At first, you might expect, like I did, that the birth months of MLB players would be relatively uniformly distributed. Being born in a particular month shouldn’t give someone an advantage in reaching the Majors. But in looking at all MLB players in history, we see a slightly different story:
August and October have produced more Major Leaguers than any other month, with September, January, and November rounding out the top 5 birth months for baseball players. So if you want your kids to grow up to be professional athletes, make sure they’re born in the fall, right? Not necessarily. Compare that chart with the ones produced in this article, which uses a population of people who have applied for life insurance. That is, not necessarily people with extraordinary athletic ability. What we find with a broader view of the population is that most people are born from July to October, during the fall months that seem to produce so many good baseball players. So it’s probably not likely that fall months birth better athletes, but rather that there are more people born in the fall months in the first place.
If we shorten the time frame that we’re looking at to players born in or after 1973, the chart looks slightly different. 1973 was selected because the Steroid Era (or the Longball Era, if you don’t want to implicitly accuse everyone of taking illegal substances) began in the early 1990s, and it takes about 20 years after birth to reach the Majors. Most of the players who have entered into today’s baseball have been born after 1973. Check out which birth months produce the most (and the fewest) professional MLB players:
Again, there’s a huge portion of modern baseball players with fall birthdays, particularly in August. August and September are far more pronounced as common birth months than in a full MLB player census, however, and July appears underrepresented.
I used a chi squared test1 to determine whether the variance in the distribution of birth months of modern-day players is greater than what would be expected due to random chance, and it came back with the surprising result of a p-value of 0: there is no chance that modern-era birth months have come to favor the fall randomly. There must be a reason why August is such a common birth month for modern players and why July is so uncommon.
I think the seeming break between July and August is significant. It’s possible that instructional or competitive baseball leagues at the youth level have a birth date cutoff to distinguish, say, Under-13 from Under-12 teams. If a U13 team’s cutoff birth date is July 31 of one year and the U12 team starts at August 1, the July players in the U13 league will be, on average, the smallest players (particularly so at this age, where 13-year olds may have entered puberty and 12-year-olds may not have). Meanwhile, the August babies on the U12 team will be the biggest and more noticeably developed than their peers – and, importantly, more likely to be noticed for their development, and more likely to receive additional instruction as a league or team All Star, and more likely to develop a confidence necessary to compete against other skilled opponents. I also noticed the frequency of each birth month of modern players dipping heading into August, which works well in my theory: June babies will be only a little older and more developed than July babies, but less so than May babies. It may be that modern baseball players – those that have likely gone through travel or competitive baseball leagues before high school – are not only terrific athletes, but the beneficiaries of the good fortune of a birth month that coincides with their little league cutoff.
Indeed, the birth date cutoff for the Little League World Series, or the earliest date that players were allowed to turn 13, was August 1 until it was changed in 2006. The oldest – and most likely to be physically and mentally more developed, and therfore most likely to receive attention for other forms of competitive play – players in the Little League World Series were then born in August of each year until 2006, when the birth date was made May 1. That’s right: if this theory holds water, get ready to see an influx of professional baseball players born in May, June, and July in the near future. The young athletes competing in the 2006 and more recent LLWS are, at the oldest, 20 right now (and turning 21 in May), and could be the next wave of prospect and MLB talent.
If you’re looking forward to an early retirement via your son’s MLB career, the fall is out of style. Make sure he’s born in the early summer.
1. In fairness, I wasn’t really testing a random sample against the population it came from here, since I specifically chose a subset of the population of professional ballplayers based on their birthdate. If there’s a better test for this, I’m happy to run it!
Patrick was the co-founder of Observational Studies, a blog which focused on the analysis and economics of professional sports. The native of Carroll County graduated with a Bachelor’s degree in Economics from Loyola University Maryland. Patrick works at a regional economic development and marketing firm in Baltimore, and in his free time plays lacrosse.