1:32pm

Mon August 5, 2013
Author Interviews

Talent Or Skill?: Homing In On The Elusive 'Sports Gene'

Originally published on Mon August 5, 2013 11:59 pm

We've all had the experience of watching a great athletic performance — from gymnast Mary Lou Retton defying gravity to Michael Jordan sinking a mind-blowing turnaround jumper — and wondered: Were they born with that talent or can you get there with hard work and practice?

Sports Illustrated senior writer David Epstein says scientists are learning a lot more about the role of genetics in athletic performance. In his new book, The Sports Gene, he looks at whether big league hitters have naturally faster reflexes, whether some people are born with speed and that delicate question of whether African-Americans are better athletes than whites. Epstein says that science now has answers, or at least insights, into all those questions. He joins Fresh Air's Dave Davies to talk about the secret to hitting a fastball and why slow dogs win the Iditarod.


Interview Highlights

On the truth about baseball and softball hitter reflexes

"Going into it, I figured that they would have these superhuman reaction speeds because they face 100-mile-an-hour pitches everyday, and [softball pitcher] Jennie Finch's fastballs take exactly the amount of time as a mid-'90s baseball does. So the baseball comes at 60 feet 6 inches, 95 miles an hour; Jennie Finch throws from about 43 feet at about 65 miles an hour. Same exact time and the ball is bigger and yet they couldn't hit it all. It turns out that even the best hitters in the world have perfectly pedestrian reaction times."

On how, then, hitters manage to hit the ball

"They pick up on cues from the players' bodies before their pitch. So for a pitcher, without knowing it, the hitters are actually focusing in on the motion of the pitcher's shoulder and the pitcher's torso and hand. And then, as soon as the ball is released, [hitters focus] on what is called the flicker, which is a flashing pattern that the [ball's] red seams make as they rotate. And it's only picking up those anticipatory cues that allows the hitter to hit the ball.

"... This is a learned perceptual skill. And in fact, if you do a digital simulation, which some scientists have done, where you delete the pitcher's shoulder, [the Los Angeles Angels'] Albert Pujols becomes me, basically. You have to delete a little more than the shoulder to get him to that novice level, but he basically becomes a novice if you do that. And you can do the same thing with tennis players."

On breeding sled dogs for the Iditarod

"As I write in the book, it's not the fastest dogs that win. So sled dogs, when they were first bred for racing, the mushers bred for speed traits, and the idea was to race between checkpoints in the Iditarod very, very quickly, and they topped out in their top speed. And then what became popular because of [four-time Iditarod champion] Lance Mackey, who I write about, who couldn't afford to breed fast dogs, he had to breed instead dogs that were slower but would just go and go and go, and had a drive to pull the sled all the time and never wanted to stop. And it turns out — and scientists look at some of those sled dogs — they've actually been bred for motivation, they've been bred for work ethic. And the speeds of the Iditarod races are getting faster because the dogs are pulling longer, not faster."

On the question of whether African-Americans are predisposed for athletics

"Most of our ancestry as humans has occurred in Africa, so people have been in Africa for far longer than they've been outside of Africa. So genes for hundreds of thousands of years were evolving, changing inside of Africa, and then just a tiny group of people — maybe no more than 150 people, or a small group — left East Africa en route to populating the rest of the world. At each stop, their genes changed to accommodate their environments and sometimes just by random chance. ... But what this means is that most of the genetic differences that have been built up in our history are all still in Africa. All of us outside of Africa are just tiny subsets of a tiny subset that left Africa. So if you got rid of everyone in the world outside of Africa you would lose a little, but you would preserve most of the genetic variation for all of humanity.

"... [For] a particular trait, you might find the most diversity within an African population, as opposed to comparing someone in an African population and someone in a European population. So you might find the fastest 10 runners and the slowest 10 runners. But nobody is looking for the slowest 10 runners."

Copyright 2013 NPR. To see more, visit http://www.npr.org/.

Transcript

TERRY GROSS, HOST:

This is FRESH AIR. I'm Terry Gross. We've all had the experience of watching a great athletic performance, a gymnast like Mary Lou Retton defying gravity, Michael Jordan sinking a mind-blowing turnaround jumper or Tiger Woods executing a seemingly impossible shot and wondering were they born with that talent, or can you get there with hard work and practice.

Our guest David Epstein says scientists are learning a lot more about the role of genetic in athletic performance, and in his new book he explores the subject in a number of ways, like whether big-league hitters have naturally faster reflexes, whether some people are born with speed, and that delicate question, are blacks, overall, better athletes than whites.

He finds that science now has answers, or at least insights, into all those questions. David Epstein is a senior writer for Sports Illustrated, where he covers sports science, medicine and Olympic sports. He spoke with FRESH AIR contributor about his new book "The Sports Gene."

DAVE DAVIES, HOST:

David Epstein, welcome to FRESH AIR. Your first chapter is called "Beat By an Underhand Girl." This is a lovely story. Tell us what happened here.

DAVID EPSTEIN: Well, thanks for having me, and that first chapter is about Major League Baseball players who are in an exhibition facing Jennie Finch, who at the time was one of the world's greatest softball pitchers. And frankly they sort of giggled their way up to bat against her because they figured, well, here comes this ball that's bigger than the one we're used to, here's this woman who's throwing it underhand, tosses, and it takes about the same time to get to them as a major league pitch does.

DAVIES: And they are hitting softballs, not hard balls, right? Yeah.

EPSTEIN: Indeed, indeed. She was pitching them softballs, exactly. And so they all felt like that would basically tee off on her and take batting practice, hit a couple balls out of the park, and it turned out that they couldn't even hit foul balls off of her, much less get any actual hits. And so it turned into actually a television show where this pitcher Jennie Finch would go around to major league camps and just whiff the best hitters in the world.

DAVIES: Right, and when you say the best hitters in the world, give us some names.

EPSTEIN: Well Albert Pujols, who was the best hitter of a generation, is one of the first people that Jennie faced, and when I talked to Jennie Finch, she actually said that she was afraid that he would hit the ball right back at her. So they thought about putting a net in front of her before she did this. And then within about two pitches, she realized that he would never even get close to the ball. I mean, he was missing by a half a foot sometimes.

Some of the major leaguers she was facing, Barry Bonds, they would swing and miss so hard they would spin around.

DAVIES: Right so the best of the best, and they can't even make contact with her pitches. Now there's a fascinating point that arises from this, and one of them kind of dispels the notion that these big-league hitters are blessed with amazing reflexes. It turns out they aren't, right?

EPSTEIN: That's correct. That's what I thought. So going into it, I figured that they would have these sort of super-human reaction speeds because they face 100-mile-per-hour pitches every day. And Jennie Finch's fastballs take exactly the same amount of time as a mid-90s baseball does.

So the baseball comes from 60 feet and six inches at mid-95 miles an hour. Jennie Finch throws from 43 feet at about 65 miles an hour, same exact time, and the ball is bigger, and yet they couldn't hit it at all.

It turns out that even the best hitters in the world have perfectly pedestrian reaction times. I actually scored better on a visual reaction time test than Albert Pujols did.

DAVIES: Ooh, there's bragging rights for you.

(LAUGHTER)

EPSTEIN: Although he was only in the 66th percentile compared to a group of college students. So I don't know that I'm special. He's just not that special in that regard.

DAVIES: OK, so it's not their reflexes, but there is an explanation, and what is it?

EPSTEIN: They pick up on cues from the player's body before their pitch. So for a pitcher, they're, without knowing it, the hitters are actually focusing in on the motion of the pitcher's shoulder and the pitcher's torso and hand, and then as soon as the ball's released on what's called the flicker, which is a flashing pattern that the red seams make as they rotate, and it's only picking up those anticipatory cues that allows a hitter to hit the ball because the - basically the reaction time of major league hitters and teachers and lawyers and doctors to a visual stimulus is about 200 milliseconds. That's one-fifth of a second.

That's half the total transit time of a fastball. We simply do not have a biological system that is capable of tracking objects moving at that speed. So once the ball is halfway to the hitter, he might as well close his eyes. He's already swinging wherever he's swinging. So in that first half of the pitch, right when the ball's out of the hand, the hitter has to have picked up cues from the pitcher's body and the movement of the ball to know where it's going ahead of time.

DAVIES: All right, so it's not that they're reacting more quickly, it's that they're reading the picture they're seeing and anticipating where the ball is going to be. It's like it's the software, not the hardware, right?

EPSTEIN: Exactly, exactly. It's - this is a learned perceptual skill, and in fact if you do a digital simulation, which some scientists have done, where you delete the pitcher's shoulder, Albert Pujols becomes me, basically. You have to delete a little more than the shoulder to get to that novice level, but he basically becomes a novice if you do that, and you can do the same thing with tennis players.

The problem, when he was facing Jennie Finch, was that he didn't know this. He didn't know how he is able to hit a baseball. And Jennie Finch, her shoulder motion is completely different from a Major League Baseball player. The seams of the ball are completely different. The rotation of the ball is different. And so he was completely stripped of the anticipatory cues that would allow him to seem like he has super-human reflexes.

DAVIES: You know, I remember reading Pete Rose's autobiography, and as a young kid he was not a particularly outstanding athlete, but he taught himself to be a switch hitter and just practiced and practiced and practiced and got really, really good. And I guess this raises the question of sort of is thousands of hours of training all you need to be a high-performing athlete.

And in fact one of the things you find that geneticists have discovered is that we don't all respond equally to training, right.

EPSTEIN: That's exactly right. In fact that is sort of - more than anything that's a message that's emerging from exercise genetics. When I looked back at older papers and particularly in the sports psychology literature, the definition of talent that would be given in papers was basically skill that pre-exists any attempt to train.

But now, what genetics is teaching us is the more important kind of talent is actually your biological setup to respond well training, your ability to get more benefit from your one hour of training than your training partner's one hour of training, and that is emerging as the real talent.

DAVIES: You are a runner yourself, and you write about another runner who - both of you trained really hard. You got a lot better, and he didn't, right. What was going on here?

EPSTEIN: Exactly, that was my college training partner who was sort of a blue chip recruit, where I was just a walk-on, and we became training partners, and he came into college and into our seasons in great shape and then sort of stagnated. And I would come in in terrible shape. I mean, I went to a pulmonologist once, and his report said that I was - my aerobic function was consistent with very early stage emphysema.

And then I'd start training, and it was like somebody had injected me with rocket fuel. I would do the same exact training as this training partner of mine, stride for stride, day after day, and I would improve every race, and he would stagnate. And what happened was that our teammates and coaches sort of foisted upon us two different narratives.

For him it was this talented guy who, you know, was kind of a head case because he wasn't improving, and for me it was I was this talentless walk-on who was so tough that I was improving. I even got this award in college for overcoming significant difficulty to achieve success.

But later on I had my genes tested, and it turns out I have genes that predispose me to responding very well to aerobic exercise. And now looking back on it, it's clear to me, we were doing the exact same thing; it's clear to me that I wasn't moving at a faster pace than him because I was tougher, it's because I was responding to the training better.

DAVIES: And when you say you were responding, what's the physiology of it? What was happening in your body that was different?

EPSTEIN: So my aerobic capacity, for example, would go up very rapidly. I started with a low aerobic baseline, which is the amount of oxygen that I can use when I'm exercising. So I started quite low, actually, you know, pretty much very, very average, even for a non-athlete.

And as I would start to train, that would go up, and it would go up quickly. I mean, when I would go back to a pulmonologist, they would do these - do tests on me and say, you know, it looked like I was cured of asthma, basically, because suddenly I could expel more air, and my endurance would get so much greater. Everything about me would respond.

I would drop weight extremely quickly, much more easily when I started training compared to guys who were doing the exact same training that I was doing. And so that sort of won me this flattering narrative that I was the tough guy while my training partner was, you know, struggling mentally even though he was talented.

DAVIES: Are there genetic traits that make us better at getting the software to perform well in a sport, that help us to see a ball coming out of a pitcher's hand and recognize what it's going to do? Are there genetic differences in our ability, you know, with practice to pick up the right kind of patterns?

EPSTEIN: Right, so if our sort of genetic setup is our hardware, and the software, the skills that we learn as we train, there are sort of two different lines of evidence there. One is yes, we are starting to find genes that speed the download of software for certain people.

For example, there are genes associated with proteins in the brain, for example, and if people have a certain version, they learn motor skills more quickly and make less mistakes in repeat tests. That said, that picture is not totally clear on the genetic level. There are a lot of genes still left to be found because it turns out genetics is just way more complicated than we thought it would be a decade ago when the human genome was sequenced.

But there are innate traits that we know factor into that. So for example with a baseball player, they have - baseball hitters in the major leagues have an average visual acuity of 20/12, which means they can see from 20 feet away what I would have to stand at 12 feet away to see. And that appears to be an innate quality. It's based on the density of cones in your eye, basically. It's like the megapixel rating on the digital camera that is your eye, and it doesn't really change.

And so even though they're relying on learned perceptual cues, they are able to pick up those cues more quickly than the average person.

DAVIES: Wow, so a baseball player is simply seeing more. I mean, you know, we interviewed Reggie Jackson once, and I asked him can you really pick up the actual rotation of a pitch coming out of a pitcher's hand by looking at those tiny red threads in the ball. He said absolutely, you have to. And so visual acuity allows you to do that.

EPSTEIN: Absolutely. I remember listening to Keith Hernandez do some commentary, and he said, you know, on a four-seam slider, you see sort of this - like a red dot in the middle of a white circle, and if you didn't see that, and a guy threw a good slider, forget about it, you'd never hit it.

DAVIES: You write about somebody testing the vision of the Los Angeles Dodgers and Tommy Lasorda, the famous manager, wanting to know what good it was to him. What did he find out?

EPSTEIN: Well yeah, so this Louis Rosenbaum(ph), this scientist, he went to Dodger Town to do not just visual acuity, not just the sort of 20/20 test on baseball players but also looking at depth perception, contrast sensitivity, all measures of visual skills. And Tommy Lasorda was sort of skeptical and asked him to - fine, if that works, then don't look at any of these minor leaguers' stats who are in our camp and just tell me who's going to be good based on just their visual test scores.

And the first year, Dr. Rosenbaum picked Eric Karros, who went on to become the rookie of the year and have a long, successful professional career. And Lasorta challenged him again the next year, and Rosenbaum picked out Mike Piazza. Mike Piazza had been drafted only as basically a favor to Piazza's father, who was a friend of Lasorta. Nobody expected him to do anything. And he went on to become the greatest hitting catcher of all time.

DAVIES: So if you want to pick a good baseball player, forget about their reflexes, look at their eyes.

EPSTEIN: Looking at their eyes is definitely going to do more for - looking at the reflexes of Major League Baseball players will do nothing for you, so you might as well not do that. They don't score particularly well. So that gets you nothing. For the most part, even looking at their visual skills, though it's important, by the time they're at the major league level, they've been culled to the point where when they're tested their average visual acuity is usually 20/11 or 20/12, so you've already sort of, just by coincidence, sifted out most of the population.

DAVIES: We're speaking with David Epstein. He is a senior writer at Sports Illustrated. His new book is called "The Sports Gene." We'll talk more after a short break. This is FRESH AIR.

(SOUNDBITE OF MUSIC)

DAVIES: This is FRESH AIR, and if you're just joining us, our guest is David Epstein. He's a senior writer for Sports Illustrated. He's looked at the genetics of athletic performance, and his new book is called "The Sports Gene."

So you write that a lot of athletic performance involves practice, spending time, you know, learning the sport that you have to learn, but there are certain - there are genetic predispositions to improve more quickly, both physically and in some cases mentally.

You have a section about sled dogs in Alaska that people, you know, that the mushers drive through the Iditarod. What kind of dogs win, and what does it tell us?

EPSTEIN: Slow dogs win. So as I write in the book, it's not the fastest dogs that win. So sled dogs, when they were first bred for racing, the mushers bred for speed traits, and the idea was sort of to race between checkpoints in the Iditarod very, very quickly. And they sort of topped out in their top speed, and then what became popular, partly because of guys like Lance Mackey, who I write about, who couldn't afford to breed fast dogs, he had to breed instead dogs that were slower but that would just go and go and go and go and had a drive, had a drive to just pull the sled all the time and never, ever wanted to stop.

And it turns out, you know, and scientists look at some of those sled dogs, they've actually been bred for motivation. They've been bred for work ethic. And the speeds of the Iditarod races are only getting faster because the dogs are pulling longer, not faster.

DAVIES: So it's the dogs that just really want to pull that sled that win, that just have to do it?

EPSTEIN: That's right. As Lance Mackey told me, it's the dogs that will bore a whole in the earth. So he said his competitors will try to race between checkpoints at 12 or 15 miles per hour, and he'll try to go seven miles per hour for 19 straight hours, and he won four straight Iditarods.

And so now people have started sort of trying to copy those styles. I actually went on a sled dog - you know, as I was learning about this, I went on a sled dog trip and had one of his dogs in my team, and it took me, you know, a couple hundred meters to brake the sled because that dog kept pulling when I had the brakes on. And as soon as I stepped off, he was gone, pulling the sled, and I had to chase them for about a half-mile until the sled got stuck between two trees, and I could catch back up.

(LAUGHTER)

DAVIES: Now is there a parallel with human beings? Is, among humans, is there a genetic predisposition to a good work ethic, and is there evidence that that makes better athletes?

EPSTEIN: There is, and this was one of the most surprising things to me when I started researching the book, which was finding that it's sort of well known in the scientific literature that not only does our biology respond to physical activity, which is quite obvious, but our physical activity responds to our biology.

And some of the first genes that are being implicated in this have to do with the dopamine system, which is the brain's pleasure and reward system. And for a long time now, scientists have bred mice for voluntary running. So you just take a group of mice, separate the ones that run a lot on their own and the ones that don't, breed the high runners with the high runners, and in just a few generations you have mice that run so much that they'll get antsy and frustrated if they're not allowed to run.

And some of those gene variants also appear in people, like Pam Reed, who I talk about in the book, who I interviewed while she was running laps around the parking structure at LaGuardia because she hates sitting still.

DAVIES: Now if we've identified genes that will help you respond physically to training more, genes that kind of tell us that you have the kind of work ethic that will make you a great athlete, are people testing for that? Can you figure out whether you're likely to be a great athlete?

EPSTEIN: Well, testing is sort of just coming online for most of this, and to be quite honest I wouldn't recommend most of it. I've seen some of the marketing for direct-to-consumer testing, and it's vastly overstated. Most of these genes are one small piece in a 1,000-piece puzzle that includes not only other genes but also your environment, and you're much better off sort of testing yourself directly, by going out and trying something.

You know, it's like why would you look for the height genes when you can just have a tape measure and measure it directly.

DAVIES: We're speaking with David Epstein. He's a senior writer for Sports Illustrated. His new book is called "The Sports Gene: Inside the Science of Extraordinary Athletic Performance."

You know, before I even read the book, when I knew you were going to be addressing the issue of, you know, to what extent athletic performance and talent is, you know, hereditary and to what extent it's based on, you know, practice and development, I thought of Dennis Rodman, who's a pretty unique character in a whole lot of ways but was not a good athlete in high school, came to basketball late and became one of the all-time greats.

And then you write about Rodman in the book. Is there a lesson in his remarkable emergence as an NBA star?

EPSTEIN: It was - I interviewed Dennis, when I interviewed him for the book, he was telling me about, you know, he had hardly played any organized basketball through high school. He was working as a janitor at the Dallas-Fort Worth airport before he got fired for, sort of, stealing watches with a broom handle from a closed shop.

And then he grew, you know, about nine inches after high school, and in his words, his body all of a sudden knew how to do all these things that it didn't know how to do before. And so I think we, sort of, see that in who he became in that there were some basketball-specific skills, like shooting, that he never really developed, but he became this amazing raw athlete, and it's - it was really interesting to me because he was obviously an incredibly skilled athlete.

At the same time without that sort of miraculous puberty, he's certainly not a guy who gets inducted into the Hall of Fame, which is what happened to him.

DAVIES: Do you think he just was better at kind of learning the patterns of the games? Because you can be big and strong and still not be a good rebounder. There's a lot going into timing your jumps and getting leverage under the basket.

EPSTEIN: In fact there are some brain imaging studies that suggest that good rebounders are also looking for anticipatory cues, that they're actually mentally rehearsing what they're watching a shooter do in an attempt to predict where the ball's going to go. And I think that probably his, sort of, focus on rebounding and sort of not worrying about developing skills in the other part of the game gave him practice at that, sort of predicting where the ball would go better than other guys did.

And when I watched clips of him playing before I would interview him, I felt like I would see him moving in a direction before somebody else, like he was not only strong and could jump well, but had excellent anticipatory skills for rebounding.

DAVIES: David Epstein will continue his interview with FRESH AIR contributor Dave Davies in the second half of the show. Epstein is the author of the new book "The Sports Gene." He's also a senior writer for Sports Illustrated. I'm Terry Gross, and this is FRESH AIR.

(SOUNDBITE OF MUSIC)

GROSS: This is FRESH AIR. I'm Terry Gross. Let's get back to Dave Davies' interview with David Epstein, author of "The Sports Gene: Inside the Science of Extraordinary Athletic Performance." It explains the latest research investigating the role of genetics in athlete ability. Epstein is a senior writer for Sports Illustrated, where he covers sports science, medicine and Olympic sports.

DAVIES: One of the more sensitive questions that arises in any discussion of genetics in sports is whether African-Americans and Africans are better athletes than others. And one of the things that you write is that there's sort of an evolutionary explanation that might account for part of this. Do you want to explain that?

EPSTEIN: Well, most of our ancestry as humans has occurred in Africa. So people have been in Africa for far longer than they've been outside of Africa. So genes, for hundreds of thousands of years, were evolving, changing inside of Africa. And then just a tiny group of people - maybe no more than 150 people, or a small group - left East Africa en route to populating the rest of the world. And at each stop their genes change to accommodate their environments, and sometimes just by some random chance.

And so there hasn't been nearly as much time outside of Africa for a lot of genetic action to take place. Certainly, there has been some, and we can see it. Only people with their ancestry outside of Africa have white skin. But what this means is that most of the genetic differences that have been built up in our history are all still in Africa. All of us outside of Africa are just tiny subsets of a tiny subset that left Africa. So if you got rid of everyone in the world outside of Africa, you would lose a little, but you would preserve most of the genetic variation for all of humanity. And...

DAVIES: Which means that those that are of the African gene pool have more diversity, which means you'd have some really, really fast people and some of the slower people - maybe not on average difference, but more diversity?

EPSTEIN: That's correct. That could be. So, of course, we don't look for the slowest people, but you might find - at a particular trait, you might find the most diversity within an African population, as opposed to comparing someone in an African population and someone in a European population. So you might find the fastest 10 runners and the slowest 10 runners, but nobody's looking for the slowest 10 runners.

DAVIES: Now, there are some interesting phenomena here, like, for example, the - how many remarkable sprinters come from Jamaica, and a particular part of Jamaica. And you describe in the book some efforts to try and get to the bottom of that, why so many of the world's fastest humans come from this one area. What were some of the theories, and what do we know about what's accurate?

EPSTEIN: Right. Well, before I even went to Jamaica, as I was just looking up sort of historical records, I saw that since the boycotted Olympics of 1980, every finalist in a 100 meter Olympic final - despite homelands spanning Canada, the Netherlands, Portugal, Nigeria, the United States - they're all from a tiny swath of West Africa. They're not just African. They're from a small area in West Africa.

DAVIES: So these are people that may have grown up in Canada, may have grown up some other place, but genetically speaking, they came from that part of West Africa.

EPSTEIN: That's correct. That's exactly correct. And so there are a couple of things we know. Those people that have their ancestry at low latitude, they have long limbs. That means long legs, proportional to their body size. And that is a genetic adaptation to cooling. It's the same reason why your radiator has a lot of coils, is to increase the surface area compared to the volume for cooling purposes. And that's true of every organism, including humans. And all other things being equal, speed sort of scales with the square root of leg length, so it's a good thing to have longer legs. So you have a population that starts with longer legs, but that's not sort of something that isn't present in other places.

One thing that studies that have been done so far suggest is that individuals from this area of West Africa have a higher proportion of fast-twitch muscle fibers, on average. Not that that doesn't exist in other populations, but a small difference in the average also makes a huge difference in the number of individuals who are at the extreme. And I think Jamaica is this phenomenon where you sort of combine a good talent pool with an unbelievably good talent filtering system, and you see what happens.

DAVIES: We were talking about so many, you know, highly successful long-distance runners coming from Kenya. When you look at the one tribe in Kenya that has produced so many successful distance runners, you know, part of it, you're right, is the physical advantages that they have. But part of it has to do with the economy and the opportunities that kids there have. Explain how that works in.

EPSTEIN: That's right. So the Kalenjin, the tribe that lives in altitude that produces most of the great runners, they also have a large population that lives in a rural area. And really, they have no opportunity cost for training. So in the United States, the best college runners put off their lives in order to pursue their dream of running. In Kenya, there's nothing else for a lot of these runners to do other than to try to train.

So I went to a dirt track in Kenya one morning to watch interval training. You know, it's right on the ledge of the Rift Valley. It's a 4,000-foot drop right over it, and there's sheep, you know, walking over the ledge, crossing the track while a gold medalist is training in the dirt with 30 guys hanging on behind him. Literally, some guys who would just walk off of a subsistence farm, try to run right beside a gold medalist, and if they did OK, maybe they'd come back the next day. And if they didn't, maybe they'd sort of slink back to the Shamba, which is a subsistence farm. So it was an amazing thing to see, whereas, most runners in the United States are joggers - recreational runners.

In Kenya, there's only two types of runners: people who are running for transportation - maybe to and from school - people who are killing themselves in training trying to be Olympians, and people who aren't running at all. That there's this huge population of people willing to try to train like Olympians because of the economic benefits that can confer.

One of the days when I was in Eldoret, one of the main training centers, I saw a milk truck go by. And then I later went to visit Moses Kiptanui, a Kenyan who was a former world record holder in the steeplechase. And he told me that he owned the dairy farm that produced the milk, the trucking company that transported the milk and the building in which the milk was sold. That would not happen to an American steeplechase runner. So the Kenyans see that happening around them. And now, of course you very rarely see the children of a great Kenyan runner be great themselves. Because as Moses Kiptanui told me, my kids, they get picked up in a car to go to school. We have money now. They prefer easier sports. That's what he said.

DAVIES: So when you turn to big American sports, I mean, you write that if you look at the National Football League, the position of cornerback - I mean, it's one of the pass defenders - that there's not a single white player among them. What do you conclude from that?

EPSTEIN: Well, I think that...

DAVIES: It's speed position, right?

EPSTEIN: That's right. And it is - it's probably the speed position in terms of stopping and starting and accelerating as quickly as possible. And certainly, the NFL combine, for all its many faults, backs that up, that cornerback is the fastest position. And there hasn't been a white player at that position for about a decade.

And I think that's because when you're looking for the extreme of humanity - look, I do think that there is an effective stereotyping at lower levels, where coaches will see a black athlete and say, oh, well, he has to go to the speed position. But at that level, it's clear that the cornerbacks are the fastest. It's not like there are people in other positions being excluded from cornerback, and I think that does have to do with some of the same traits that cause all of the Olympic 100-meter finalists to be West African. It's long legs proportional to body and high proportion of fast-twitch muscle fibers. And if that average is a little higher in a certain population, then at the extremes, it makes a huge difference. And you're only looking for the extremes in the NFL.

DAVIES: Did you find when you - I mean, you spoke to a lot of geneticists and scientists as you researched the book. Were they reluctant to talk about race and sports?

EPSTEIN: Reluctant would be an understatement. It almost scared me out of writing the book, to be honest. And at the American College of Sports Medicine Conference in San Francisco last year, actually, the head of a kinesiology department at a major research university confessed to me that he was actually hiding data that he had on ethnic differences.

And his study was about the response of exercisers to a dietary supplement. It had nothing to do with anything that I would consider taboo, and yet he was worried that by publishing it and conceding that there might be some biological ethnic differences, that somehow this could lead to speculation that there are also innate intellectual differences, as if the two had anything at all to do with one another. The problem is hiding data like that can, in this case - may be not a big deal - but can be really can and has been really dangerous, as I write in the book.

DAVIES: You know, when I look at the book overall, it sort of seems to be asking the question: How much of athletics is nature and how much is nurture? And it seems like the answer is plenty of both.

EPSTEIN: The answer is plenty of both. So the job of the scientists - one of the things about 10,000 hours rule is it's become shorthand for saying practice is important. And that is something that was never controversial among scientists. If we didn't believe practice - I think it was always a straw argument that we thought all these gifts were just completely innate. If we didn't think practice was important, we could have athletes to show up on game day, and nobody would've ever had to practice.

But the job of scientists is to find out how important practice is exactly, and how important it is in one sport compared to the next. And sort of as I go through in the book, I think in different sports, a sort of suggest that it's more important in some and less important than others.

I mean, in the chapter about NBA body types, right, if you know an American man who's at least seven feet tall between the ages of 20 and 40, there's a 17 percent chance he's in the NBA right now. So those genetics would be pretty darn important. But in some other sports, it's not quite that outlandish.

And, actually, in a way, it gets to your specialization question. Because I think sometimes we sort of do a disservice by pushing every young athlete into trying the most popular sport. Whereas one thing that's clear from genetics is no two people respond to one type of training the same way, but it also looks like no two people don't respond to any type of training at all. So no person doesn't respond to any type of training at all. So there is probably a training out there and a sport out there that people are better fit for, and I think we'd do well to sort of expose young athletes to the range of sports instead of just presenting them all with football.

DAVIES: You know, it's obvious that someone who's very tall has an advantage at basketball. And if you're big and bulky, that gives you advantage - some advantages in certain football positions. Are there some sports in which the skills are simply more likely to be - you're more likely to be born with them?

EPSTEIN: Well, certainly, in a high skill sport, any high skill sport you're going to have to have to learn the skill. But if you think of basketball not only being tall, but a jumping ability, and jumping ability is something you can improve, but it's also a function of the land and stiffness of your Achilles tendon. And you can make your Achilles tendon stiffer through training, but you can't lengthen it. That's just a function of the distance between your calf and your heel bone.

So skills like that, absolutely, they're very important sort of innate qualities, even though you can improve them, to a degree, with training. In terms of the sports that we would call sort of more skill-centric, like golf and things like that, nobody's born knowing it.

So as I wrote, one of the things that's often omitted from Tiger Woods' life story, where his father presented opportunities, allowed him to train furiously from a young age, is Tiger was able to balance on his father's palm at six months old, when most infants are just struggling to stand. So I don't think that destined to him, necessarily, to be a great golfer, but it certainly allowed him to practice earlier, so that he was hitting balls by the time he was 11 months old. And that's an advantage in itself.

DAVIES: Well, David Epstein, thanks so much for spending some time with us.

EPSTEIN: Thank you for having me. It's a pleasure.

GROSS: David Epstein spoke with FRESH AIR contributor Dave Davies. Epstein is the author of the new book "The Sports Gene." You can read an excerpt on our website, freshair.npr.org.

Coming up: Google Glass. Geoff Nunberg has been thinking about whether it's cool, creepy or both.

This is FRESH AIR. Transcript provided by NPR, Copyright NPR.