Genes play a significant role in the development of psychopathy. However socialization and other environmental factors interact with genetics, so genes are not the only determinant in whether one has psychopathic traits. Studies on the heritability of psychopathy have focused primarily on identical twins (100% shared genes) and fraternal twins (50% shared genes). One study (Larsson, Andershed, & Lichtenstein, 2006) which examined the heritability of psychopathy in twins reported that genetics accounted for approximately half of the variation in the features of psychopathy (as assessed by a self-report measure, the Youth Psychopathic Traits Inventory). Other studies have also reported substantial heritability to psychopathic traits when these are assessed using self-report measures (Blonigen, Hicks, Krueger, Patrick, & Iacono, 2005; Brook, Panizzon, Kosson, Sullivan, Lyons, Franz, Eisen, & Kremen, 2010).