Aggression: Neural and hormonal explanations

Question 1

What is aggression?

Answer 1

A feeling of anger resulting in hostility or violent behaviour, readiness to attack or confront.

Question 2

What are two types of aggression?

Answer 2

• Proactive
• Reactive

Question 3

What is proactive aggression?

Answer 3

Instrumental aggression that has been planned to get what you want

Question 4

What is reactive aggression?

Answer 4

Aggression in reaction to something is often impulsive and accompanied by physiological arousal.

Question 5

What is the limbic system and its role?

Answer 5

The limbic system is part of the brain that includes structures such as the amygdala, hypothalamus, basal ganglia and hippocampus that are implicated in reactive aggression.

Question 6

What is the amygdala and its role in aggression?

Answer 6

• The most important part of the limbic system for understanding aggression is the amygdala. The amygdala takes information from the thalamus and interprets it as a threat or not; it produces fear or aggression, the famous “fight or flight” response. The amygdala can be stimulated when faced with a perceived threat. If in a threatening situation, the amygdala will send information to other parts of the brain to prepare the body to either face the situation, or to get away from it.
• This fight-or-flight response is triggered by emotions of fear, anxiety, aggression, and anger. It is beneficial that the amygdala is working correctly in order to act appropriately in threatening or stressful situations.

Question 7

What is cortisol and its role in aggression?

Answer 7

• Cortisol is commonly referred to as the ‘stress hormone’, although it is a steroid hormone with multiple effects across the body and is not purely related to stress. It is produced in the cortex of the adrenal glands, regulated by the pituitary gland, which the hypothalamus controls (for hormone release). As part of the glucocorticoid family of steroid hormones, once cortisol is produced, it is released into the bloodstream, where it travels around the body.
• Cortisol acts as an inhibitor of aggression, a modulator compared to testosterone, which usually facilitates aggressive behaviours and impulses. When cortisol levels are high, testosterone is inhibited, which in turn reduces aggressive behaviours in the person.

Question 8

What is the hypothalamus and its role in aggression?

Answer 8

The hypothalamus is responsible for the regulation of the autonomic nervous system, which in turn regulates responses to emotional circumstances. Therefore, damage to this area can result in an inappropriate aggressive response to a perceived threat.

Question 9

What is the pre-frontal cortex and its role in aggression?

Answer 9

The prefrontal cortex is crucial for regulating social behaviour and aggressive responses. Damage to the prefrontal cortex would reduce the inhibition of the amygdala resulting in higher levels of aggression.

Question 10

What is serotonin and its role in aggression?

Answer 10

Serotonin has a calming influence and low levels of serotonin mean that people can’t control their impulsive and aggressive behaviour. Serotonin also regulates the pre-frontal cortex; therefore, lower levels of serotonin affect our response to external stimuli, meaning the person becomes aggressive easily and can’t control their responses in a ‘normal’ way. They can’t anticipate risk and therefore impulsively engage in aggressive behaviour.

Question 11

What is testosterone and its role in aggression?

Answer 11

Testosterone is an androgen which produces male characteristics, although it is present in both males and females and is linked to aggression. It is suggested that testosterone acts with androgen or oestrogen receptors in the brain, affecting neural transmission and the number of neurotransmitters that are released at the serotonergic synapses. It appears to lower the amount of serotonin that is available, thereby leaving people less able to control their aggressive responses to situations and people.

Question 12

What is progesterone and its role in aggression?

Answer 12

Low levels of this female ovarian hormone have been linked to aggression.

Question 13

What is the MAOA gene?

Answer 13

Monoamine oxidase A (MAOA) is an enzyme that breaks down important neurotransmitters in the brain, including dopamine and serotonin; low levels of serotonin have been associated with impulsive and aggressive behaviour. MAOA is regulated by the MAOA gene and humans have various forms of the gene, resulting in different levels of activity of the enzyme.

Several studies have now found a correlation between the low-activity form of the MAOA gene and aggression.

Question 14

A03: Neural and hormonal explanations of aggression

Answer 14

• Other brain structures neural
  + Animal research hormonal
• Dual hormone hypotheses hormonal
  + Direct or indirect? neural

Question 15

Other brain structures (neural)

Answer 15

More recent research shows that non-limbic brain structures are also shown in aggression. Limbic structures eg the amygdala function together with the orbital frontal cortex which is not part of the limbic system. The orbital frontal cortex is involved in regulating and inhibiting aggressive behaviour. According to Emil Cocaro et al orbital frontal cortex activity is reduced in those psychiatric disorders that feature aggression. This shows neural regulation of aggression is more complex than theories that focus on the amygdala suggest.

Question 16

Animal research (hormonal)

Answer 16

Support from research with non-human animals. Giammanco et al review of studies confirm the role of testosterone. eg in male rhesus monkeys there is an increase in both testosterone levels and mouse-killing behaviour. Injecting female rats with testosterone increases mouse-killing. These findings show the role of testosterone in a range of animal species.

Question 17

Dual hormone hypotheses (hormonal)

Answer 17

Mixed evidence of the link between aggression and testosterone levels in humans. Pranjal and Mehta claim that a high level of testosterone leads to aggressive behaviour but only when cortisol is low. When cortisol is high testosterone’s influence on aggression is blocked. The hormone cortisol plays a central role in the body’s response to chronic stress. Therefore the combined activity of testosterone and cortisol may be a better predictor of aggression than either hormone alone

Question 18

Direct or indirect? (Neural)

Answer 18

The argument is that neural factors are directly linked to aggression. Evidence to support this is Gospic et al study of amygdala reactivity and benzodiazepines. Serotonin also reduces aggression by inhibiting neuronal activity. However, the role of neuronal factors may be indirect. Eg Denson et al found a link between serotonin and aggression. But this is an indirect link because other factors eg social and psychological may influence it.

Question 19

A03 for serotonin:

Answer 19

+ 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).

+ Evidence to support the link between low serotonin levels and aggression comes from animal studies. Raleigh et al (1991) found that vervet monkeys fed on a diet high in tryptophan (which increases serotonin levels in the brain) exhibited decreased levels of aggression and vice versa. This is a strength because it shows that low levels of serotonin are related to aggression, since the calmer monkeys had more of it, and the aggressive monkeys had less. This, therefore, suggests that aggressive behaviour is biologically determined.

• A weakness of Raleigh’s research is that the study is carried out on animals. For example, Raliegh uses vervet monkeys in order to investigate the link between serotonin and aggressive behaviour. This is a weakness because vervet monkeys and humans are biologically and physiologically different, the biological changes in a monkey’s body that lead to aggression may not necessarily lead to aggression in humans. As a result, the findings from this research cannot be generalised/extrapolated.

Question 20

Evaluation for testosterone:

Answer 20

+ Evidence to support the link between low testosterone levels and aggression comes from animal studies. Connor & Levine (1969) found that rats who had been castrated when they were young had lower levels of testosterone and also displayed lower levels of aggression. This is a strength because it supports a link between testosterone and aggression, the higher the level of testosterone, the more aggressive behaviour is demonstrated.

+ There is empirical evidence by Dabbs et al (1987) supporting the association of testosterone with the explanation of aggression. Dabbs et al measured salivary testosterone in violent and non-violent criminals. It was found those with the highest levels of testosterone levels has a history of primarily violent crimes, whereas those with low levels committed non-violent crimes. The findings show that high levels of testosterone levels increased the violence levels of the individuals, just as the theory suggests. Consequently, the study’s supportive findings increase the internal validity of the theory that high levels of testosterone levels are an explanation for aggressive behaviour.

• The theory that testosterone explains aggression is however opposed by empirical evidence from Albert et al (1993). Albert et al found that despite, any studies showing a positive correlation between testosterone and aggression some studies found no relationship. The study shows that not all studies prove testosterone explains aggression and thus weakened the theory as an explanation for aggression in all individuals. As a consequence, the internal validity of the theory is weakened as it is not fully supported and can be argued against.

Question 21

What are the genetic factors?

Answer 21

Twin studies

Adoption studies

MAOA gene

Question 22

What’s the role of the MAOA gene in aggression?

Answer 22

Monoamine oxidase A (MAOA) is an enzyme found at the synapse that breaks down important neurotransmitters in the brain, including noradrenaline, dopamine and serotonin; low levels of serotonin have been associated with impulsive and aggressive behaviour. MAOA is regulated by the MAOA gene and humans have various forms of the gene, resulting in different levels of activity of the enzyme. One variant of the gene is associated with high levels of MAOA (MAOA-H) and another variant is associated with low levels (MAOA-L). Several studies have now found a correlation between the low-activity form of the MAOA gene and aggression.

Imbalances in serotonin and dopamine are associated with aggression. A variant of the MAOA gene leads to low MAOA activity in the brain, which is associated with aggressive behaviour.

Question 23

How do twin studies play a role as a genetic factor of aggression?

Answer 23

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.

Several studies have suggested that heritability accounts for about 50% of the variance in aggressive behaviour. Monozygotic (identical) twins share 100% of their genes, while dizygotic (non-identical) twins share only about 50%, as a result we would expect to find greater similarities in aggressive behaviour between MZ twins if aggression is mostly influenced by genetic factors. Examples of concordance rates for different types of aggression are below.
Physical aggression MZ=50% and DZ=19%, Verbal aggression MZ=28% and DZ=7%.

Question 24

How do adoption studies play a role as a genetic factor of aggression?

Answer 24

Adoption studies can also help to separate out the effects of genes and environment. If a positive correlation is found between aggressive behaviour in adopted children and their biological parents, then this is suggestive of genes because the environmental factor has been removed. Meta-analysis was carried out looking at adoption and its relationship to direct aggression and antisocial behaviour. They found that genetic influences accounted for 41% of the variance in aggression.

Question 25

A03 for the genetic explanations of aggression:

Answer 25

+ Further support for twin studies
+ Supporting evidence for the role of MAOA in aggressive behaviour
- Issues of measuring aggression:
- Focus on animal research

Question 26

Further support for twin studies: Genetic explanation

Answer 26

Twin studies provide further support for the genetic explanation of aggressive behaviour. For example, McGue et al (1992) found a correlation of +0.43 for MZ twins and +0.30 for DZ twins on aggression scales. This research demonstrates a stronger link to aggressive behaviour in identical twins. This suggests that genes play some part in aggressive tendencies as MZ twins share more genes than DZ twins, therefore if there was no genetic element to aggression we would not see any difference in the correlation for MZ and DZ twins. This is positive as it suggests genes may at least play a part in aggression and therefore that the cause of aggression is, in some part, biological.

Question 27

Focus on animal research: genetic explanation

Answer 27

A considerable amount of research into genetic factors in aggressive behaviour was conducted on animals, such as Lagerspetz’s research on mice. For example, this means that the research only investigates the genetic factors in the development of aggression for animals and fails to generate any information regarding human aggression and links to genetics. This is a weakness because the findings from this research cannot be extrapolated (generalised) to humans. Mice and humans are physiologically different and so, although there seems to be a genetic basis for aggression in mice, it doesn’t mean that such findings can be generalised to humans.

Question 28

Issues of measuring aggression: genetic explanation

Answer 28

There are potential issues in how aggressive behaviour is measured. For example, aggression is often measured through questionnaires or responses to a hypothetical scenario, and therefore predictive validity is poor. This is a weakness because such measures of aggression are subjective (often influenced by individual judgement) and therefore it is difficult to draw conclusions about aggression from research that has used such measures.

Question 29

Supporting evidence for the role of MAOA in aggressive behaviour: genetic explanation

Answer 29

Supporting evidence for the role of MAOA in aggressive behaviour. Brunner looked at 28 male members of a Dutch family who were repeatedly involved in murder, rape and physical assaults. They were found to have abnormally low levels of the enzyme MAOA in their brains and have the high-activity version of the gene. This increases the validity of the genetic explanation, by showing a link between MAOA and aggression