The second relevant advantage of a relatively prosperous upbringing is height. The economist Robert W. Fogel has demonstrated the impact of improved early life nutrition on adult height over successive generations. Poor children in contemporary America still have substandard nutrition, holding back their development. They have higher infant mortality rates and lower average birth weights, and recent research has found that poverty in modern America inhibits height. In basketball, the importance of every inch is enormous. I estimate that each additional inch almost doubles your chances of making the N.B.A.

The N.B.A.’s changing demographics may also reflect the advantages of growing prosperity. Even casual fans will have noticed the difference the past 30 years have made: In 1980, fewer than 2 percent of N.B.A. players were foreign-born; now more than 20 percent are.

Much of this is surely because of the increased international popularity of basketball. In 1992, a 14-year-old German, Dirk Nowitzki, watched the American team starring Mr. Jordan, Larry Bird and Magic Johnson win a gold medal in the Barcelona Olympics. Mr. Nowitzki, a childhood tennis and handball star, decided he wanted to become a basketball player. But there is a somewhat surprising factor that may also be contributing to the game’s internationalization: Foreign countries are producing taller men.

From 1900 to 1980, the average American adult male’s height rose to 5-feet-10 from 5-feet-6. But American height has leveled off since 1980. The number of American-born 7-foot N.B.A. players, which increased from 1 in 1946, the N.B.A.’s first year, to 16 in 1980, has leveled off as well (there were 20 last year). Western countries that have health patterns similar to those of the United States have also produced a fairly constant number of 7-foot N.B.A. players during the past few decades.

Meanwhile, other countries have caught up to the United States in health and height. A widely available proxy for early life conditions is infant mortality. In the United States, roughly 20 fewer infants per 1,000 births died in 2012 than in 1960. In other countries, declines have been much larger. In Turkey, over the same period, the rate dropped by a staggering 159 per 1,000 births. Even some Western European countries, like Spain, Greece and Portugal, had declines more than twice as large as those in America. All of these countries, recent research finds, have grown taller.

Take every country with bigger health improvements than the United States. Suppose they grew an inch on average in the past 30 years. This would most likely increase the proportion of 7-footers in these countries fivefold, and indeed these fast-improving countries have about five times as many N.B.A. 7-footers now as they did 30 years ago.

Or look at it from the other direction. Suppose Omer Asik, a 27-year-old Turkish player on the Houston Rockets, was born 25 or 30 years earlier, when Turkey’s children were much worse off. Perhaps he would have peaked as a 6-foot-10 forward in Ankara, not as a 7-foot center in Houston.

After winning his second N.B.A. championship last June, Mr. James was interviewed on television. He said: “I’m LeBron James. From Akron, Ohio. From the inner city. I am not even supposed to be here.” Twitter and other social networks erupted with criticism. How could such a supremely gifted person, identified from an absurdly young age as the future of basketball, claim to be an underdog? The more I look at the data, the more it becomes clear that Mr. James’s accomplishments are more exceptional than they appear to be at first. Anyone from a difficult environment, no matter his athletic prowess, has the odds stacked against him.