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Aggressive behaviour lies within an individual’s genetic makeup.

Individuals are predisposed to act aggressively as a result of inherited genetic abnormalities.


Twin studies compare the concordance rates (degree of similarity)

between monozygotic twins and dizygotic twins.


If MZ twins are more similar in their aggressive behaviour than DZ twins, then this could be attributed to their genetic make-up rather than environmental factors

seeing as MZ share 100% genetic similarity and DZ share 50%


Several twin studies have suggested that heritability accounts for

about 50% of the variance in aggressive behaviour.


Adoption Studies

Adoption studies involving investigating the likelihood of aggressive behaviour being displayed in adopted children who have aggressive biological parents but have not shared the same environment can help to separate genetic and environmental factors in aggression.


If a positive correlation is found between aggressive behaviour in adopted children and aggressive behaviour in their biological parents

a genetic component is implied


If a positive correlation is found between the adoptee’s aggressive behaviour and the rearing family

then an environmental effect is implied


Meta-analysis of adoption studies of direct aggression and anti-social behaviour reveal that

genetic influences account for 41% of the variance in aggression.


The “Warrior” gene MAOA has been linked to aggressive behaviour.

The gene produces an enzyme (protein) called Monoamine Oxidase A (MAO-A)
Regulates levels of serotonin, dopamine and noradrenaline


Aggressive individuals inherit a GENETIC ABNORMALITY ‘MAOA-L’ variant which produces LOW levels of this enzyme.

Low levels of the enzyme means excess neurotransmitters CANNOT be broken down after transmission (stops effective communication between neurons in the brain)


Those individuals predisposed to aggressive behaviour inherit a genetic abnormality known as MAOA-L

whereby they have low MAOA activity. Without this enzyme the individual is likely to act aggressively.


MAOA-L is much more frequent in populations with a history of warfare

with two-thirds of people in these populations having this version of the gene compared to only one-third of people in Western populations.


There is supporting evidence from twin and adoption studies for genetic factors
Hutchings and Mednick

An investigation assessed aggression levels in pairs of 54 MZ twins and 79 DZ using a personality scale
MPQ. Results found that in MZ twins there was a concordance of 0.43 for aggressive behaviours, whereas for DZ the concordance was much lower at 0.30
Meta analysis investigated aggression in 14,000 individuals who had been adopted in Denmark. They found a significant postive correlation between the number of convictions for criminal violence among the biologicialparent (particularly the fathers) and the number of convictions for criminal violence among their sons that had been adopted
add to val of genetic companant in agg explanation


There is supporting evidence from for the role of MAOA in genetic factors

28 male members of a Dutch family who were repeatedly involved in murder, rape and physical assaults. found to have abnormally low levels of MAOA in their brains, and have the low activity version of the gene MAOA-L
97 males who were involved in a treatment programme for offenders who committed violence against partners, showed that the males with the low activity version of the gene MAOA-L engaged in the highest level of violence and inflicted the worst injuries on their partners
validity of genetic componant in agg explanation


There are gene-environment interactions which must be considered

found an association between higher levels of antisocial behaviour and the low-activity MAOA gene variant in adult males, but only for those who had experienced a significant trauma (i.e. sexual or physical abuse) during the first 15 years of life. Those who had not experienced such childhood trauma did not have particularly high levels of aggression even though they possessed the low activity MAOA variant
May possess a gene associated with aggression but behaviour only expressed under certain environmental conditions minimises the role of genetic factors


methodological issues with some evidence investigating the role of genetics
measuring aggression

focus on aggressive individuals convicted of crimes, unrepresentative because many violent individuals are not commited, only represents a minority

differences in genetic varience shown depending on which self report method is used observations indicate less genetic contribution than self reports. findings from self reports may have exaggerated the role of genetics

make it difficult to assess the relative contributions of genetic factors in aggression, reducing validity