A recent paper on the genetics of attention deficit and hyperactivity disorder (ADHD) notes that the some of the genetic variants predisposing to disease are fixed or at high frequency in some populations, suggesting the work of natural selection (see here for molecular evidence and review). The authors note that "it is unclear if ADHD should be considered as a nosological entity or as a common variant of human behavior".

Of course, if it were just a common variant of human behavior, no one would get money to study it. This encourages the pharmaceutical company approach to an area of interest-- instead of studying the variation is social aptitude, people study "social anxiety disorder". Instead of behavioral morphs, "attention deficit disorder". This is problematic, because pathology is largely an extreme on a continuous spectrum-- in that spectrum lies the bulk of the information.