Original sentence: A major competitor, probably more widely accepted, is that ADHD is merely the co-occurrence of several genetic variants.An important view, with considerable genetic backing, is that some of these genetic variants may have value in certain kinds of social groups, such as those that have migrated (Chang et al 1996 Human Genetics 98; Grady et al 2003 Molecular Psychiatry 8).Harpending and Cochran (PNAS, Jan 8 2002) offer a 3-phase view of history that includes hunter-gathering; female farming; and then intensive agriculture; they suggest that ADHD increased reproductive fitness in the second phase.It should be understood that this hypothesis is only one of several that attempt to account for the origins of ADHD