Abstract

This paper performs a critical review of twin and adoption studies looking at possible genetic factors in criminal and antisocial behavior. While most modern researchers acknowledge that family studies are unable to separate possible genetic and environmental influences, it is argued here that twin studies are similarly unable to disentangle these influences. The twin method of monozygotic–dizygotic comparison is predicated on the assumption that both types of twins share equal environments, and it is argued here that this assumption is false. Adoption studies have been promoted as a better way of separating genetic and environmental influences. However, there is good reason to believe that adoption studies of criminal and antisocial behavior were confounded by selective placement factors. In addition, these studies suffered from bias and serious methodological errors. In spite of these problems, no adoption researcher claimed to have found evidence of a genetic predisposition for violent crime. It is concluded that the weight of the evidence from family, twin, and adoption studies does not support a genetic basis for any type of criminal or antisocial behavior. The historical background of genetic theories of criminality is also discussed