The reasons for criminal and aggressive behaviour in humans are complex and include both biological and nonbiological explanations. Twin studies estimate that up to half of the variance in aggressive behaviour can be attributed to genetic factors. Genes play a role in aggression, and both human and animal studies suggest that common gene variants predispose some people to violent actions and criminality. Several studies have found associations between aggressive behaviour in males and a low-activity variant of the monoamine oxidase A (maoa) gene. These associations are often dependent on environmental insults, which dependent on maoa genotype may increase susceptibility for aggressive behaviour. These and other findings pose a challenge in determining criminal responsibility as the complex interplay of genetic and environmental factors in the aetiology of violence becomes better understood.

Key Concepts:

• Aggression has an evolutionary basis with both positive and negative aspects.
  
• Criminal behaviour falls under a larger phenotype of antisocial and aggressive behaviour, marked by an inability to conform to social norms.
  
• Twin and adoption studies demonstrate that roughly half of the individual variance in aggressive behaviour is due to genetic factors.
  
• Despite well-known gender differences in aggressive behaviour, biological evidence in humans tying differences in aggression to sex hormones remains equivocal.
  
• There is a robust literature demonstrating the association between serotonin and aggressive behaviour, highlighted by candidate genes along the serotonergic pathway.
  
• A number of studies have identified interactions between environmental adversity and the low-activity variants of the MAOA gene as a predictor of aggressive behaviour.