A twin study of self-reported criminal behaviour

Twin studies can be used to investigate the contributions of genetic factors, the common or shared environment, and the unique or non-shared environment to individual differences in a measurable characteristic. This paper reports the results of preliminary analyses of self-reported data on arrests and criminal behaviour from the Vietnam Era Veteran Twin Registry. The subjects for the study were 3226 male twin pairs in which both members served in the military during the Vietnam era. There were significant influences from both genetic factors and the common environment on early arrests. Genetic factors, but not the common environment, significantly influenced whether subjects were ever arrested after age 15, whether subjects were arrested more than once after age 15, and later criminal behaviour. The common environment, but not genetic factors, significantly influenced early criminal behaviour. The environment shared by the twins has an important influence on criminality while the twins are in that environment, but the shared environmental influence does not persist after the individual has left that environment. Genes are likely to influence the occurrence of criminal behaviour in a probabalistic manner by contributing to individual dispositions that make a given individual more or less likely to behave in a criminal manner.