There is growing evidence of a genetic basis for antisocial traits. In a 2008 twin study by Mats Forman and colleagues, all 1,480 twins born in Sweden from 1985-1986 were administered the Youth Psychopathic Traits Inventory. By comparing correlations between identical and fraternal twins, the authors concluded that psychopathic traits have a strong genetic basis and that this is especially true for the impulsive/irresponsible subscale versus the grandiose/manipulative and callous/unemotional subscales. Some authors estimate that genetics account for 40 to 50 percent of antisocial behavior. However, it is difficult to know for sure how much of antisocial behavior is due to genetics and how much is due to environment. For one, estimates of heritability (or genetics) rely heavily on twin studies, in which the genetic differences are carefully measured but the environmental differences are not. Moreover, twins tend to have quite similar environments. Thus, environmental contributions are probably underestimated.