Genetic Factors in Agression

Genetic Factors in Agression

1. Twin Studies

   • Monozygotic twins (MZ) share all of their genes, while Dizygotic (non identical) twins share only 50% . In twin studies, researchers compare the degree of similarity for a particular trait (such as aggression) between sets of MZ twins of the similarity between sets of DZ twins. If the MZ twins are more alike in terms of their aggressive behaviour, then this should be due to genes rather than environment (both types of twins share the same environment as each other but MZ twins are more genetically alike). Most twin studies have focused on criminal behaviour generally, but one of the few studies to specifically study aggressive behaviour ising adult twin pairs found that nearly 50% of the variance in direct aggressive behaviour (i.e aggression toward others) could be attributed to genetic factors (COCCARO et al).

2. Adoption Studies

   • Adoption studies can help to untangle the relative contribution 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. If a positive correlation is found between the adoptees aggressive behaviour and the rearing family, then an environemtal 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 (particularly fathers) with criminal convictions (HUTCHINGS & MEDNICK), providing evidence for a genetic effect.

3. A gene for agression?

   • The role of MAOA
     • Although no individual gene for aggression has been identified in humans, a gene responsible for producing a protein called monomanie oxidase A (MAOA) has been associated with aggressiveness behaviour. MAOA regulates the metabolism of serotonin in the brain, and low levels of serotonin are associated with the impulsive and aggressive behaviour. In the 1980s a study of a Dutch family found that many of its male members behaved in a particularly violent and aggressive manner and a large proportion had been involved in serious crimes of violence, including rape and arson. These men were found to have abnormality low levels of MAOA in their bodies and a detect in this gene was later identified (BRUNNER)
   • Gene-environment interaction
     • A second study (CASPI) linking MAOA to aggressive behaviour, involved 500 male children. Researchers discovered a variant of the gene associated with high levels of MAOA and a variant associated with low levels. Those with low levels of MAOA were significantly more likely to grow up to exhibit antisocial behaviour but only if they had been maltreated as children. Children with high levels of MAOA who were maltreated and those with low levels who were not maltreated, did not display antisocial behaviour. This shows that it is the interaction between genes and environment that determines behaviours such as aggression.

4. Genetics and Violent Crime

   • Researchers do not suggest that there is a gene fro violent crime per se. Rather it is claimed that inherited temperamental or personal characteristics place some individuals more at risk of communicating violent crime. Adoption studies have shown that the highest rates of criminal violence in adopted children occur when both biological and adoptive parents have a history of violent crime - clear evidence of a gene environment interaction. However, a series of adoption studies in which the criminal history of both his biological and his adoptive fathers, found that genetic influences were significant in cases of property crime but not in cases of violent crime (BRENNAN & MEDNICK).

5. Evaluation

   • Difficulties of determine the role of genetic factors
     • We have discussed the role of genetic factors in aggression, but what does this really mean? The connection between genetic factors and aggression is far from straightforward because of problems determine what is, and what is not, a product of genetic inheritance. It is difficult to establish genetic contributions to aggressive behaviour for the following reasons:
       • More than one gene usually contributes to a given behaviour.
       • As well as genetic factors there are many non genetic (i.e environmental) influences on the manifestation of aggressive behaviour.
       • These influence may interact with each other. Genetic factors may affect which environmental factors have an influence and vice versa (gene environment interaction). This is clearly demonstrated by CASPI study.
   • Problems of assessing aggression
     • Many of the reported studies of aggression have relied on either parental or self reports of aggressive behaviour, whereas other studies have made use of observational techniques. In the MILES & CAREY meta analysis, mode of assessment was found to be a significant moderator of aggressive behaviour in the 24 studies that made up their analysis. They found that genetic factors explained a large proportion of the variance in aggressive behaviour in studies that has used parental or self reports.
       • However, those that had made use of observational ratings showed significantly less genetic contribution and a greater influence of environmental factors. For example, in a replication of Bandura's bobo doll study, twin pairs were encouraged to act aggressively towards the doll by being exposed to an adult model who also acted aggressively towards it. Researchers found no difference in correlations between MZ and DX twin pairs, suggesting that individual differences in aggression were more a product of environmental influences (e.g family upbringing) than genetic influences (PLOMIN)
   • inheritance of criminal violence
     • Methodological Limitations
       • Studies that have investigated the role of genetic factors often fail to distinguish between violent and non violent crime, making it more difficult to untangle the role of genetics factors in specifically aggressive violence. These studies also often fail to distinguish between criminals who are habitually violent and those for whom their violent crime is a one off.
     • Inconclusive evidence
       • The evidence for violent crime being inherited is also far from conclusive. A meta-analysis of studies in this area (WALTERS) found only a low to moderate correlation between heredity and crime, with better designed and more recently published studies providing less support for the gene crime hypothesis than more poorly designed and earlier published studies. A more recent review of published studies of youth crime (Surgeon general report on youth violence, 2001) concluded: 'The data do not suggest a strong role for heredity in violence'.
   • Research Support
     • MILES & CARY carried out a meta analysis of 24 twin and adoption studies that supported the genetic basis of aggression. Most studies had relied on parental or participants' self-reports of aggressive tendencies, although some involved observation of aggressive behaviour. The results suggested a strong genetic influence that could account for as much as 50% of the variance in aggression. Age differences were notably important, with both genes and family environment being influential in determining aggression in youth, but at later ages the influence of rearing environment decreased and then influence of genes increased.
       • A later meta analysis by RHEE & WALDMAN combined the results of 52 twin and adoption studies and also concluded that aggressive antisocial behaviours was largely a product of genetic contributions. However in this study, as with MILES & CAREY study above, several variables, including age of participant and assessment method for aggression, moderated the genetic influence on aggression, suggesting that although genetic factors play a significant part in the development of aggressive behaviours, the influence of other factors affects their expression.
   • Problems of sampling
     • Many of the studies in this area have focused on individuals convicted of violent crime. Two particular difficulties arise when trying to draw meaningful conclusions form these studies. The first problem lies with the participant themselves. Convictions for violent crime are relatively few compared to the vast number of violent attacks by individuals that never result in a conviction. They therefore represent just a small minority of those regularly involved in aggressive behaviour. Second, country to popular belief, offenders are not necessarily the most serious, persistent offenders. For example, a convicted murderer would be designated as violent for one offence despite, perhaps, having otherwise had a lifetime free from crime. This might explain why so many studies have found little or no evidence of heritability for violence.

6. IDA's

   • The value of animal research
     • Studies of aggressive behaviour is non human animals have an important role in helping us understand aggressive behaviour in humans. For example, rodents offer the advantage if experimental manipulation to test the effects of specific genes on aggressive behaviour. Manipulations may involve selective breeding programmes and ;knockout' techniques (where a single gene is eliminated from a group of experimental animals in order to study its effect). An example,of such a study that has potential for an understanding of human aggression was by YOUNG et al. These researchers claim to have identified a genetic mutation that causes violent behaviour in mice. This mutation that causes violent behaviour in mice. This mutation, nicknamed 'fierce', has a range of effects in mice, including extremely violent behaviour towards other mice. A counterpart of the gene does not exist in humans, although its precise function is not known.
   • Real world application
     • Although studies of the biological basis of aggression interest nearly everyone, research findings are far too uncertain to be valuable in understanding the causal factors affecting those who engage in violent activities. Nevertheless, there have been suggestions that public policy should be informed by the results of this research. If people are predisposed towards aggressive behaviour or violent crime, then questions about the treatment of such behaviours inevitably arise. Some commentators advocate genetic engineering but others go much further. As long as violence remains at the forefront of public concern, ways of dealing with it that address the problem, more directly (e.g through 'chemical castration') remain an attractive option to many.
       • Given the extremely tentative nature of conclusions that can be reached from this research and the far reaching ethical consequences of labelling an individual ad a threat to society on the basis of their genetic inheritance, an awareness of the limitations of these studies is extremely important.