Performance of Older vs. Younger Siblings

I just came across a very interesting research study that compared hitting and pitching records and career lengths of 312 sets of brothers who become major league baseball players.

Researchers found out that older siblings at non-pitching positions had signficantly higher batting averages and longer career than their younger siblings. However, the differences for pitchers were not statistically signifiant.

These results corroborates other studies that have shown that first borns are more likely than later born siblings to be higher achievers.

In the case of pitching, the authors think that many different factors may be operative for pitchers.

I would be really curious to learn more about the reasons why first borns are more likely to be dominant players and have longer careers.

Any ideas? Feel free to comment below.

Reference:

Abel EL, Kruger ML. Performance of older versus younger brothers: data from major league baseball. Percept Mot Skills. 2007 Dec:105(3 Pt 2):1117-8.