Aasai

Twin studies

Both identical (monozygotic or MZ) and fraternal (dizygotic or DZ) twins have been studied in an attempt to examine the relationship between inheritance and criminal behaviour. MZ twins develop from the splitting of a single egg at the time of conception and so share a very high proportion of the same genes. 

It is often thought that they are totally identical but there is the possibility for random variations and different influences in the womb that make them appear different. DZ twins, though, just like any other siblings, share about 50 per cent of their genes. So, when DZ twins share the same environment, any major differences between them are very likely to be due to genetic variation. Thus any differences between MZ and DZ twins are usually regarded as an indication of the power of genetics.

Lange’s (1931) twin study was one of the first to demonstrate concordance (the degree to which related pairs of subjects within a study population display the same behaviour) for criminal behaviour. He reported 77 per cent for 13 pairs of identical twins and 12 per cent for 17 pairs of fraternal twins. 

Other studies have found differences in the same direction but not always to the same degree. In general, the average concordance rate for MZ twins is 55 per cent and for DZ twins 17 per cent (Bartol, 1999), favouring the idea of some genetic influence on criminality. These findings, though, are inconclusive in the absence of data on twins reared apart. Critics argue that the phenotypic similarities of MZ twins entail a closer relationship, more similar treatment by parents and therefore similar interests, behaviour and social responses. As a consequence, it may be these social processes that give rise to the concordance rather than genetics.

Adoption studies

Another strategy that helps to determine the impact of genetics is to consider people who have been adopted and therefore brought up by people who are not their biological parents. The view is that to some degree this separates out the influence of genetics from the influence of family environment. 

This may not be as straightforward as may seem at first sight. The whole process of adoption and indeed how people come to be adopted into families that are criminal, or adopted from families that are criminal, raises many questions that go beyond genetics.

One of the first studies of criminality in relation to adoption was by Crowe (1974) who found that in a sample of adopted children whose biological mothers had a criminal record, almost 50 per cent of adoptees had a criminal record by 18 years of age, while only about 5 per cent of the adoptees whose mothers had no criminal record had been convicted for a criminal offence. 

In a much larger study Hutchings and Mednick (1975) looked at over 1000 adoptees. They found that 21.4 per cent of sons became criminal when the biological father was criminal, 11.5 per cent when the adoptive father was criminal and 10.5 per cent when neither father was criminal. However, if both fathers had criminal records the percentage of adoptees with criminal records was higher at 36.2 per cent. 

Mednick et al (1984), in their study with a sample of 14,427 adoptees, also showed that the percentage of criminal adoptees is higher when both fathers are criminal . All these studies imply a modest genetic contribution to crime and show that interaction between genetic and environmental factors appears to be crucial.

In reality no one can predict the consequence of family history on likely criminality with any certainty. There are children with criminal parents who experience adverse environments and yet do not commit crimes, and there are children with non-criminal parents, raised in seemingly good environments, that become criminals. ‘The question of just what is inherited remains unanswered; it is unlikely to be criminality as such’ (Rutter and Giller, 1983: 179).