Conduct disorder clusters in families. Until the 1980s most twin studies of antisocial behaviour showed a high concordance for monozygotic pairs at around 70 to 80 per cent, but also high concordance for dizygotic pairs at around 60 to 70 per cent, suggesting a major shared familial influence, a moderate unique environmental contribution for the individual, and only a limited genetic component. (19> However, the subjects were mostly in their late teens, when the adolescent-onset type outnumbers the early-onset type by about 3:1. More recent twin studies, which subtype conduct problems, suggest that in older adolescents with pure conduct problems family environment indeed has the overwhelming effect, but in younger conduct-disordered children with comorbid hyperactivity and other problems, genetic influences predominate.'20) French and American adoption studies show two- to threefold higher rates of criminality in adolescence in infants adopted into lower socio-economic status families. Danish and Swedish adoption studies have confirmed the effect of the socio-economic status of the rearing environment, but have also investigated the adoptee's genetic potential for antisocial behaviour, as indicated by the birth parent's criminality. These studies show a marked genetic effect as well as the environmental one. What appears to be particularly damaging is the combination of a genetic predisposition plus an adverse upbringing, which interact to give a high rate of antisocial activity.(21) Adoption studies are likely to underplay the importance of adverse environments, since adoption agencies select out prospective adoptive families who are notably dysfunctional and disadvantaged.
Genetic influences play a stronger role in the development of adult antisocial personality and criminality than in adolescent antisocial behaviour, due in part to the greater proportion of early-onset types in this group. Cytogenetic and molecular genetic studies have not yet identified specific conditions associated with antisocial behaviour, except for the XYY karyotype where affected individuals show about three times the rate of criminal behaviour than controls or individuals with other chromosomal anomalies such as XxY, even allowing for their reduced IQ.
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