I recommend reading the entirety of My Genome, My Self — by Steven Pinker (The Stuff of Thought, The Blank Slate) — but I’ll excerpt one particular passage to provide a taste:
[T]here is nothing like perusing your genetic data to drive home its limitations as a source of insight into yourself. What should I make of the nonsensical news that I am “probably light-skinned” but have a “twofold risk of baldness”? These diagnoses, of course, are simply peeled off the data in a study: 40 percent of men with the C version of the rs2180439 SNP are bald, compared with 80 percent of men with the T version, and I have the T. But something strange happens when you take a number representing the proportion of people in a sample and apply it to a single individual. The first use of the number is perfectly respectable as an input into a policy that will optimize the costs and benefits of treating a large similar group in a particular way. But the second use of the number is just plain weird. Anyone who knows me can confirm that I’m not 80 percent bald, or even 80 percent likely to be bald; I’m 100 percent likely not to be bald. The most charitable interpretation of the number when applied to me is, “If you knew nothing else about me, your subjective confidence that I am bald, on a scale of 0 to 10, should be 8.” But that is a statement about your mental state, not my physical one. If you learned more clues about me (like seeing photographs of my father and grandfathers), that number would change, while not a hair on my head would be different. Some mathematicians say that “the probability of a single event” is a meaningless concept.Even when the effect of some gene is indubitable, the sheer complexity of the self will mean that it will not serve as an oracle on what the person will do. The gene that lets me taste propyl thiouracil, 23andMe suggests, might make me dislike tonic water, coffee and dark beer. Unlike the tenuous genes linked to personality or intelligence, this one codes for a single taste-bud receptor, and I don’t doubt that it lets me taste the bitterness. So why hasn’t it stopped me from enjoying those drinks? Presumably it’s because adults get a sophisticated pleasure from administering controlled doses of aversive stimuli to themselves. I’ve acquired a taste for Beck’s Dark; others enjoy saunas, rock-climbing, thrillers or dissonant music. Similarly, why don’t I conform to type and exploit those fast-twitch muscle fibers (thanks, ACTN3 genes!) in squash or basketball, rather than wasting them on hiking? A lack of coordination, a love of the outdoors, an inclination to daydream, all of the above? The self is a byzantine bureaucracy, and no gene can push the buttons of behavior by itself. You can attribute the ability to defy our genotypes to free will, whatever that means, but you can also attribute it to the fact that in a hundred-trillion-synapse human brain, any single influence can be outweighed by the product of all of the others.
Even if personal genomics someday delivers a detailed printout of psychological traits, it will probably not change everything, or even most things. It will give us deeper insight about the biological causes of individuality, and it may narrow the guesswork in assessing individual cases. But the issues about self and society that it brings into focus have always been with us. We have always known that people are liable, to varying degrees, to antisocial temptations and weakness of the will. We have always known that people should be encouraged to develop the parts of themselves that they can (“a man’s reach should exceed his grasp”) but that it’s foolish to expect that anyone can accomplish anything (“a man has got to know his limitations”). And we know that holding people responsible for their behavior will make it more likely that they behave responsibly. “My genes made me do it” is no better an excuse than “We’re depraved on account of we’re deprived.”