As a part of my NBA preview feature focused on maps that explain the league, I looked at the birthplaces of American players. I found that if you looked both at prior generations of players and current players, you found a simple pattern: large cities produce most of the league's players. For example, here's the heatmap that shows birthplaces of the NBA players born since 1980.

The strongest producers aren't much of a surprise: New York City, Los Angeles, Chicago, Washington, D.C., and Philadelphia, with nods to Atlanta, Dallas, Houston, Indianapolis, Detroit, New Orleans, the Bay Area and Seattle. These are the major population centers of the nation, so it's expected to see most players come from those areas.

But I wanted to find out if some areas produce NBA players at a higher rate than expected historically. So I adjusted for population and produced this map based on all-time NBA player counts. (Note that a Hall-of-Famer counts the same as a 10-day call-up in this map. There's no weighting for games played or quality of the player. Just raw numbers of players born in a state or territory.)

As it turns out, Washington D.C. has an extraordinary record of producing NBA players. With 68 current and former players born in Washington, the District's rate per million of current resident is around 125. That's five times higher than the nearest state. It's so high that it's eyebrow-raising, in fact. Here are some potential explanations.

* Washington is only urban, and we discovered from our previous maps that cities tend to produce most NBA players. Therefore, while other states have rural populations less likely to produce NBA players that in theory drag down their per-capita rates, D.C. does not.

basketball is a game deeply embedded in certain diverse populations.

* These maps rely on birthplaces only. (Thank you as always to basketball-reference.com for the data.) Perhaps an outsized number of suburban Maryland and Virginia children are born in D.C. such that the District's birth rate is abnormally high given its population. As it turns out, D.C.'s birth rate is among the highest in the nation, but not abnormally so.

* Washington is overwhelmingly black. The NBA is overwhelmingly black. This seems like the most simple explanation, and it's backed up when you see what state follows D.C. on the list: Mississippi. D.C.'s population is 50 percent black. Mississippi leads the 50 states with a 37 percent black population. The U.S. Virgin Islands, which has produced three NBA players despite a tiny current population of 100,000 and follows Mississippi closely in per-capita NBA player production, is 76 percent black.

Here's a U.S. census map from 2000 showing percentage of black population in each U.S. county.

It doesn't match up with our per-capita NBA birthplace map perfectly. But the broad strokes are pretty close in the Deep South and along the eastern seaboard.

What stands out are Indiana and Kentucky. Those two states have reputations as basketball wonderlands but middle-of-the-pack black populations. That suggests that local culture plays a major role in basketball player development. And in fact, that cultural argument reinforces the correlation between black populations and higher rates of NBA player production we see in D.C., Mississippi and the U.S. Virgin Islands: basketball is a game deeply embedded in certain diverse populations. It builds strongly local cultures, and that ultimately leads to success.

Some other notes on this subject:

* One of the pitfalls here is that we're using data on NBA player birthplaces collected since the late 1920s, but only accounting for current population. Obviously, population has shifted dramatically over 90 years. That means that the American West may be underrated on our map since it didn't have high levels of population for most of the NBA's life. We'll see if Arizona and Nevada, for example, starts producing more NBA players.

* Vermont is the only U.S. state to never produce an NBA player. Neither Guam nor the Northern Mariana Islands have done so either.

* Alaska can only claim one NBA player who was born there, Mario Chalmers. Carlos Boozer was born in Germany and Trajan Langdon was born in California. Such is the limit of using birthplace data. High school data might be more representative if so many players didn't attend prep academies in Virginia, Connecticut and Florida.

* In raw numbers, California (the most populous state) leads the way with 352. New York follows with 310.

* No, I'm not sure how Montana has produced nine NBA players either. It has the smallest black population in the nation (4,000 black residents for 0.67 percent of the population as of the 2010 census) and no major basketball culture as far as anyone outside Montana can tell. (For the record, all nine of the NBA players born in Montana are white. Phil Jackson is the only one in the Hall of Fame.)

* The three players born in the U.S. Virgin Islands: Tim Duncan (of course), Raja Bell and Charles Claxton, who played seven minutes for the Celtics in 1996.

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