Genetic Factors

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Twin and Adoption Studies

Heritability (genetic) factors have been linked to aggressive behaviour with many studies focussing on twin and adoption studies.

Twin studies allow researchers to examine the role of genetics.
- Identical, or monozygotic (MZ) twins share 100% of their genes.
- Non-identical, or dizygotic (DZ) twins share 50% of their genes.
If a closer genetic relationship is related to higher or lower levels of aggression then aggression can be said to have a heritable component.

Coccaro et al. (1997) studied aggressive behaviour (defined as direct physical assault) in adult male MZ and DZ twins.
They found concordance rates of:
- 50% for MZ twins.
- 19% for DZ twins.

Christiansen (1977) investigated the role of genetics in aggressive criminal behaviour.
Christiansen (1977) conducted a case study of 3500 twins in Denmark and looked at both MZ and DZ twins
For MZ male twins the concordance rate was 35% and MZ females 21%, compared to 13% DZ males and 8% DZ females.
Since MZ share 100% DNA this gives some support to the genetic explanation of aggressive behaviour.

Adoption studies have found that similarities in aggressive behaviour between an adopted child and their biological parents.
This suggests that genetic influences are operating.
Similarities with the adoptive parents suggest that environmental factors also influence aggressive behaviour.

Rhee and Waldman (2002) carried out a meta analysis of adoption studies relating to direct aggression and antisocial behaviour.
They discovered that genetic influences accounted for 41% of the variance in aggression.
This is more or less in line with findings from twin studies.

The MAOA gene could be linked to aggressive behaviour.

The MAOA, or monoamine oxidase A, is an enzyme whose role is to mop up neurotransmitters in the brain after nerve impulses have been transmitted from one neuron to another.
MAOA does this by breaking down the neurotransmitter (especially serotonin) into constituent chemicals to be recycled or excreted, a process known as catabolism.

The production of the enzyme is determined by the MAOA gene and dysfunction in the operation of this gene may lead to abnormal activity of the MAOA enzyme.
This can affect serotonin levels in the brain.

One variant of the MAOA gene, often known as the warrior gene, leads to low MAOA activity in certain areas of the brain.
This has been associated with various forms of aggressive behaviour.

Brunner et al. (1993) studied 28 members of a large Dutch family who were repeatedly involved in impulsive aggressive violent criminal behaviours such as rape, attempted murder and physical assault.
They found that these men had abnormally low levels of MAOA in their brains and the low activity version of the gene.

Cases et al (1995) provided evidence via an animal experimentation to support Brunner’s findings about the MAOA gene and its link to aggression.
Transgenic mice were studied who had a deletion in the gene that encodes MAOA.
MAOA deficient males showed increased aggressive behaviour.
This shows that MAOA deficiency is linked to aggression.

Stuart et al. (2014) studied 97 men who (because they had been involved in inflicting intimate partner violence) (IPV), were part of the batterer treatment program.
Men with the low activity MAOA gene were found to be the most violent perpetrators of IPV, engaging in the highest levels of physical and psychological aggression and inflicting the worst injuries on their partners.

If the low activity variance of the MAOA gene is associated with greater aggression, then the converse must also be true.
Merins et al. (2011) studied participants with low-activity and high-activity variance of the MAOA gene in a money distributing game where participants had to contribute money for the good of the group.
The researchers found that males with a high-activity variant were more cooperative and made fewer aggressive moves than the low-activity participants.

Genes interact with the environment to produce aggression; this is known as gene-environment (GxE) interaction.

Genes influence aggressive behaviour but they do not function in isolation.
Low MAOA gene activity only appears to be related to adult aggression when combined with a early traumatic life events.

Frazzetto et al. (2007) found an association between higher levels of antisocial aggression and the low activity MAOA gene variant in adult males.
However this was only the case in those who had experienced significant trauma, such as sexual or physical abuse, during the first 15 years of life.

Those who had not experienced such childhood trauma did not have critically high levels of aggression as adults, even if they possessed the low-activity MAOA variant.
This would suggest a gene environment interaction or diatheses-stress.