Genetic explanations for aggression are concerning, as highly aggressive people often become violent
criminals, are we saying some people are just born criminals?
Well in this second aggression video we will look at the theory and some gene candidates.
As well as asking if the evidence stacks up for this disturbing claim.
Genetic factors in aggression If we are giving a genetic explanation for
aggression, then of course we are saying aggression is inherited.
If we are making the argument aggression is innate then we should see similarities in
aggression levels between people who are closely related, and for that psychologists will often
turn to twins, with monozygotic twins sharing all of their genes, so genetically identical,
and dizygotic twins as related as siblings, sharing 50% of their genes with their twin.
One twin study did find a difference between monozygotic and dyzygotic twins and aggression.
Coccaro’s research on Vietnamese twins found the genetic difference accounted for 47% of
the difference in assault.
We can also use family studies in this kind of research, looking at the concordance rate
in aggression.
So the likelihood of two people showing the same behaviour.
With closer genetic relations within a family being more likely to both show aggression,
or not show aggression, a higher concordance rate.
But of course there are issues with both twin and family studies, identical twins may be
treated differently to non identical twins, having more similar experiences and there
may be social pressures making them act more alike.
The same is likely true for family dynamics, with siblings having a more similar environment
than cousins.
Well there is a way to remove this issue, adoption studies.
With adoption studies we can see if people develop aggressive behaviours due to their
genes, even if they are raised in non aggressive families.
Well there isn’t much research on aggression and adoption, but there is adoption evidence
by mednick and Hutchings that a related factor, criminality, is linked to genetics.
Looking at the criminal records of adoptees, they found if they had a biological criminal
father 21.4% of the adoptees had a criminal record, if their father was non criminal only
10.5%.
So what's going on?
How could someone be aggressive genetically?
Well genes code for the biological processes that happen in your body, so everything we
discussed in the previous video, limbic system development, how much testosterone is made
in the testis, and how much serotonin is produced, broken down and how receptors detect serotonin
in the brain.
Genetic factors in aggression Now let me say right away, there is no one
gene, or combination of genes that has been identified that always results in aggression,
it's not that simple, any potential link found is always weak and there must be an interaction
with the environment.
But some psychologists do think that genetic codes, genotypes are associated with the display
of aggressive behaviour, so the phenotype.
Now there have been a couple of gene variations that seem to be linked to aggression, the
first one was fairly famous at the time and got a fair amount of media coverage, it suggested
the XYY chromosomal disorder resulted in aggression.
This isn’t a variation in a part of the genetic code, this is an entire extra Y chromosome.
Most biological males are XY.
Early researchers suggested these males were taller, more aggressive and had low empathy.
A few case studies suggested these males may be overrepresented in the prison population.
In fact this research was flawed, and while XYY individuals do develop faster and tend
to be a little taller on average they are not more aggressive.
However there is more convincing evidence that a variation in the MAOA gene is linked
to aggression.
The MAOA enzyme breaks down serotonin in the brain, and its production is controlled by
the MAOA gene.
The MAOA- L version produces low amounts of enzyme, resulting in excess serotonin in the
brain.
Now this genetic variation has been given the nickname the warrior gene, it's actually
fairly common, found in one third of all western men and in some countries over 60% of males.
As not all these men are aggressive it's suggested that a combination of early child abuse and
having MAOA-L as a vunerability is needed to trigger aggression.
This process is a diathesis stress model of aggressive behaviour.
Genetic factors in aggression - Evaluations A family with a large number of particularly
aggressive males was researched for a case study by Brunner.
These males were found to all have the low activity MAOA gene, and it's suggested that
their extreme levels of aggression was due to their genetic makeup.
Another researcher, Stuart, also provides evidence for MAOA leading to aggression.
The low activity version of the gene was found in the most physically and psychologically
aggressive members of a sample of 97 men, all convicted of intimate partner violence.
Now the MAOA-L gene results in high levels of serotonin, but in the previous video I
told you low levels of serotonin is linked to high levels of aggression.
These contradictory results mean it's likely serotonin has a role to play in aggressive
behaviours, but a complex one, with too much, or too little being problematic.
A major issue with the MAOA hypothesis is a third of western men have the gene, and
the vast majority are not aggressive, if MAOA has a real connection with people showing
aggressive behaviours, then this biological process is likely not the full story, it must
work in combination with social
psychological factors.