As an 800-meter runner at Columbia University, David Epstein often tested himself against his training partner, Scott. Mr. Epstein was a walk-on who had clawed his way onto the varsity team. Scott, the son of two Canadian track standouts, had set a national age-group record in 1997, the same year David had taken up track as a high school junior.  Where Scott performed like a natural, David was a âtalentless dufferâ who would âchew through a crowbarâ to gnaw a quarter-second off his time. âI just had to be tougher than him, I thought, because I didnât have the talent,â Mr. Epstein writes.  But what exactly differentiated the two runners? Was Scott endowed with some gene for speed? Could Davidâs grit and determination overcome his apparent lack of innate ability? Where does the intersection between talent and practice lie?  These are the questions Mr. Epstein seeks to answer in this captivating book, which began as a feature in Sports Illustrated, where he is a senior writer. The bookâs title misleads, since he forcefully argues that no single known gene is sufficient to ensure athletic success. His answer to the question âNature or nurture?â is both.  If that sounds like a hedge, it isnât: instead, itâs a testament to the authorâs close attention to nuance. He approaches his subject like a scientist, stopping to examine the uncertainties and taking care not to overgeneralize.  The narrative follows Mr. Epsteinâs search for the roots of elite sport performance as he encounters characters and stories so engrossing that readers may not realize theyâre receiving an advanced course in genetics, physiology and sports medicine.  To illustrate that learned perceptual skills can trump innate reaction speed, for example, he revisits the slugger Albert Pujolsâs humiliating strikeout against Jennie Finch, an Olympic softball pitcher who throws underhand. While investigating a theory that practice is what distinguishes professional athletes from amateurs, Mr. Epstein meets a golf novice, Dan McLaughlin, who quit his job to test whether 10,000 hours of practice â called the âmagic number for true expertiseâ in Malcolm Gladwellâs best-seller âOutliersâ â could transform him into a P.G.A.-worthy professional. (He started in 2010 and is less than halfway to 10,000 hours; by June, his handicap was 5.5.) Whether (and how fast) chumps can become champs depends on their baseline ability and how rapidly they improve â factors highly influenced by genetics. After months of identical training, some exercisers make almost no fitness gains, while others increase their aerobic capacities by 50 percent or more. Scientists have identified more than 20 gene variants that can separate high responders from low ones.  Mr. Epstein argues that we often confuse innate talent with spirit or effort. Even traits like desire may arise from DNA (see the Iditarod dogs selectively bred for enthusiastic pulling), but that does not mean they come down to any single gene. Whether itâs running faster, standing taller or jumping higher, multiple genetic pathways may lead there.  In a particularly fascinating chapter, Mr. Epstein investigates an old theory that purports to explain why one small country, Jamaica, produces so many Olympic sprinters. The notion is that strong Africans were selected as slaves, that the strongest of them survived the voyage to Jamaica, and that the strongest survivors eventually escaped slavery and cloistered themselves in a remote region to form an isolated âwarriorâ gene stock that now produces world-class athletes.  It makes a convenient story. But it is belied by the DNA research of Yannis Pitsiladis, a biologist at the University of Glasgow, who finds no genetically distinct subgroup of Jamaican sprinters. It appears that Jamaica churns out sprinters because almost everyone on the island tries the sport.  Recent positive drug tests from two prominent sprinters may also help explain Jamaicaâs dominance, and itâs disappointing that the book devotes little space to the interplay of doping and genetics. But the omission is easy to forgive in a book that covers so much ground.  In light of the research on trainability, Mr. Epstein revises his own history. He and his college training partner ran the same workouts and pushed themselves to the point of vomiting, so perhaps Scott wasnât more talented after all. They were simply born with different gifts.  Mr. Epstein speculates that he had low baseline ability but a rapid training response that allowed him to improve quickly, while Scott began with a high level of baseline talent but less potential to improve. Yet it was David who got credit for guts and grit, while Scott was written off as a head case who was squandering his talent. Narratives matter, Mr. Epstein argues, and many of the ones we tell are flat wrong.  Correcting these false stories wonât be easy. In the final chapter, we meet Eero Mantyranta, a Finnish cross-country skier and three-time Olympic gold medalist born with a genetic mutation that gives him an extraordinarily high red-blood-cell count. Though the geneticist who discovered the mutation insists that it confers a significant advantage by allowing his blood to carry more oxygen, Mr. Mantyranta could not be convinced. It was his âdetermination and psycheâ that made him a champion, he says â his blood had nothing to do with it.  Christie Aschwanden is a science writer and a contributing editor at Runnerâs World.
David Epstein, a senior writer at Sports Illustrated and an 800-meter runner in college, searches for answers about athletic ability and genetics.