scenario outlined. A visit to any gay sauna will demonstrate that the big-dicked attract far more partners than the small-dicked. That suggests that a preference for bigger sexual body parts (breast, dicks, butts) is somehow not gender specific, but common to both males and females.

Miller says very little about homosexuality in his book, other than to point out that evolution must work via reproduction. So although humans may have displayed a range of bisexual behaviors over history, it's the heterosexual encounters that had the strong impact on human evolution. Fair enough-- those who have kids influence the genes of the future generation, while those who don't have kids can influence the culture they grow up in.

But something in this evolutionary story may also give us a moment's pause. Sexual attraction operates largely on a preconscious and irrational level-- we can't explain why we like big muscles or big dicks other than to say that we like them because we like them.

But sexual desire may drive a large part of our lives-- leading us to choose certain partners because of the shapes of their bodies, rather than the natures of their minds. Would we be happier if we thought more carefully and consciously about our sexual attractions, or are we-- like the peacock-- destined to live with the weighty plumage of our evolutionary heritage?