Behavioral Genetics, Molecular Genetics, and Criminal Behavior

Twin Studies: Identical twins share $100\%$ of their genes. Comparing twins raised together isolates the role of environmental influences when criminal outcomes differ.
Adoption Studies: Measure criminal behavior correlations between adopted children and both their biological and adoptive parents. Higher correlation with non-custodial biological parents indicates a genetic influence.
Gene-Environment Interaction: Genetics and environment jointly influence criminal behavior. Genes do not directly cause crime, but act through mediational traits (such as impulsivity) that predispose individuals to criminal behavior.
Childhood Maltreatment Example: Physical, sexual, or emotional abuse and neglect increase adult criminal likelihood; however, $50\%$ of maltreated children do not commit crime, proving environment alone does not dictate outcomes.

Molecular Genetics: Pinpoints specific individual genes associated with criminal behavior.
MAOA Gene Function: Encodes the MAOA enzyme, which breaks down neurotransmitters tied to aggression and antisocial behavior, specifically norepinephrine, serotonin, and dopamine.
2002 Longitudinal Study: Followed a cohort of male children from birth, conducting assessments at ages $3$ , $5$ , $7$ , up to age $26$ . Focused on diagnoses of conduct disorder, a precursor to adult antisocial personality disorder.
Study Results:
- Males with low MAOA activity who experienced severe maltreatment had an approximate $80\%$ rate of conduct disorder diagnosis.
- Males with high MAOA activity who experienced severe maltreatment had an approximate $40\%$ rate of conduct disorder diagnosis.
Conclusion: Demonstrates a clear interaction between a specific genetic factor (MAOA enzyme activity level) and an environmental risk factor (severity of childhood maltreatment) in predicting antisocial outcomes.