It is not a 'law' in the Physics sense. People have done some very interesting things with selective breeding in plants and animals. It will largely work for humans too just that some effects may be dampened (or accelerated) by culture.
Explain
It's a regression to the mean argument. Tall people won't have even taller children. They'll have shorter children because the average must be maintained. So very exceptional people will have less exceptional children. Giraffes disagree.