Birthday discrimination against young kids

reported by Greg Spira (via Steve Levitt):

The pattern is unmistakable. From August through the following July, there is a steady decline in the likelihood that a child born in the United States will become a major leaguer. Meanwhile, among players born outside the 50 states, there are some hints of a pattern but nothing significant enough to reach any conclusions. An analysis of the birth dates of players in baseball's minor leagues between 1984 and 2000 finds similar patterns, with American-born players far more likely to have been born in August than July. The birth-month pattern among Latin American minor leaguers is very different—if anything, they're more likely to be born toward the end of the year, in October, November, and December.

The magical date of Aug. 1 gives a strong hint as to the explanation for this phenomenon. For more than 55 years, July 31 has been the age-cutoff date used by virtually all nonschool-affiliated baseball leagues in the United States. The result: In almost every American youth league, the oldest players are the ones born in August, and the youngest are those with July birthdays. For example, someone born on July 31, 1990, would almost certainly have been the youngest player on his youth team in 2001, his first year playing in the 11-and-12-year-olds league, and of average age in 2002, his second year in the same league. Someone born on Aug. 1, 1989, by contrast, would have been of average age in 2001, his first year playing in the 11-and-12-year-olds division, and would almost certainly be the oldest player in the league in 2002.

Twelve full months of development makes a huge difference for an 11- or 12-year-old. The player who is 12 months older will, on average, be bigger, stronger, and more coordinated than his younger counterpart, not to mention more experienced. And those bigger, better players are the ones given opportunities for further advancement. Other players, who are just as skilled for their age, are less likely to be given those same opportunities simply because of when they were born. Alex Rodriguez would've been a star no matter his birth month, but a player like Juan Pierre, who has less natural aptitude for the sport, might have gotten a small leg up over similarly skilled players because he's an August baby. It's clear from the chart above that this small advantage can have an impact that lasts a lifetime.