One of the more interesting pieces of evidence in the nature or nurture debate is the that athletes on professional sports teams tend to have a higher proportion of players born early in the year. Malcolm Gladwell documented this phenomenon for ice hockey players in his book Outliers. The basic idea is that when young, those born earlier in the year are bigger and faster than their peers and, as a result, tend to get more game time, are selected for further development and so on. This ongoing cycle amplifies the original difference. (The precise time of the year can change if the age cut-off is based on another date, but the same concept still holds.)
This morning I went through the current player lists of the Australian Rules Football teams and saw a similar result. Around 30 per cent more players were born in the first quarter of the year than the final quarter (although as a quick google discovered, I was not the first to have done this). Still, there is nothing like playing with the data and seeing it with your own eyes.
This is clearly evidence in favour of the nurture side of the debate. There are also a few possible policy responses. Gladwell talks of setting up parallel sports leagues for children, with one having a different birthday cut-off (say, mid year). Another suggestion might be to spend less time trying to pick future stars at such a young age. Within a single country, there is probably not much benefit to these policies, but for international sports, could this give a country an edge by ensuring that talented individuals born later in the year have a chance?
There is some evidence for this effect in academic pursuits, with the difference in test results between children born earlier and later in the year enough to differentiate who might be selected in a gifted academic program. The longer term effects of this is not so clear however. What would be a good dataset to test whether someone born later in the year is under-represented in an academic field?
To me, whether this all matters depends on whether this effect extends beyond the top end of the bell curve. Whether a few dozen professional sports players make it to the big league or not is of no great social consequence. If this effect was throughout all levels of society, with those born later in the year being invested in less than the early birds, there may be some serious misallocation of resources and sub-optimal use of human resources.
I have one further hypothesis regarding this scenario: the long term effect is larger in sport than in other areas. This is based on two ideas. Firstly, it comes from some scepticism on my part about the benefits of “gifted child” programs and the like. Secondly, in the area of sports, if you are shorter or slower, you tend not to be picked for the sports team or not given the ball. In academic areas, you still have to do your math problems, sit through the test and undergo most of the academic training that the “gifted children” do. I’m not fully convinced of my hypothesis, but if there was a dataset to examine the birth effect for academic related outcomes in adulthood, it would be worth a look.