From the WSJ:

Why Basketball Runs in the Family A new WSJ study finds 48.8% of players are related to an elite athlete—that number is 17.5% for the NFL and 14.5% for MLB More than any other professional sport, NBA basketball is a family business. For evidence, look no further than the reigning champion Golden State Warriors. The father of two-time MVP Stephen Curry, Dell, played in the NBA for 16 seasons as an expert outside shooter. His brother, Seth, plays for the Sacramento Kings. Curry’s backcourt mate Klay Thompson is the son of Mychal Thompson, a former No. 1 draft pick of the Trail Blazers and two-time NBA champion. Warriors guard Brandon Rush has two brothers—JaRon and Kareem—who played in the NBA. Forwards Draymond Green and Andre Iguodala both had brothers play in college. And guard Shaun Livingston’s uncle played pro basketball in Germany. This web of familial relationships is hardly unique to Golden State. In fact, it exists on every single NBA team, with athletic bloodlines that aren’t just limited to basketball. The Oklahoma City Thunder, who led Golden State 2-1 in the Western Conference Finals before Tuesday’s Game 4, have a center, Steven Adams, whose sister has two Olympic gold medals in shot put. Nearly everyone in the family of Thunder teammate Kyle Singler is an elite athlete, including his father, Ed, who was a quarterback at Oregon State. According to a Wall Street Journal analysis of biographical data on every NBA player, 48.8% are related to current or former elite athletes—defined as anyone who has played a sport professionally, in the NCAA or at national-team level. While other leagues feature notable dynasties—the Manning’s of the NFL or the Griffey’s in baseball—only about 17.5% of NFL players and 14.5% of MLB players are related to other elite athletes, based on a similar study.

It would be helpful if they mentioned how close a relationship is required for their study.

The connectedness in the NBA likely comes down to the importance of height in elite basketball. The average NBA player is about 6-feet, 6-inches tall, which is 11 inches taller than the average American male, according to Census data.

Actually, young whites and blacks are taller than that. The federal government’s NHANES study of 2003-2005 found that the average 20-39 year old white male is 5’10.4″ in his bare feet, the average youngish black male is 5’10.0″, and the average youngish Hispanic is 5’7.1″.

The other factor is the intense over-representation of blacks relative to the entire American population, so that NBA players are drawn overwhelmingly from tall, athletic black families. (Also blacks tend to have more half-siblings and half-cousins, which might affect the data.)

Dynasticism in major league baseball increased in the late 20th Century, but I haven’t checked for the last decade or so. Perhaps the two top stars of the 1990s, Barry Bonds and Ken Griffey Jr., were sons of All-Stars. Before about the 1980s that kind of thing was rare in baseball. As the national pastime earlier in the 20th Century, most American boys got exposed to baseball, so baseball had a wide pick of talents. But in recent decades, other sports have become high paying as well (although big league baseball remains a very lucrative career), so early specialization has become more important.