Reign of Errors: Why Birthdate Is So Influential In Baseball

  Malcolm Gladwell in his book Outliers described the relative age effect in sports; athletes born in the month(s) immediately following the age-cutoff date are disproportionately overrepresented in the sport.

For example, data kept from 1950-2005* shows that most American Major League Baseball players were born in the month of August (503). Further, every month after August of a particular birth year until July 31 of the following year,  (313) there was a steady decline in the likelihood that an American child would become a major leaguer.

What is so magical about August? For more than 55 years (beginning in 1950 when organized Little League baseball got its start), July 31 had been the age-cutoff date used by virtually all non-school affiliated baseball leagues in the U.S.

The Baseball Info Solutions (BIS) database contains birth information about all major and upper-level minor league players active since 2002. According to the data, 18.6 major league players were born per day in August, compared to just 12.5 players per day in July. The BIS data also shows a 26.6% spike in August while July comes in at 15.4% below average.

The result:

In almost every American youth league, the oldest players had been the ones born in August, and the youngest were those with July birthdays.

For example, someone born on July 31, 2000, would almost certainly have been the youngest player on his youth team in 2011, his first year playing in the 11-and-12-year-olds league, and of average age in 2012, his second year in the same league. Someone born on Aug. 1, 1999, by contrast, would have been of average age in 2011, his first year playing in the 11-and-12-year-olds division, and would almost certainly be the oldest player in the league in 2012.

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. Bryce Harper would've been a star no matter his birth month, but a player like Dustin Pedroia (8/17/83) 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 by the numbers that this small advantage can have an impact that lasts a lifetime.


In 2006, USA Baseball changed the domestic age determination date to April 30 of the current year. The age of a baseball player as of April 30 is that player's "league age" for the season.

According to USA Baseball:

The primary reason to change the domestic determination age is so most players on a team will spend the majority of the regular season at the same chronological age as their league age. Currently, more than 95 percent of all local Little League programs start their seasons before May 1.

 Most youth baseball organization members under the jurisdiction of USA Baseball have adopted the April 30 age determination date.


With the new rule it will take a while to shake out how this affects rosters at the Major League level but I think we should definitely be aware of the historical data. Those who fail to acknowledge history could lose out on a significant pool of talent if we don’t think this thing through a little more critically.

For example, does moving the cutoff date from July 31 to April 30 really matter? It doesn’t’ appear that this will actually fix anything over the long-term. All that will likely happen is that kids born in the month of May will now have a big advantage. After all, players born immediately after the age determination date have been shown to have a decided advantage over players born in the months just before it because they are more physically mature and able to dominate their competition at the youth level.

Allan Simpson in the February 2005 edition of Baseball America wrote-

Research has shown that a majority of players on youth league all-star teams of all age groups are born in the four months immediately after July 31. That advantage carried forward to the major league level, as more 2004 big leaguers were born in August (123) than any other month, and the fewest were born in July (89).

If we chose to we could acknowledge that cutoff dates matter. We could set up different tracks of development, one for the early developers and one for the potential late bloomers. Keep those late bloomers involved during their developmental years and continue to provide them with quality coaching and instruction.  The results should be significant.   In 2 or 3 years, you would have a much larger talent pool to choose from.

According to Bill James online:

If organized baseball could manage to develop these overlooked ballplayers as well as it develops August-born (now May) talent, there would be 25% more MLB-caliber players. The league could expand to 7 or 8 additional markets without a drop in quality, leading to more revenues for the league, owners, players, and individual cities.

The solution won’t be easy but even a small group of committed individuals can make a big difference. By placing an emphasis on athlete development rather than arbitrarily rewarding those with the happy fortune of being born in August… or now May we can start to create the necessary social proof that will be vital for getting the attention of the entire baseball community.

The organization that decides and acts to serve in the best interest of their young athletes will undoubtedly become an Outlier, an organization with humble intentions that will become the beacon for others to follow.

*In 2006, the age cutoff date moved from July 31 to April 30.