Why I’m not a professional sportsperson, maybe?

Following up on yesterday’s deep dive into NBA birthdays I’ve been reading more about the relative age effect. This is the apparent phenomena whereby “older” players are over represented in professional sports. By older I mean that professional athletes are more likely to be born at the beginning of the year.

There’s actually a remarkable amount of research on this phenomena, and it appears to hold for some sports, in some countries, especially in Europe. One paper notes:

During the last three decades, researchers have identified overrepresentations of athletes born in the first quartile of the selection year (i.e. January to March if the cut-off date is 1 January) across cultural contexts in sports such as football, ice hockey, handball, baseball, basketball, rugby, volleyball, tennis, ski sports and swimming, Till et al.. demonstrated the possible extent of such over representations of relatively older players in rugby: 47.0 % of the regional and 55.7 % of the national junior representative players were born in the first 3 months of the selection year

It does not appear to hold for the NBA, as I discovered yesterday. That basketball finding was in France. And many of these studies found the phenomena reduces with age. It’s more prevalent in teams of teenagers than the higher grades, for instance.

But what about cricket? Again, a thought that hit me today as I was planning attending some upcoming test matches. One study of Australian cricket players again found no significant difference in relative age among state level players, but did for lower grades.

Players born in the first quartile of the cricket season were significantly over- represented in both male Under-15, Under-17, Under-19 and female Under-15 and Under-18 levels. However, there was no significant difference at the senior state level for either male or female cricketers.

What about Sri Lanka? Where schools play an outsized role in player development and the club system is a mess. I haven’t been able to find research on this so I decided to scrape the birthdays for all 150 Sri Lankan test players from Wikipedia:

Not much to go on here. Not much of a difference. And not enough data points to say the variation is much more than randomness. I may try to find a larger database of Sri Lankan club players.

But, have I been cursed for my October birth? It seems to depend on a lot. Given I grew up in Australia probably not.

Does it matter when NBA players were born?

I once read something about how professional athletes were more likely to be born at the beginning of the year. I don’t know the provenance of this. The source is probably questionable. But I was thinking about it today while discussing birthdays with a friend.

So, I decided to check it out. For one sport anyway. I scraped birthdays on Basketball Reference, giving me several thousand ABA/NBA players going back to the 1940s.

So, this isn’t too promising. As I remember, the theory says players born in the beginning of the year would have some advantage as children. Someone born in January would have ten months more physical maturity than someone born in October, for instance. This could be significant when you’re eight years old, setting you up for life.

This doesn’t seem to bear out by the time they get to the NBA. There might be some noise in here from international players – seasons and school terms differ, especially between hemispheres. But international players are a small minority, especially over the life of the ABA/NBA.

Some interesting research suggests that peak birth rates are influenced by latitudes. The bulk of the United States lands between July and September. As we can see, NBA players don’t seem to follow this pattern.

So, I decided to slice the data a few other ways. Maybe the theory might hold among elite players? Let’s look at NBA Hall of Famers.

This is getting closer to the distribution we’d expect in the United States. But we’re only playing with about 150 people here, so let’s not read too much into it. Anyway, it doesn’t really support the theory. The peak comes around the beginning of the basketball season in America.

The last notion I have is that older players are throwing us out. Over the past couple of decades there’s been a lot of infrastructure built to target young prospects – camps and tournaments etc. It stands to reason those born in the past thirty years could be more affected by birth month.

So, here are all the players born since 1990. Again, this distribution looks a bit more promising (for the theory) than the entire dataset. But we’re still only dealing with about 600 players. I want to look more into this.

Why I love Isaiah Thomas

I’ve been playing with Python on my iPad, scraping Basketball Reference. The dots represent all the NBA players drafted in the second round (draft picks 31-60) between 2010 and 2019. Those that have actually played, that is.

You can see plenty of players drafted early in the second round go on to play major roles for their team. Khris Middleton, for example, drafted 39th in 2012 and playing 31.5 minutes a game for his career.

But Isaiah Thomas is the clear outlier. The little guy. Drafted last in 2011. He’s made himself indispensable. Made himself an All Star.