In Major League Baseball, there are a lot more Juan Pierres than there are players like Alex Rodriguez. Yes, Pierre is a light hitter, and A-Rod is one of the greatest sluggers of all time, but there's another important difference between the two: Pierre was born in August, and Rodriguez was born in July.
In 2000, John Holway argued in a book called The Baseball Astrologer that the sign under which an individual was born played a significant role in whether he made it in pro ball. Holway identified a real phenomenon, but the explanation does not lie in the stars. Since 1950, a baby born in the United States in August has had a 50 percent to 60 percent better chance of making the big leagues than a baby born in July. The lesson: If you want your child to be a professional baseball player, you should start planning early. Very early. As in before conception.
The table below lays out the full month-to-month data. As of the 2005 season, 503 Americans born in August had made it to the major leagues compared with 313 American born in July. (In this article, the United States refers to the 50 states and the District of Columbia but not territories like Puerto Rico. And again, these are players born since 1950, and the data are current through 2005.)
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. Youth baseball organizations including Little League, Cal Ripken/Babe Ruth, PONY, Dixie Youth, Hap Dumont, Dizzy Dean, American Legion, and more have long used that date to determine which players are eligible for which levels of play. (There is no such commonly used cutoff date in Latin America.) 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.
This phenomenon will not come as news to social scientists, who have observed the same patterns in a number of different sports. The first major study of what has become known as the "relative age effect" was published in the Journal of the Canadian Association for Health, Physical Education, and Recreation in 1985. This study, by R.H. Barnsley, A.H. Thompson, and P.E. Barnsley, determined that NHL players of the early 1980s were more than four times as likely to be born in the first three months of the calendar year as the last three months. In 2005, a larger study on the relative age effect in European youth soccer was published in the Journal of Sports Sciences by Werner F. Helsen, Jan Van Winckel, and A. Mark Williams. This study found a large relative age effect in almost every European country, though it seems to shrink in adult leagues and is less significant in women's soccer. (Stephen J. Dubner and Steven D. Levitt of Freakonomics fame wrote about the age effect in European soccer for the New York Times Magazine.)