After the First World War, the U.S. War Department published a report called “Defects Found in Drafted Men,” which detailed how the incidence of goitre varied from state to state, with rates forty to fifty times as high in places like Idaho, Michigan, and Montana as in coastal areas.

The story is not dissimilar from Epstein’s account of Kenyan distance runners, in whom accidents of climate and geography combine to create dramatic differences in abilities. In the early years of the twentieth century, the physiological development of American children was an example of the “fantastic menagerie that is human biological diversity.”

In this case, of course, we didn’t like the fantastic menagerie. In 1924, the Morton Salt Company, at the urging of public-health officials, began adding iodine to its salt, and initiated an advertising campaign touting its benefits. That practice has been applied successfully in many developing countries in the world: iodine supplementation has raised I.Q. scores by as much as thirteen points—an extraordinary increase. The iodized salt in your cupboard is an intervention in the natural order of things. When a student from the iodine-poor mountains of Idaho was called upon to compete against a student from iodine-rich coastal Maine, we thought of it as our moral obligation to redress their natural inequality. The reason debates over élite performance have become so contentious in recent years, however, is that in the world of sport there is little of that clarity. What if those two students were competing in a race? Should we still be able to give the naturally disadvantaged one the equivalent of iodine? We can’t decide.

Epstein tells us that baseball players have, as a group, remarkable eyesight. The ophthalmologist Louis Rosenbaum tested close to four hundred major- and minor-league baseball players over four years and found an average visual acuity of about 20/13; that is, the typical professional baseball player can see at twenty feet what the rest of us can see at thirteen feet. When Rosenbaum looked at the Los Angeles Dodgers, he found that half had 20/10 vision and a small number fell below 20/9, “flirting with the theoretical limit of the human eye,” as Epstein points out. The ability to consistently hit a baseball thrown at speeds approaching a hundred miles an hour, with a baffling array of spins and curves, requires the kind of eyesight commonly found in only a tiny fraction of the general population.

Eyesight can be improved—in some cases dramatically—through laser surgery or implantable lenses. Should a promising young baseball player cursed with normal vision be allowed to get that kind of corrective surgery? In this instance, Major League Baseball says yes. Major League Baseball also permits pitchers to replace the ulnar collateral ligament in the elbow of their throwing arm with a tendon taken from a cadaver or elsewhere in the athlete’s body. Tendon-replacement surgery is similar to laser surgery: it turns the athlete into an improved version of his natural self.

But when it comes to drugs Major League Baseball—like most sports—draws the line. An athlete cannot use a drug to become an improved version of his natural self, even if the drug is used in doses that are not harmful, and is something that—like testosterone—is no more than a copy of a naturally occurring hormone, available by prescription to anyone, virtually anywhere in the world.

Baseball is in the middle of one of its periodic doping scandals, centering on one of the game’s best players, Alex Rodriguez. Rodriguez is among the most disliked players of his generation. He tried to recover from injury and extend his career through illicit means. (He has appealed his recent suspension, which was based on these allegations.) It is hard to think about Rodriguez, however, and not think about Tommy John, who, in 1974, was the first player to trade in his ulnar collateral ligament for an improved version. John used modern medicine to recover from injury and extend his career. He won a hundred and sixty-four games after his transformation, far more than he did before science intervened. He had one of the longest careers in baseball history, retiring at the age of forty-six. His bionic arm enabled him to win at least twenty games a season, the benchmark of pitching excellence. People loved Tommy John. Maybe Alex Rodriguez looks at Tommy John—and at the fact that at least a third of current major-league pitchers have had the same surgery—and is genuinely baffled about why baseball has drawn a bright moral line between the performance-enhancing products of modern endocrinology and those offered by orthopedics.

The other great doping pariah is Lance Armstrong. He apparently removed large quantities of his own blood and then re-infused himself before competition, in order to boost the number of oxygen-carrying red blood cells in his system. Armstrong wanted to be like Eero Mäntyranta. He wanted to match, through his own efforts, what some very lucky people already do naturally and legally. Before we condemn him, though, shouldn’t we have to come up with a good reason that one man is allowed to have lots of red blood cells and another man is not?

“I’ve always said you could have hooked us up to the best lie detectors on the planet and asked us if we were cheating, and we’d have passed,” Lance Armstrong’s former teammate Tyler Hamilton writes in his autobiography, “The Secret Race” (co-written with Daniel Coyle; Bantam). “Not because we were delusional—we knew we were breaking the rules—but because we didn’t think of it as cheating. It felt fair to break the rules.”

“The Secret Race” deserves to be read alongside “The Sports Gene,” because it describes the flip side of the question that Epstein explores. What if you aren’t Eero Mäntyranta?

Hamilton was a skier who came late to cycling, and he paints himself as an underdog. When he first met Armstrong—at the Tour DuPont, in Delaware—he looked around at the other professional riders and became acutely conscious that he didn’t look the part. “You can tell a rider’s fitness by the shape of his ass and the veins in his legs, and these asses were bionic, smaller and more powerful than any I’d ever seen,” he writes. The riders’ “leg veins looked like highway maps. Their arms were toothpicks. . . . They were like racehorses.” Hamilton’s trunk was oversized. His leg veins did not pop. He had a skier’s thighs. His arms were too muscled, and he pedalled with an ungainly “potato-masher stroke.”

When Hamilton joined Armstrong on the U.S. Postal Service racing team, he was forced to relearn the sport, to leave behind, as he puts it, the romantic world “where I used to climb on my bike and simply hope I had a good day.” The makeover began with his weight. When Michele Ferrari, the key Postal Service adviser, first saw Hamilton, he told him he was too fat, and in cycling terms he was. Riding a bicycle quickly is a function of the power you apply to the pedals divided by the weight you are carrying, and it’s easier to reduce the weight than to increase the power. Hamilton says he would come home from a workout, after burning thousands of calories, drink a large bottle of seltzer water, take two or three sleeping pills—and hope to sleep through dinner and, ideally, breakfast the following morning. At dinner with friends, Hamilton would take a large bite, fake a sneeze, spit the food into a napkin, and then run off to the bathroom to dispose of it. He knew that he was getting into shape, he says, when his skin got thin and papery, when it hurt to sit down on a wooden chair because his buttocks had disappeared, and when his jersey sleeve was so loose around his biceps that it flapped in the wind. At the most basic level, cycling was about physical transformation: it was about taking the body that nature had given you and forcibly changing it.