Personal genomics is here to stay. The science will improve as efforts like the Personal Genome Project amass huge samples, the price of sequencing sinks and biologists come to a better understanding of what genes do and why they vary. People who have grown up with the democratization of information will not tolerate paternalistic regulations that keep them from their own genomes, and early adopters will explore how this new information can best be used to manage our health. There are risks of misunderstandings, but there are also risks in much of the flimflam we tolerate in alternative medicine, and in the hunches and folklore that many doctors prefer to evidence-based medicine. And besides, personal genomics is just too much fun.

At the same time, there 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 propyl­thiouracil, 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.”

Many of the dystopian fears raised by personal genomics are simply out of touch with the complex and probabilistic nature of genes. Forget about the hyperparents who want to implant math genes in their unborn children, the “Gattaca” corporations that scan people’s DNA to assign them to castes, the employers or suitors who hack into your genome to find out what kind of worker or spouse you’d make. Let them try; they’d be wasting their time.

The real-life examples are almost as futile. When the connection between the ACTN3 gene and muscle type was discovered, parents and coaches started swabbing the cheeks of children so they could steer the ones with the fast-twitch variant into sprinting and football. Carl Foster, one of the scientists who uncovered the association, had a better idea: “Just line them up with their classmates for a race and see which ones are the fastest.” Good advice. The test for a gene can identify one of the contributors to a trait. A measurement of the trait itself will identify all of them: the other genes (many or few, discovered or undiscovered, understood or not understood), the way they interact, the effects of the environment and the child’s unique history of developmental quirks.

It’s our essentialist mind-set that makes the cheek swab feel as if it is somehow a deeper, truer, more authentic test of the child’s ability. It’s not that the mind-set is utterly misguided. Our genomes truly are a fundamental part of us. They are what make us human, including the distinctively human ability to learn and create culture. They account for at least half of what makes us different from our neighbors. And though we can change both inherited and acquired traits, changing the inherited ones is usually harder. It is a question of the most perspicuous level of analysis at which to understand a complex phenomenon. You can’t understand the stock market by studying a single trader, or a movie by putting a DVD under a microscope. The fallacy is not in thinking that the entire genome matters, but in thinking that an individual gene will matter, at least in a way that is large and intelligible enough for us to care about.

So if you are bitten by scientific or personal curiosity and can think in probabilities, by all means enjoy the fruits of personal genomics. But if you want to know whether you are at risk for high cholesterol, have your cholesterol measured; if you want to know whether you are good at math, take a math test. And if you really want to know yourself (and this will be the test of how much you do), consider the suggestion of François La Rochefoucauld: “Our enemies’ opinion of us comes closer to the truth than our own.”