What I should have said is that it is a heuristic, which is generally believed to hold in all cases where between group variation is less than within-group variation. (Which is why it's okay that athletics are gender segregated)
Well, depending on the metrics you use, within-group variation for men's and women's athletic ability is much greater than between-group variation. After all, the difference between the most and least athletic women, or between the most and least athletic men, is much greater than the average difference between men and women, or the difference between the top men and the top women.
Between-group differences can be small relative to within-group differences (certainly significantly smaller than the difference in average athletic ability between men and women,) while still dominating representation of the groups at the tail ends of an activity.
People want to tell everything instead of telling the best 15 words. They want to learn everything instead of the best 15 words. In this thread, instead post the best 15-words from a book you've read recently (or anything else). It has to stand on its own. It's not a summary, the whole value needs to be contained in those words.
I'll start in the comments below.
(Voted by the Schelling study group as the best exercise of the meeting.)