Genome sequencing is getting cheaper and faster, and more and more people are having it done. A new addition to the ranks is Steve Pinker, who contemplates the details of his personal genome in an interesting essay. It’s got to be fascinating, in a terribly self-centered way — I’d love to have a copy of mine someday. It’s an opportunity to see a manifestation of one’s own lineage, your biological history all laid out for you. There’s the ability to compare with others, and see hints of statistical correlations and associations with specific traits and even, unpleasantly, diseases. Pinker also makes the point that you are not determined by your genome — the man famously has a wild head of hair, and as it turns out, he’s also carrying a bit of sequence that seems to predispose carriers to baldness.

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.

Another thing I should think having a copy of your genome should drive home is how much of it is incomprehensible; we simply don’t know what most of it does, and even in the example mentioned above, we don’t have a causal relationship between one variant of the rs2180439 SNP and head hair, only a rough correlation. That’s the promise of the future, that we can now get copies of this book of our genome…we just have to get to work learning how to read it.

I’ve got my eye on the progress in genome sequencing. When the price hits $1000 (which isn’t at all unlikely to occur in my lifetime), I know I’m going to have it done, just because it’s a book I’ve been waiting most of my life to read.