Genes and aggression: MAOA

How genes could affect aggression

Genes carry instructions for making proteins, including enzymes and receptors in the brain. A gene does not 'make' a behaviour; it can raise or lower the likelihood of a behaviour, especially in certain environments.

Evidence that aggression is partly inherited comes from selective breeding in animals and from twin studies comparing identical (MZ) and non-identical (DZ) twins.

The MAOA gene

The MAOA gene codes for the enzyme monoamine oxidase A, which breaks down noradrenaline, serotonin and dopamine after synaptic transmission.

A low-activity version (MAOA-L, sometimes called the 'warrior gene') produces less of the enzyme, so these neurotransmitters are not broken down as efficiently. This may affect impulse control and reactions to threat.

McDermott et al. (2008) found that men with MAOA-L were more likely than MAOA-H men to punish a provoker with hot sauce when a lot of money was taken from them (75% versus 62% gave some hot sauce when 80% was taken).

Gene-environment interaction

McDermott et al. found the groups differed more under high provocation, which suggests genes and environment interact: the gene matters most when the person is provoked.

Brendgen et al. (2005) found physical aggression was strongly influenced by genes, while social aggression was influenced more by the environment.

Evaluation

• Strength: twin and gene studies use objective measures and support a genetic contribution.
• Strength: gene-environment interaction gives a more complete explanation than genes alone.
• Weakness: no single gene explains aggression; many genes and environments are involved.
• Weakness: twin studies assume MZ and DZ twins share environments equally, but MZ twins may be treated more alike, inflating genetic estimates.
• Weakness: ethical and social risks, such as labelling people with a gene as violent.

Worked example

Use McDermott et al. (2008) to explain gene-environment interaction.

1. At 80% taken (high provocation), 75% of MAOA-L men gave some hot sauce compared with 62% of MAOA-H men, a significant difference.
2. At 20% taken the groups differed less (40% versus 34%) and the difference was not significant.
3. So the gene's effect shows mainly when the environment provokes.

Answer: The MAOA-L gene predicted aggression mainly under high provocation.

Check yourself

1. What does the MAOA gene code for?
   • The hormone testosterone
   • The myelin sheath
   • Melatonin receptors in the pineal gland
   • An enzyme that breaks down neurotransmitters such as serotonin
   Show answer

   An enzyme that breaks down neurotransmitters such as serotonin. Monoamine oxidase A breaks down monoamine neurotransmitters.

2. Which MAOA variant is linked to higher aggression?
   • The high-activity variant (MAOA-H)
   • The low-activity variant (MAOA-L)
   • A variant found only in women
   • A variant that increases cortisol
   Show answer

   The low-activity variant (MAOA-L). MAOA-L produces less enzyme and is linked to aggression.

3. In McDermott et al., 75% of MAOA-L and 62% of MAOA-H men gave some hot sauce when 80% of their money was taken. What is the difference in percentage points?
   Show answer

   13 percentage points. 75 - 62 = 13 percentage points.

4. Why does McDermott et al. support gene-environment interaction?
   • The MAOA groups differed most when provocation was high
   • Every MAOA-L man was aggressive in every round
   • Provocation had no effect
   • The study used identical twins
   Show answer

   The MAOA groups differed most when provocation was high. The gene's effect depended on the level of provocation.

5. Which assumption of twin studies may inflate estimates of genetic influence?
   • Twins are always raised apart
   • DZ twins share 100% of their genes
   • MZ and DZ twins share their environments to the same extent
   • Aggression cannot be measured
   Show answer

   MZ and DZ twins share their environments to the same extent. MZ twins may be treated more alike, so greater MZ similarity is not only genetic.

6. Explain how the MAOA gene might lead to aggressive behaviour.
   Show answer

   The low-activity MAOA gene produces less monoamine oxidase A, so serotonin, dopamine and noradrenaline are not broken down efficiently, which may reduce impulse control and increase aggression, especially when provoked.. Link the gene, the enzyme and the neurotransmitters.

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Sources

• Pearson Edexcel International Advanced Subsidiary/Advanced Level in Psychology specification, Issue 5 (February 2021)
• Pearson IAL Psychology component guide: WPS02 (Issue 2, November 2024)
• Pearson IAL Psychology summary of studies: WPS02

Last updated 8 November 2026. Spotted a mistake? Tell us.