Biological Explanations of offending Twin Studies and Adoption Studies

Genetic Transmission Recent genetic research has moved away from the more simple theories put forward by Lombroso and Sheldon and instead focuses on genetic contributions to criminal behaviour. The main research areas include twin and adoption studies.

Twin studies The rationale underlying the use of twins in explaining criminal behaviour is that if one MZ twin displays aggressive and criminal tendencies and so does the other twin, then that behaviour must be innate. However, if one MZ twin displays aggressive and criminal tendencies but the other twin does not, then the environment may be more responsible. The expression of the degree of similarity in twin pairs is by concordance rate.

Examples One of the earliest twin studies was reported by the German physician Johannes Lange (1929). He found that MZ twins showed a much higher degree of concordance than dizygotic (DZ) twins for criminal behaviour. Christiansen (1977) studied 3,586 twin pairs from the Danish islands and found concordance rates of 35% (MZ) and 13% (DZ) for male twins and 21% (MZ) and 8% (DZ) for female twins. Similarly, Dalgaard and Kringlen (1976) in a Norweigen study found concordance rates of 26% (MZ) and 15% (DZ).

Evaluation Whilst these studies seem to suggest that criminal tendencies are inherited, a number of important points must be considered. First, the concordance rates in these studies are low, indicating that environment has a substantial influence. Secondly, a more plausible explanation for the concordance rates in MZ twins is that they share a similar and close environment, where they may be treated more alike because they look similar, whereas DZ twins tend not to.

Adoption Studies The rationale underlying the use of adoption studies in explaining criminal behaviour is the comparison of criminals with both their biological and adoptive parents. If, in criminal behaviour, the child is more similar to their biological parents than to their adoptive parents, with whom they share the same environment, a genetic basis of criminality may be suggested. Conversely, if the child is more similar to their adoptive parents than their biological parents, an environmental argument for criminality is favoured.

Examples Almost 50% of children in a sample of adopted children whose biological mothers had a criminal record themselves had a record by the age of 18 (Crowe,1972). In a matched control group of children whose biological mothers did not have a criminal record, only 5% of the adopted children had been convicted of a criminal offence. Hutchings and Mednick (1975) found that, if both biological and adoptive fathers had a criminal record, 36.2% of sons also became criminals. When only the biological father was the criminal, 21.4% of sons were criminal. When only the adoptive father was the criminal, 11.5% of the sons were criminal. When neither the biological nor the adoptive father had a criminal record, only 10.5% of the sons went on to have a criminal record.

The findings of Hutchings and Mednick’s study show that, whilst genetic factors clearly play a role in influencing criminal behaviour, environmental influences cannot be neglected. Finally, Stott (1982) argues that the prenatal environment influences how the child develops. A high degree of stress on the mother, particularly if she is from a low socio- economic class where conviction rates are high, can result in a variety of developmental disorders. If the adopted child shows criminal tendencies as an adult, this may be because of problems incurred during pregnancy and not as a result of the environment.

Evaluation As with twin studies, adoption studies have a number of limitations. First, children who are adopted tend to be placed in environments that are very similar to those of their biological parents. Secondly, some children are adopted at a far later age – months or even years after birth. It may be that their early life experiences are the cause of their criminal behaviour.