The Makings of a Criminal

It is a well established scientific fact that crime often runs in families. Osborn and West found that 40% of the sons of criminal fathers had criminal convictions, compared with a figure of 13% for the sons of non-criminal fathers. 32 Similarly, David Farrington found that 6% of families were responsible for 50% of all crime in a 30 year longitudinal study. All of this strongly suggests that there is a cause of crime to be found within the normal family dynamic. However, the question remains whether the cause is biological or sociological. This obvious corollary could easily be the result of genetic transmission. However, it could just as easily be said to be the result of the parents’ social influence upon their children. Thus the relationship alone can tell us nothing. Because they work to clarify this issue twin and adoption studies are seen to be extremely valuable.

Twin studies often employ both identical (monozygotic or MZ) and fraternal (dizygotic or DZ) twins in their comparisons. Identical twins share the same genes while fraternal share only about 50% of their genetic makeup. It is thought that since any given pair of twins share the same environment any major difference between the two should be the result of genetic difference.

Researchers investigate this to find the degree to which twins differ in the behaviour. A 50% concordance would indicate that in half of the total sample, both members of a twin pair displayed the same characteristics. One study of Farrington’s does suggest that biological factors play a role. 33 Although, specifically speaking, this role is more significant in relation to adult crime than to juvenile delinquency. This would suggest that the difference here is primarily the result of the twins’ differing environments; that their actions only begin to differ markedly after they have left home and had their own experiences. All in all, twin studies are ambiguous, in places supporting biological theories, while in others suggesting their inaccuracy.

Further, twin studies suffer from a large number of limitations. Consider a few:

• Random mating. Twin researchers assume that people are as likely to choose partners who are different from themselves as they are to choose partners who are similar for a particular trait. If, instead, people tend to choose mates like themselves, then fraternal twins could share more than 50 percent of their genes-and hence more similarities on genetically influenced traits-because they would receive similar genes from their mothers and fathers.
• Equal environments. Twin researchers also assume that fraternal and identical twins raised in the same homes experience equally similar environments. But some research suggests that parents, teachers, peers and others may treat identical twins more similarly than fraternal twins.

Because of all of this, if we are seeking clearer understanding of the reality of the situation, we must look elsewhere, to adoption studies.