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Original Articles
Introduction
Understanding the roots of human criminal and violent behavior has been a fundamental question in the social and biomedical sciences since the mid-19th century (Lilly et al. 2010). In light of the strong evidence that criminal and violent behaviors 'run in families' (Burt, 1925; Glueck & Glueck, 1950; Farrington et al. 1975), a central subject in the large resulting research literature has been determining the magnitude of the genetic contributions to the propensity to crime. A long tradition of genetic epidemiological research has addressed this question (Lange, 1929; Rosanoff et al. 1934). For example, classic twin studies using national registers in Denmark (Christiansen, 1974) and Norway (Dalgard & Kringlen, 1976) found heritable influences on broadly defined criminal behavior (CB) (including both violent and non-violent CB; VCB and NVCB, respectively), and many other investigators found genetic influences on a range of antisocial/aggressive disorders and traits (Mason & Frick, 1994; Rhee & Waldman, 2002; Frisell et al. 2011). However, findings on criminality from adoption studies, the most powerful design in humans to separate 'nature and nurture', have been surprisingly inconclusive. Given the potential methodological limitations of the non-experimental designs possible in human genetics, it is particularly important to attempt to validate findings using different and complementary methods.
The first adoption study of broadly defined CB identified 52 adopted-away offspring of women with criminal offences in Iowa and 52 matched adoptive controls, and reported a significant excess of criminal records and incarceration in the index versus control adoptees (Crowe, 1972). The largest study to date, performed with the Danish adoption register and examining only males (13 194 adoptees), found evidence for genetic transmission of risk to property crime but not to violent crime as well as an association between all convictions and the adoptive parents' social class (Gabrielli & Mednick, 1984; Mednick et al. 1984). The Stockholm adoption study (2000 adoptees) found that criminality alone was not transmitted from biological parents to adoptees, but did find elevated rates of criminality in adopted-away offspring of biological parents with alcohol use disorders (AUD) alone, or with both AUD and criminality (Bohman, 1978). Very recently, self-report measures of CB were assessed in a small US sample of adoptees (about 250 subjects)...