In IB Psychology you need to be able to explain at least one example of how behaviour could be influenced by genetic factors. In this post, we’ll look at why a variation of the MAOA gene (a.k.a the “warrior gene”) could be linked with antisocial behaviours like aggression. There are two explanations: a simple one and a complex one. We’ll look at the simple explanation first.


Simple Explanation: Warrior gene affects the brain

Studies have shown that a variation of the MAOA gene (MAOA-L) can influence brain activity in certain situations. Meyer-Lindenberg et al. (2008) showed this in their study that compared the brain activity of two groups of healthy participants. One group had high expressing MAOA gene (MAOA-H), whereas another group had the variant that has low expression of the MAOA gene, which is the type correlated with aggressive behaviour (MAOA-L). When viewing angry and fearful faces in an fMRI, the MAOA-L group had significantly increased activity in their amygdala, and reduced activity in their ventromedial prefrontal cortex. Similar results have also been found in other studies (e.g. Raine, 2008).

This relationship between the MAOA-L and brain activity could provide us with an insight as to why MAOA-L is correlated with antisocial behaviour: we know that one important function of the ventromedial prefrontal cortex is to inhibit our impulsive behaviour and to allow us to think through our actions. So if someone has reduced activity in the vmPFC and they are threatened, they might act on impulse by reacting aggressively. The chances of this are increased if they have high activity in their amygdala, since activity in the amygdala is associated with high levels of emotion and arousal, which are important components of anger and aggression.


More Evidence: Caspi et al.’s “Dunedin Study (2002)

This famous longitudinal study followed over 1,000 children in Dunedin, NZ over 25 years and took measures every few years. They found that the MAOA-L gene moderated the effects of experiencing child abuse on adult aggression – that is to say, those participants with the MAOA-L gene variant and who were abused as children, were more likely to be antisocial and aggressive adults, compared with just those with the MAOA-L gene by itself.

What this study shows us is that once again, the MAOA-L is associated with antisocial behaviour. However, it’s not just the gene that is responsible: our childhood and environmental factors are also important factors.

Exam Tip: If you are asked “to what extent genetics influences behaviour” you can explain the relationships between MAOA-L and aggression as your central argument. Your counter-argument (critical thinking) can include other possible variables that are important (e.g. culture, childhood, neurotransmitters, social learning, etc.)

This video explains more about the Dunedin study.


The more complex explanation:

Let’s now look at a more detailed explanation of how the MAOA-L can affect brain activity. 

A human has about 20,000 individual genes. Genes send messages from the cell, which is called “gene expression.” When a gene is expressed it leads to other physiological processes. This is how genes can affect behaviour: their expression (or lack thereof) will influence our physiology (e.g. neurotransmitters, hormone levels or brain activity) which can affect our behaviour.

Our Student Workbook in the Criminology Support Pack includes all this information about the MAOA gene. 

More about MAOA

MAOA stands for “monoamine oxidase A (alpha).” Monoamine oxidase A (MAOA) is an enzyme that breaks down neurotransmitters in the synapse. So the job of the MAOA-gene is to send signals from the cell to produce the MAOA enzyme. However, some humans have a low expression variant of this gene (known as MAOA-L). If they have the low expressing variant it means that there will be less MAOA produced. This can affect neurotransmitter levels (including serotonin and dopamine) which affects brain activity in important parts of the brain, like the amygdala and the prefrontal cortex. This could explain how the MAOA-L affects brain activity.

I wouldn’t recommend trying to go further to explain how or why less MAOA will influence neurotransmitter levels, unless you’re really keen.