We describe an integrated theory of individual differences that traces the behavioral development of life history from genes to brain to reproductive strategy. We provide evidence that a single common factor, the K-Factor, underpins a variety of life-history parameters, including an assortment of sexual, reproductive, parental, familial, and social behaviors. We explore the psychometrics and behavioral genetics of the K-Factor and offer a speculative account of the proximate mediation of this adaptive patterning of behavior as instantiated in well-established functions of specific areas of the human brain, including the frontal lobes, amygdala, and hippocampus. We then apply Life History Theory to predict patterns of development within the brain that are paedomorphic (i.e., development begins later, proceeds at a slower rate, and has an earlier cessation) and peramorphic (i.e., development begins early, proceeds at a faster rate, and has a later cessation).