Birth Month Effect On Baseball Performance, Part II

In our first post, we showed the percentage of players in Major League Baseball based on birth month.  As I showed, there is a much higher percentage of players born in August, largely due to the Little League Baseball cutoff date of July 31.

In order for players to reach the majors, they had to have been performing at a higher level through various levels of baseball.  But once they reach the major league level, is there any correlation between birth month and performance? Let’s take a look.


Note: Batting Performance Chart Returning Soon

Interestingly, the statistics show that, if anything, players born in JULY outperform those born in August.  While 2 HR and 5RBI isn’t huge, it still interesting to see that players born in July outperform those born in August.


Note: Pitching Performance Chart Returning Soon

There is very little difference in pitching statistics relative to birth month.  Players born in August pitched more innings, and had a slightly lower ERA, but no extreme differences between people born in July and August.


Wile there was a huge difference in the percentage of players born in July (6.4%) compared to August (12.2%), the difference in performance was fairly small.  How could this be?

The theory is that by the time a player reaches the Major League, they are an expert, and thus in aggregate the statistics are very similar.  Any other thoughts?  There are more players born in August, yet their performance isn’t any better.  What other explanations can you think of?


5 responses to “Birth Month Effect On Baseball Performance, Part II”

  1. Ryan Avatar

    While HR, RBI, and AVG are good barometers to consider, I think it would be interesting to look at OBP and OPS as well. Perhaps those figures would shed some more light on the analysis and give you more data to see if there is a reliable trend in offensive statistics based on birth month. Also, why not compare August to every other month, as opposed to just July? Its interesting that there seems to be a very real correlation between birth month with regard to making the bigs, but I would doubt there's any real correlation to birth month and performance once a player makes it.

  2. Christopher Lee Avatar
    Christopher Lee

    I agree that OBP and OPS would better illustrate offensive success. I used AB, HR, RBI and only showed July/August to make it easier to understand for the casual fan (or blog reader). Given the interest in the topic, I'll likely post a Part III that goes into greater depth with OBP, OPS and detailed breakdown by month.

  3. Kevin Avatar

    Well Nolan Ryan, Babe Ruth, and Hank Aaron were born 7 days apart. Jan 31 to Feb 6th. 🙂

    The fact is that if someone loves the game and wants to play, they will have a greater level of commitment and that commitment separates the good ones from the great ones. So in essence, there is no replacement for having a passion to play baseball and the desire to be great.

  4. Larry A Avatar

    This is interesting, I heard of a similar effect regarding Hockey except the cutoff is Jan. Something like there are several times more professional Hockey Players born 3 months after the new year vs before. Yet when you look at the Hockey Hall of Fame, it is evenly distributed indicating that the pre-new year players have a better chance of reaching the Hall of Fame. So I was expecting to find July baseball players being significantly better than August. I wonder if you get a different result with Baseball Hall of Fame?

  5. Chuck Avatar

    The issue isn’t really birth month, but physical development, which happens to run in correlation to birth month. Those who are physically stronger tend to hit and throw better than those who are not. As a result, those kids tend to be selected for all-star play, as an example. Through extended play, those kids are early-defined as the top players, whether or not they have proper technique. Of those kids, those with better technique and athleticism tend to be the younger kids who make it into the group (this, of course, is a subjective observation, but learned eyes would probably tend to pick out those with better technique and athleticism on every team and, you will find, that they are the youngest — this, of course, makes sense because they must make up for lack of physical strength thor gun technique and athleticism).

    At this stage, an important factor comes to play as other kids begin to drop out of the sport. (70% of all kids drop organized sports at age 13 in one well-known study). Kids drop out for a variety of reasons, but an important ones relate to self-belief, frustration, and discouragement by untrained coaches who tend to favor their own kids without regard to the psychological damage done to other kids who have yet to physically develop.

    Over time, kids mature in high school. At that point, kids who were larger at, say, age 10 are now no larger than kids who were smaller. Left behind at this point are kids who might have greater strength or athleticism, but left the sport by age 13 as expalined above. Once in a while, you hear an anecdotal story about a kid who takes up baseball in high school and excels based on physical gifts, but you would also find that the particular boy likely did not have those gifts in younger years, thereby explaining how it is that he came from nowhere. (Interestingly, sports like football that are less skill-specific, but based more on athleticism and strength, are sports that favor late development or early development.)

    At this point, kids moving to college or pro baseball are all about the same in physical strength and all have developed similar skill — it’s just that, by month, there is a pattern as to how many are still in the system. It’s not really an issue of having become an expert as all players are experts at the pro level. Similarly, the age pattern would make no sense at that point because expertise surely couldn’t have been achieved at age 13 or younger. What makes sense is that, likely, an almost identical percentage of remaining players move on to pro baseball from each month, but it was the early weeding out (the likely fundamentally wrong weeding out) that occurred much earlier in life that leads to the pattern at the MLB level.

    Interestingly, some of the greatest players fall outside the pattern — probably because, regardless of physical strength and early athleticism, they developed superior technique at an earlier age. In that sense, while a boy faces psychological unfairness and difficulty being born in the last month of age cutoff (now, April 30, but possibly moving to the international December 31 standard), he might just as well have an advantage on larger players whose technique might not be good and which is ignored in favor of glowing results. That is, then, a warning to all that you should look beyond results and, instead, at technique and form. Unfortunate, too many valid dreams are destroyed on youth fields by unlearned coaches and ravenous parents seeking what they see — winning results — and ignoring what they should notice — better ballplayers.

    As a final caveat, note that international cutoff is December 31, so recent MLB data of the last 30 years, with the tremendous influx of Hispanic and Japanese players, needs to be bisected between the American players and the foreign players to truly see the impact of the psychological weeding out effect I described above. It is real and, sadly, its cause is an arbitrary date — even December 31 is arbitrary. The answer is too complex for a world that wants simple — select subjectively for form and technique while ignoring results. The result of that would be a far better system that matches dreams and development, forcing coaches to learn their crafts and allowing all players to develop appropriately. Of course, that invites a whole new world of arbitrary selection in a world of men…