Few of us are as smart as we’d like to be. You’re sharper than Jim (maybe) but dull next to Jane. Human intelligence varies. And this matters, because smarter people generally earn more money, enjoy better health, raise smarter children, feel happier and, just to rub it in, live longer as well.

But where does intelligence come from? How is it built? Researchers have tried hard to find the answer in our genes. With the rise of inexpensive genome sequencing, they’ve analyzed the genomes of thousands of people, looking for gene variants that clearly affect intelligence, and have found a grand total of two.

One determines the risk of Alzheimer’s and affects I.Q. only late in life; the other seems to build a bigger brain, but on average it raises I.Q. by all of 1.29 points.

Other genetic factors may be at work: A report last year concluded that several hundred gene variants taken together seemed to account for 40 to 50 percent of the differences in intelligence among the 3,500 subjects in the study. But the authors couldn’t tell which of these genes created any significant effect. And when they tried to use the genes to predict differences in intelligence, they could account for only 1 percent of the differences in I.Q.