Epstein talked to me recently about why Usain Bolt is so fast, a little-discussed reason so many boxers and football players end up with brain injuries, and why it's dangerous to believe that intellect and athleticism—brains and brawn—are mutually exclusive.

Responses have been edited for clarity and length.

You've written about sports medicine and sports science before. At what point did you say to yourself, "Well, this is becoming a book"?

I started thinking about these things, not quite in genetics terms but in nature-vs.-nurture terms, back when I was in high school. We had a big Jamaican population in my high school, and that made track and field a really popular sport. That phenomenon made me curious. And later, I was running in college at Columbia against a lot of Kenyan guys—who I discovered were largely from the same part of Kenya. And since I would do the same training as the guys I was running with, I would have expected that we would all cross the line in the race at more or less the same time. But it wasn't like that at all: Instead, it would get more and more different. I started to marvel at that, and I got interested in training theology and those things.

Then, my sophomore year of college, my former training partner dropped dead during a track meet. It turned out he had this genetic disease related to sudden cardiac death, and that's what got me into writing about sports science in the first place—wanting to write about sudden cardiac death in athletes.

Your book lays out a lot of heavy-duty science, but what's so effective about it, I think, is that it also puts a memorable, sometimes already-recognizable human face on it. You talked to so many athletes and read up on so much research—which one of these cases you studied taught you the most?

Ooh. The contrast between Donald Thomas and Stefan Holm, the high jumpers, probably—that really stuck with me.

Stefan Holm is this highly dedicated Swedish high jumper who had started inching up year after year after year until he was a world champion and an Olympic gold medalist. I think most people think of jumping as a thing you either have or you don't, but the degree to which he transformed himself, I didn't know was possible. He was this epitome of the "nurture" approach. He had some talent innately, just not as much you'd have thought.

And then to see him get beat by Donald Thomas in 2007, a guy who doesn't even really like the high jump, who had only been doing it for about a year, but who's just got the right genes and the right build to respond incredibly well to training. It just made me realize that there are so many different paths to the same basic physical outcome.

One of the most intriguing things you talk about in The Sports Gene is why it seems like Jamaican runners just seem to be really fast.