Genetic Causes of Aggression

XYY Karyotype

Sandberg (1961): First identified what is scientifically known as the 47 XYY Karyotype. Most individuals have 46 chromosomes (23 from each parent). Female have XX and male XY. It is possible for males to have an extra Y chromosome (making XYY).

The link between XYY males and increased aggressiveness was an early assumption that Court-Brown made prior to the examination of the patients. This point was later retracted, although it was too late because many scientists had already incorporated this incorrect view into their thinking. While the gene has actually only been proved to have effects on height and acne, but the stigma is still there and men with the XYY chromosome are assumed to be more aggressive despite research opposing this.

Research in this  area is conflicting so it is not possible to conclude that the XYY gene causes aggression. Using projective tests, Theilgaard showed that XYY men tend to give more aggressive interpretations of the images compared to XY males.

Research: 
- Court-Brown (1965-1967)
- Theilgaard (1984) 


Twin Studies

Monozygotic twins share all of their genes.
Dizygotic twins share 50% of their genes.

If a researcher compares the similarity between sets of MZ twins to the similarity between DZ twins for  a trait like aggression and finds that  MZ twins are more alike, then this likeness should be due to genetics rather than environmental factors. Most twin studies have focused on criminal behaviour.

Research: 
- Berkowitz (1993) 
- McGuffin and Gottesman (1985) 
- Meta analysis by Mason and Frick (1997) 
- Coccaro et al (1997) 


Adoption Studies

A way of studying genetic factors in aggressive behaviour is by studying children who have been brought up by adults who aren't their biological parents. Adoption studies can help to untangle the relative contributions of environment and heredity in aggression. If a positive correlation is found between aggressive behaviour in adopted children and aggressive behaviour in their biological parents, a genetic effect is implied.

Research
- Mednick et al (1975) 
- Miles and Carey (1997) 
- Brennan and Mednick (1993)