Why Do Former High-School Athletes Make More Money? - The Atlantic:
We parents joke about that too, no matter it is baseball, basketball, swimming or scouts, we almost always meet the same bunch of parents, from one activities to another. So my hunch is that the extracurriculars at elementary school years is good indicator of commitment of the parents. At the same time, early start in sports would lead to higher participation in later years.
To measure how much kids "acquired" from sports, we need to control for other factors.