Genetic Factors in Aggression Flashcards
Twin Studies
MZ twins share all of their genes, whilst DZ twins share only 50%. In twin studies, researchers compare the degree of similarity for a particular trait between MZ twins and DZ twins. If the MZ twins are more alike in terms of their aggression, then this should be due to genes rather than the environment. Most twin studies have focused on criminal behaviour, but one of the few studies to specifically study aggression using adult twin pairs found that nearly 50% of the variance in the direct aggressive behaviour could be attributed to genetic factors.
Adoption Studies
If a positive correlation is found between aggression in adopted children and aggression in their biological parents, a genetic effect is implied. If a positive correlation is found between the adoptee’s aggression and the rearing family, then an environmental effect is implied. A study of over 14,000 adoptions in Denmark found that a significant number of adopted boys with criminal convictions had biological parents with convictions for criminal violence (Hutchings and Mednick).
Research on Genetic Influences in Aggression
- Miles and Carey did a meta-analysis of 24 twin and adoption studies that demonstrated the genetic basis of aggression. The results showed a strong genetic influence that could account for as much as 50% of the variance in aggression. Age differences were important, with both genes and family environment being influential in determining aggression in youth, but less influential in adults.
- Rhee and Waldman’s later meta-analysis combined the results of 51 twin and adoption studies and also concluded that aggressive anti-social behaviour was a product of genetic contributions. However, in this study, several variables, including age and assessment method for aggression, moderated the genetic influence on aggression, suggesting that although genetic factors play a significant role in aggression, the influences of other factors affect their expression.
The role of MAOA
Although no individual gene for aggression has been identified in humans, a gene responsible for producing an enzyme called Monoamine Oxidase A (MAOA) has been associated with aggression. MAOA regulates the metabolism of serotonin in the brain, and low levels of serotonin are associated with impulsive and aggressive behaviour. In the 1980s, a study of a Dutch family found that many of its male members were particularly aggressive and violent, and a large proportion had been involved in serious crimes of violence, including rape and arson. These men were found to have abnormally low levels of MAOA, and a defect in this gene was later identified (Brunner).
MAOA-H and MAOA-L
Caspi linked MAOA to aggression, involving 500 male children. Researchers discovered a variant of the gene associated with high levels of MAOA (MAOA-H), and low levels of MAOA (MAOA-L). Those with the MAOA-L gene were more likely to grow up to be anti-social but only if they had been mistreated as children. Children with the MAOA-H gene who were mistreated, and those with the MAOA-L gene who were not mistreated, did not display anti-social behaviour.
The Warrior Gene
MAOA-L is much more frequent in populations with a history of warfare, with about two thirds of people in these populations having this version of the gene. By way of contrast, only one third of people in Western populations have this gene. This is referred to as the warrior gene. McDermott found that MAOA-L ppts displayed higher levels of aggression (forcing a fictional ppt to eat unpleasantly hot and spicy sauce), when provoked, than did the MOAO-H ppts.
Problem of Sampling (AO3)
Many studies in this area have focused on those convicted of violent crimes. There are two main issues with this. Convictions for violent crime are relatively few compared to the vast number of violent attacks by those who never end up being convicted. Therefore, ppts represent a small minority of those regularly involved in aggressive behaviour. Secondly, offenders designated as violent on the basis of a court conviction are not the most serious, persistent offenders. E.g. a convicted murderer would be designated as violent for one offence despite having otherwise had a lifetime free from crime. This may explain why so many studies have found little or no evidence of heritability for violence.
Difficulties Determining the Role of Genetic Factors
