Behavioral variation in human beings encompasses wide differences in personality and susceptibility to psychiatric illness arising from both genotype and experience. Long-lasting behavioral differences generally have heritabilities of 30% or more, and such inheritance is ultimately attributable to functional variants of genes programming brain development and function. The sequencing of the human genome is revealing a pattern of gene sequence variation. The ability of sequence variants to affect neural function either alone or in concert may reveal effects of behavioral selection on the human genome over evolutionary time frames. Dopamine and serotonin are phylogenetically ancient neurotransmitters intrinsic to brain function and behavior. Dopamine and serotonin receptor and transporter genes have been an early focus for efforts to identify and functionally characterize sequence variation. The purpose of this article is to present a preview of a developing new perspective in human behavior: the genetic variation of the brain or neurochemical individuality. Arch Gen Psychiatry. 2000;57:1105-1114.