Defining Aggression

Aggression is an act carried out with the intention to harm another person- this harm can be physical or psychological. There are different types of aggression, for example:

  • Hostile/reactive aggression: driven by anger and hurt, where the goal is to harm another for the sake of getting even with them. Characterised by displays of rage and impulsivity (screaming, shouting, etc)
  • Instrumental/Proactive Aggression: serves as a means to an end, where aggression is carried out to solve a problem. This is cool, emotionally detached and often pre-mediated (military actions, for example)

The Limbic System

The limbic system is a network of subcortical structures (deep inside the brain) linked with more primitive, emotional behaviours. Structures include the hypothalamus, hippocampus, the thalamus, and the amygdala- this is the key structure in aggression. Increased amygdala activity is associated with aggression, as it has a role in the assessment of threats and challenges. Gospic et al (2011) discovered that amygdala activity increased in response to an aggressive game in which participants how to decide to split money with a confederate. If both agree to share money, it is split between them. If the participant refuses the offer, both leave with nothing. More activity was seen in the amygdala when the confederate made an unfair offer, which the participant then rejected. Taking drugs to lower this activity also lowered aggression- the participant was less likely to reject the offer.

Aggression, figure 1


Serotonin is a neurotransmitter thought to inhibit aggressive responses to emotional stimuli and increase self-control. Low levels of serotonin are associated with increased susceptibility to impulsive behaviour, aggression and violent suicide, and a general lack of self-control. Mann et al (1990) found that dexfenfluramine- a drug which reduces serotonin- increased measures of hostility and aggression in males (but not females), as assessed through questionnaires. Virkkunen et al (1994) found lower levels of a serotonin breakdown product, and increased sleep disruption, in impulsive offenders compared to non-impulsive offenders, suggesting a low level of serotonin is linked with aggression.


  • There is evidence that the amygdala acts alongside other structures in aggressive behaviour, for example the orbitofrontal cortex (OFC). This weakens the explanation, as it suggests that the role of brain structure is more complex than first thought, involving many different structures.
  • Berman et al (2009) found that participants given a drug which boosted serotonin activity were less likely to give electric shocks in a laboratory-based game than a placebo group, but only where the participants had a prior history of aggressive behaviour, supporting the role of serotonin in aggression (to an extent).
  • Raleigh et al (1991) found that vervet monkeys fed on serotonin-increasing diets showed reduced aggression, supporting the serotonin explanation (although there may be difficulties generalising to humans).


Testosterone is thought to act on areas of the brain which control aggression from young adulthood onwards, and is regarded as the primary biochemical influence on aggression- perhaps explaining why males are generally more aggressive than females. The ‘challenge hypothesis’ suggests threats to male status should produce a surge in testosterone. Dabbs et al (1987) looked at salivary testosterone levels, and were able to differentiate between violent and non-violent criminals just on the basis of the levels found. Dolan et al (2001) found a positive correlation between testosterone levels and aggression in 60 male offenders, suggesting a link.


  • Wagner et al (1979) found that castrated male mice showed decreased aggression, which then increased when given testosterone again, supporting the link between testosterone and aggressive behaviour.
  • Archer (1991) analysed five studies involving 230 males, and found a low positive correlation between testosterone and aggression, again supporting the link.
  • Popma et al (2007) found that high levels of testosterone only lead to aggression when levels of cortisol are low, suggesting that the combination of testosterone and other hormones may be a better explanation of aggression than testosterone alone.

Genetic Factors in Aggression

Twin studies: These are often used to establish if there is a genetic basis for aggression. Coccaro et al (1997) found that up to 50% of the variance in aggressive behaviour can be explained by genetics. There was a 50% concordance rate amongst identical (MZ) twins, and a 19% concordance rate for non-identical (DZ) twins for physical aggression. For verbal aggression, the concordance rates were 28% for MZs and 7% for DZs. This suggests a genetic link, as the twins would share environments, but MZs share a greater proportion of genes.

Adoption studies: These are useful for separating the effects of genetics (nature- biological parents) and upbringing (nurture- adoptive parents). Waldman and Waldman (2002) found that 41% of the variance in aggressive behaviour can be attributed to genes, through their meta-analysis of adoption studies, suggesting a genetic link.

Aggression, figure 1

The MAOA gene: MAOA (Monoamine oxidase A) is an enzyme responsible for ‘mopping up’ neurotransmitters after they have been transmitted across a synapse. MAOA works on serotonin to break it down once it is transmitted. MAOA production is controlled by a particular gene, one variant of which has been nicknamed the ‘warrior gene’. Dysfunction in this gene leads to dysfunction in MAOA production, which affects the levels of serotonin in the brain, which increases the likelihood of aggressive behaviour. Brunner et al (1993) found a link between a gene responsible for producing MAOA and aggression. A study of a Dutch family found that many of its male members behaved in a particularly violent and aggressive manner, and a large proportion had been involved in serious crimes of violence including rape and arson. These men were found to have abnormally low levels of MAOA and a defect in this gene was later identified. It could be that the low levels of MAOA caused more serotonin to be present, reducing its inhibitory effect on other neurotransmitters.

Gene-environment interactions: It seems that low MAOA activity is linked with aggression only in combination with an environmental influence, for example a traumatic life event. Frazzetto et al (2007) found a link between the low-activity MAOA gene variant and aggression, but only in those who experienced childhood sexual or physical abuse. Those who had not been abused did not show high levels of aggression, even with the low-activity MAOA gene. This suggests an interaction between genes and the environment, supporting a diathesis-stress explanation.


  1. It is hard to isolate the role of genes from environmental influences. Genes may only create a vulnerability to aggression, when combined with certain environmental influences. Therefore, the genetic explanation of aggression seems not to be a complete one.
  2. There are likely to be multiple genetic influences on aggression, for example genes involved in MAOA production, serotonin production, and many others. This decreases the usefulness of this explanation, as a particular genetic cause is too hard to identify.
  3. There are different methods of measuring ‘aggression’ in studies in this area, for example self-reports, observations, and others. This creates questions over the validity of the research findings, so reducing the support for genetic factors in aggression.

Exam Question

Eduardo is at secondary school and has been suspended for being in a fight. This isn’t the first time this has happened. His younger brother Felipe is also showing signs of aggression, for example trying to slap another child in his class. Eduardo’s school are aware that his father has spent time in prison for assault.

Using your knowledge of genetic factors in aggression, explain the behaviour of Eduardo and his brother. (4 marks - 1-2 paragraphs)
Your answer should include: Genetic / 50% / Influence / MAOA