The evolution of attention-deficit disorder (ADHD)

ADHD trait is very common in north america. It is logical to assume that this trait has some adaptive advantages in some settings. The genetics of that advantage is being actively studied:

John Hawks Anthropology Weblog : Recent human brain evolution and population differences:

Geneticists are increasingly finding genetic variants that affect behavior. Several of these variants are now known to vary in frequency in different human populations. These alleles are two; the 7r allele of the dopamine receptor D4 (DRD4) gene is another that influences ADD/ADHD susceptibility (Harpending and Cochran 2002). The selective structure underlying DRD4 variation may be frequency-dependent, with different alleles correlating with alternative behavioral strategies that pose greater or lesser advantages in some populations.

A trait which is a disability in some educational and employment settings may be an advantage in other times and places.