I always find the 10,000 hours theory developed by the likes of Ericsson fraught with endogeneity. It's entirely possible that those who practice their craft most are those for whom the barriers to entry are lowest--they find it easier, more pleasurable, etc. than others. Constellations of ability are heterogeneous, and selection bias might be what is being observed.
Specifically in these tests of musical ability in twins, the authors forget the fact that simply hearing music has a training effect on the brain.
