Genetic Factors in Aggression

• Genetic explanations of aggression assume that aggressive behaviour is the result of inherited traits/characteristics that have adapted across generations because of their evolutionary usefulness (i.e. aggression is a trait necessary for survival)
• About 50% of the variance in aggressive behaviour is explained by genetics, ergo 50% of variance is explained by environmental factors (Tuvblad & Baker 2013)
• Genetic inheritance of traits is generally investigated using twin studies which are ideal as MZ twins share 100% of their DNA and they not only provide a control sample for each other but can be compared to DZ twins
• Christiansen (1974) found 35% concordance for aggression between MZ twins but only 13% concordance for DZ twins; Rushton et al. (1986) found a correlation coefficient of 0.40 for MZ twins but only 0.04 for DZ twins 
• Adoption studies are also used to investigate genetic factors in aggression as they may highlight differences between biological and adoptive parents and their children
• Rhee & Waldman’s (2002) meta-analysis of 52 twin and adoption studies showed that environmental factors can only be an explanation for aggression if the specific genetic factors are already present i.e. environment alone is not enough to trigger aggression 

• The MAOA (monoamine oxidase) gene (also known as the ‘warrior gene’) is an enzyme which is implicated in the regulation of key neurotransmitters, particularly serotonin
• Serotonin has been linked to mood, specifically in the balancing of mood i.e. low or disrupted levels of serotonin may be a factor in affective disorders such as depression and also in aggressive behaviour
• A dysfunctional MAOA gene may lead to an imbalance of serotonin and, possibly, to an increase in aggression
• Brunner et al. (1993) which was also cited in the revision note on Neural Mechanisms of Aggression found that aggressive males in one family all shared the same dysfunctional gene which led to low levels of MAOA and, by extension, low levels of serotonin
• Some research suggests that a faulty MAOA gene alone is not enough to produce aggressive behaviour, rather it is the combination of a traumatic childhood with the faulty gene that is responsible for extreme aggression (Caspi et al. 2002)

She clearly has the warrior gene!

Research which investigates genetic factors in aggression

• Miles & Carey (1997) - Meta-analysis of 24 twin and adoption studies which found strong support for a genetic explanation of aggression; additionally they found that environment was more influential during a person’s youth and genetics were more important (in determining aggressive behaviour) during adulthood
• Reif et al. (2007) - 45% of aggressive participants had a faulty MAOA gene (low-activity, short allele) which was also linked to childhood trauma i.e. there are complex interactions between genes, serotonin and environmental factors 

Evaluation of genetic factors in aggression

Strengths 

• Biological evidence e.g. saliva samples, DNA analysis, is objective and can be measured by blind researchers making it  reliable 
• Twin and adoption studies tend to use large samples (e.g. Christiansen’s study above used 7000 twin pairs) which generates robust quantitative data

Weaknesses

• If genetic factors alone were responsible for aggression then concordance for MZ twins should always show a 100% concordance rate which has never been the case
• It is unlikely that the MAOA gene is solely responsible for aggression and, in fact, research in trying to establish which other genes may be involved in aggression is ongoing

Link to Issues & Debates:

Twin and adoption studies take a nomothetic approach as they use large samples, quantitative data and they attempt to establish general laws. An advantage of taking such an approach is that trends and patterns in behaviour are easier to spot and may then be linked to a biological basis for behaviour.