This is the idea that behaviours, such as aggression, can be inherited from parents. There is a very complex relationship between genes and behaviour, but research into behavioural genetics has found that many aspects of human behaviour can, to some degree, be influenced by the genes we inherit. There is certainly no single gene that will make someone aggressive, but there may be a type of genetic make up that interacts with the environment to make someone aggressive in certain situations. Understanding the interplay between genetic make up and the environment (the nature-nurture debate) requires research methods that can directly compare both factors, such as twin and adoption studies.
Twin studies generally focus on identical (monozygotic or MZ) twins and non-identical (dizygotic or DZ) twins raised in the same family environment. MZ twins in the same environment therefore share all their genes and their environment, whereas DZ twins in the same environment only share the same environment. Therefore if the concordance rate for a behaviour (the proportion to which both twins exhibit the same behaviour) is higher for MZ twins than DZ twins, it is probably mostly genetically influenced; and if the concordance rate for MZ and DZ twins is similar, the behaviour is probably environmentally influenced.
Adoption studies compare the rates at which adopted children share behaviours with their adopted parents (who they only share the environment with) and their biological parents (who they share genes but not environment with). If the concordance rate between child and adopted parent is higher then it is probably an environmentally influenced behaviour, but if concordance for the behaviour is higher between child and biological parents then it is probably a genetically influenced behaviour.
Evidence from twin and adoption studies
The concordance rates for aggressive and anti-social behaviours tends to be highest between monozygotic (identical) twins than dizygotic (non-identical) twins. McGuffin and Gottesman (1985) found a concordance rate of 0.87 (or 87%) between MZ twins and 0.72 between DZ twins. This shows that genes have the greatest influence on aggressive behaviours, but the high concordance rate for DZ twins shows that the environment is also an extremely influential factor.
Coccaro (1997) compared MZ and DZ twins for aggressive behaviour and found that genes were able to account for around 40% of the differences between individuals in aggressive behaviour; but that the environment accountd for 50% of individual differences in physical aggression and 70% of individual differences in verbal aggression. In other words, genes accounted partly for whether or not a person became aggressive, but the tyoe of aggression was heavily influenced by the environment a person is brought up in.
Hutchings and Mednick (1973) found a significant correlation between the criminal convictions of biological fathers and the number of criminal convictions of their sons who had been adopted into other families. This suggests there is a genetic factor in aggressive behaviour, and that it is not simply the process of being adopted that causes aggressive behaviours because the correlation between adopted parents and adopted children was not significant.
A meta-analysis by Miles and Carey (1997) of 24 twin and adoption studies found a significant genetic influence on aggressive behaviour. They also found that there was a strong influence from environmental and genetic factors, but that the environmental influence became less and the genetic influence became greater as the children grew older.