• The role of, and research into hormones and genes in aggression.

Hormones & Aggression

Testosterone: Testosterone is a male sex hormone that is also found in females, but in lower levels. Studies have shown that higher levels of testosterone are associated with increased aggression in both males and females. This may be because testosterone affects the development and function of the brain regions that are involved in aggression, such as the hypothalamus and amygdala. Exposure to higher levels of testosterone in the womb affects the developing brain and can lead to an increase in aggression. Chang et al. (2012) found that that the more testosterone a fish had in its blood the more aggressive it was so showing the that aggression and testosterone are linked. 

Cortisol: Cortisol is a hormone that is produced by the adrenal gland in response to stress. Studies have shown that high levels of cortisol are associated with increased aggression, possibly because cortisol can affect the activity of brain regions that are involved in aggression and stress response. Burnett et al. (2000) found that boys who had lower levels of cortisol in their saliva were three times more aggressive than boys with higher levels of cortisol, so cortisol does seem to play a role in aggressive behaviour.

Serotonin: Serotonin is a neurotransmitter that is involved in regulating mood, anxiety, and impulse control. Studies have shown that low levels of serotonin are associated with increased aggression and impulsivity. Researchers have studied the relationship between serotonin and aggression is by administering drugs that affect serotonin levels and observing changes in aggressive behavior. For example, selective serotonin reuptake inhibitors (SSRIs), which are commonly used antidepressants, increase the levels of serotonin by blocking the reuptake of serotonin, so it stays active in the brain for longer. Studies have found that SSRIs can reduce aggressive behavior in certain populations, such as those with impulsive aggression or aggression related to certain disorders like borderline personality disorder. There are also animal studies that support the relationship between serotonin and aggression. Studies have found that manipulating serotonin levels in animals can affect aggressive behavior. For example, when researchers blocked the action of serotonin in the brain of animals, they found that the animals became more aggressive.

Oxytocin: Oxytocin is a hormone that is involved in social behavior, including bonding and trust. Studies have shown that oxytocin may play a role in reducing aggression, by enhancing social behaviors and decreasing the response to threatening or aggressive stimuli. Low levels of oxytocin are thought to decrease trustworthiness and so increase levels of aggression. Lane et al. (2014) found more oxytocin led to an increase in trustworthiness.


  • Chang et al. (2012) found that when aggression, exploring and boldness increased in fish so did the amount of testosterone in their blood. In general animal studies have shown that increased testosterone leads to aggression/castration leads to lowered aggression. Animal studies may not be generalisable to humans as aggression in humans is can be social whereas in animals this is rarely the case.
  • Dabbs et al. (1987) found that the level of testosterone was higher in the males who had committed violent crimes compared to those who had committed non-violent crimes. Dabbs et al. (1987) had 89 participants which could be considered generalisable due to the large sample size.
  • Studies on humans may be correlations that find a relationship between hormones and aggression. Correlations cannot tell us if an increase in hormones causes aggression or vice versa. For example, it may be that aggression increases the levels of testosterone, rather than testosterone increasing aggression so testosterone does not play a role in causing aggression.
  • Whilst Lane et al. (2014) found more oxytocin did lead to an increase in trustworthiness, they did not study whether lack of the hormone caused aggression so it may not be a factor in aggressive behaviour.
  • Psychological research into the influence of hormones on aggression does not consider other explanations social learning theory would suggest that aggression is the result of imitating aggressive role model and not a failure of hormone regulation.

Genes & Aggression

Psychologists have used selective breeding in animals and have found that aggression is a trait that can be transmitted from parent to offspring, thus supporting the genetic explanation.

Research with human subjects has focused on twin studies that have looked at the incidence of aggression displayed by monozygotic (MZ or identical) and by dizygotic (DZ or non-identical) twins. Differences in rates (concordance) of aggression between these sets of twins have indicated that aggression has a genetic element, with heritability estimates ranging from 30% to 50%.

The MAOA gene codes for the enzyme monoamine oxidase A, which is responsible for breaking down neurotransmitters such as serotonin and dopamine in the brain. Variations in the MAOA gene have been linked to increased aggression in some individuals. Specifically, a variant of the gene called MAOA-L has been shown to be associated with increased aggression in men, particularly when combined with certain environmental factors such as childhood abuse or neglect.

Studies have found that individuals with the MAOA-L variant who have experienced childhood abuse are at a higher risk for aggressive and impulsive behaviour. This is thought to be because the MAOA-L variant leads to less efficient breakdown of the neurotransmitters noradrenaline, serotonin and dopamine, which can disrupt the balance of these chemicals in the brain and lead to increased aggression.

If adrenaline isn’t metabolised, then we end up with too much adrenaline. This can cause hypersensitivity in the fight or flight response and individuals may overreact to an external stimulus and perceive a threat where one does not exist. Furthermore, if dopamine is not broken down, increased or excessive levels of dopamine are also linked to aggressive behaviour. Serotonin has a calming influence, and low levels have been implicated in a reduction of control over impulsive behaviour.


  • Brendgen et al. (2005) studied physical aggression and found that the concordance rate for monozygotic twins was nearly double the concordance rate for dizygotic twins. Brendgen et al. (2005) used both teacher ratings and peer ratings of aggression, both scores were similar suggesting the data is reliably measuring the participants’ aggression.
  • McGuffin and Gottesman (1985) found a concordance rate of 87% for monozygotic twins and aggression and 72% for dizygotic twins, suggesting genes influence aggression. As the concordance rate was 87%, and not 100% in McGuffin and Gottesman (1985) it suggests there must be other factors apart from genes that influence aggression.
  • Twin studies are often used to study the genetic influence on aggression by comparing the concordance rates of monozygotic and dizygotic twins. Twin studies may not be valid, as it could be that monozygotic twins have a more similar environment than other siblings and this could be a reason for the higher concordance rates in aggression.
  • Other research into aggression has bred animals, such as mice, to investigate whether aggressive animals will have aggressive offspring or not. As animals are kept under controlled conditions researchers can assume the aggression in any offspring is due to the breeding of two aggressive parents, and not to environmental factors.
  • Stuart et al. (2014) studied 97 men who had been involved in severe domestic abuse to their partners. They found the most violent men had the faulty MAOA gene. These men engaged in the highest level of physical and psychological aggression and inflicted the worst injuries on their partners.

Past Paper Questions

2 Markers

  • Describe one hormone that may cause aggressive behaviour in males. (2) January 2017
  • Describe the role that one hormone may play in Kirk’s aggression. (2) January 2020
  • Explain one weakness of the role of genes in aggression. (2) October 2017
  • Explain one weakness of the role of genes in explaining the family’s aggression. (2) January 2021

3 Markers

  • Tariro has been fighting. His mother thinks that Tariro’s aggression is genetic as his father and grandfather have both been in trouble with the police for fighting. Describe the role of genes in Tariro’s aggression. (3) October 2017
  • Rosa is a psychologist who works with families who need help. She has assessed a family who have been referred to her due to their aggression. The father of the family frequently shouts at the children and throws objects at walls. The eldest two children often fight with each other, which led to one of the children having to go to hospital for treatment. Rosa believes that genes play a role in the aggression displayed by the family. (a) Describe the role genes may play in the family’s aggression. (3) January 2021

4 Markers

  • Describe the role of the limbic system in aggression. (4) June 2021
  • Explain two weaknesses of the limbic system as an explanation for aggression. (4) June 2021
  • Explain one strength and one weakness of research into the influence of hormones on aggressive behaviour. (4) January 2017
  • Explain one strength and one weakness of research into the role of hormones in aggression. (4) January 2021
  • Describe research into the role of hormones in aggression. (4) January 2021

8 Markers

  • Evaluate research into the role of hormones in aggression. (8) June 2018
  • Assess how far the role of hormones can explain aggression. (8) January 2019
  • Evaluate research into the role of genes in aggression. (8) January 2020

12 Marker

  • To what extent can genes be used as an explanation of aggression? (12) January 2022