Kluge: The Haphazard Construction of the Human Mind (Houghton Mifflin; 2008)

have evolved a "taste" for fruit; all well and good. But those sugar sensors can't tell the difference between a real fruit and a synthetic fruit that packages the flavor without the nutrition. We humans (collectively, if not as individuals) have figured out thousands of ways to trick our pleasure centers. Does the tongue like sweetness of fruit? Aha! Can I interest you in some Life Savers? Orange soda? Fruit juice made entirely from artificial flavors? A ripe watermelon may be good for us, but a watermelon-flavored candy is not.

And watermelon-flavored candy is only the start. The vast majority of the mental mechanisms we use for detecting pleasure are equally crude, and thus easily hoodwinked. In general, our pleasure detectors tend to respond not just to some specific stimulus that might have been desirable in the environment of our ancestors, but to a whole array of other stimuli that may do little for our genes. The machinery for making us enjoy sex, for example, causes us to revel in the activity, just as any reasonable evolutionary psychologist would anticipate, but not just when sex might lead to offspring (the narrowest tuning one might imagine), or even to pair bonding, but much more broadly: at just about any time, under almost any circumstance, in twos and threes and solo, with people of the same sex and with people of the opposite sex, with orifices that contribute to reproduction and with other body parts that don't. Every time a person has sex without directly or indirectly furthering their reproductivity, some genes have been fooled.

The final irony, of course, is that even though sex is incredibly motivating, people often have it in ways that are deliberately designed not to produce children. Heterosexuals get their tubes tied, gay men continue to have unprotected sex in the era of HIV, and pedophiles pursue their interests even when they risk prison and community censure. From the perspective of genes, all of this, aside from sex for reproduction or parental pair bonding, is a giant mistake.

To be sure, evolutionary psychologists have tried to find adaptive value in at least one of these variations (homosexuality), but none of the explanations are particularly compelling. (There is, for example, the "gay uncle" hypothesis, according to which homosexuality persists in the population because gay people often invest considerable resources in the offspring of their siblings.)* A more reasonable accounting, in my view, is that homosexuality is just like any other variation on sexuality, an instance of a pleasure system that was only broadly tuned (toward intimacy and contact) rather than narrowly focused (on procreation) by evolution, co-opted for a function other than the one to which it was strictly adapted. Through a mixture of genetics and experience, people can come to associate all manner of different things with pleasure, and proceed on that basis, t

The situation with sex is fairly typical. A substantial portion of our mental machinery seems to exist in order to assess reward (a proxy for pleasure), but virtually all of that machinery allows a broader range of options than might (from a gene's-eye view) be ideal. We see this with enjoyment of sugar — a hot fudge sundae just about always brings pleasure, whether we need the calories or not — but also with more modern compulsions, like addiction to the Internet. This compulsion presumably begins with an ancestral circuit that rewarded us for obtaining information. As the psychologist George Miller put it, we are all "informavores," and it's easy to see how ancestors who liked to gather facts might have outpropagated those who showed little interest in learning new things. But once

*The trouble is there's no evidence that all that good uncle-ing (for relatives that are only one-eighth genetically related) offsets the direct cost of failing to reproduce. Other popular adaptationist accounts of homosexuality include the Sneaky Male theory (favored by Richard Dawkins) and the Spare Uncle theory, by which an uncle who stays home from the hunt can fill in for a dad who doesn't make it home.