Genetic explanations propose that offending behaviour is inherited as one or more genes predispose people to commit crimes. The evidence comes from family studies (which have shown that certain types of crime run in the family) and twin studies (which show higher concordance rates for offending in MZ twins compared to DZ twins). For example, Raine (1993) reviewed research looking at the delinquent behaviour of twins, finding 52% concordance for MZ (identical) twins and 21% concordance for DZ (non-identical) twins. Some researchers have attempted to identify genes that might be linked to crime, and one example is the MAOA gene, which causes a deficiency in monoamine oxidase A (MAOA), an enzyme responsible for the metabolism of neurotransmitters such as serotonin (which has links to aggression).