Corrected on 2021-06-16T00:00:00-04:00
An earlier version of this story incorrectly identified David Epstein as a senior editor at Sports Illustrated. At the time this story was published, he had recently changed jobs from senior writer at Sports Illustrated to reporter with ProPublica.
An earlier version of this story also stated incorrectly that David Epstein told NPR that if one could measure the ankles and calves of runners at the Olympic starting line, one could statistically predict the winner. Epstein said that the Kalenjin tend to have extremely thin ankles and extremely thin calves, which improves their running economy.
An earlier version also stated academics told Epstein they had evidence of genetic advantage among different groups they wouldn't share. In fact, Epstein said academics had evidence of genetic differences unrelated to ability they wouldn't publish.