A01

• Twin studies indicate genetics play a substantial role in aggressive behaviour, with heritability estimated at around 50%. Coccaro (1997) compared aggression in monozygotic (MZ) and dizygotic (DZ) twins, finding higher concordance rates in MZ twins, supporting genetic influence. Adoption studies by Rhee and Waldman (2002) found genetics accounted for 41% of aggression variance, aligning with twin study findings. 
• The MAOA gene, dubbed the 'warrior gene', regulates serotonin and its low-activity variant (MAOA-L) is linked to aggression. Studies investigating New Zealand’s fierce Maori men and Brunner’s (1993) Dutch family with violent criminal behaviours both support this link.
• However, genetics interact with environmental factors, particularly early trauma, influencing adult aggression. Frazzetto (2007) found that while MAOA-L carriers exhibited higher aggression levels, this manifested mainly in those with childhood trauma, highlighting a gene-environment interaction termed diathesis-stress. This reveals the complex interactions between genes and environment in shaping aggressive behaviour, emphasising that genetic predispositions often require environmental triggers to manifest fully.