[bcg1]

The first chapter of Malcolm Gladwell's book Outliers has a plausible explanation... he gives an example of how successful hockey players are disproportionately born in the months of January, February, and March. To poorly summarize the explanation, it is that the kids who started out with a slight advantage (i.e., being slightly older and bigger than the other kids) have their advantages multiplied over time because they are singled out for extra training and opportunities for growth (all-star games, travel teams, etc). In the case of chess, it is possible that the initial slight advantage (being a boy) does not even exist, but is only perceived, but if boys receive more encouragement, training, and opportunities than girls, the end result could be the same.