Biological Explanations of Aggression

  • Neural Mechanisms and Aggression:
  • Research in the 1930’s pointed to the involvement of a circuit of structures in the brain with emotional behaviours, including aggression. Specifically there is a circuit from the areas of the amygdala, down to the hypothalamus and from there to the periaqueductal grey (PAG). This system appears to be organised in a hierarchical way, for example. Aggression responses produced from the amygdala depend o the normal functioning of the hypothalamus and PAG but PAG is not dependent on the others.
  • The amygdala has long been associated with aggressive behaviour in both animals and humans. A link was establish by Kluver & Bucy (1930s). They removed part of the temporal lobes of Rhesus monkeys, destroying the amygdala. This resulted in a loss of fear and a taming effect.
  • It is the role of the prefrontal cortex to control the outward expression of aggression. Damage to it results in a range of responses, including loss of control, impulsivity, immaturity and altered emotionality, all of which are associated with aggressive acts. (Damasio et al 1994)
  • Evaluation:
  • There is supporting evidence for the claim that damage to the prefrontal cortex can result in increased aggression. Anderson et al (1999) have shown that individuals with damage to the frontal cortex during infancy are at a greater risk of aggressive behaviour as adults. They conducted case studies on two individuals who had received damage to their frontal lobes before the age of 16 months. Both cases showed a range of anti-social behaviour, including aggression, beginning in childhood and extending into adulthood. However, it could be argued that case studies aren’t reliable as they are individual situations and aren’t generalisable, or applicable to the wider population.
  • Evidence suggests that stimulation of the amygdala can cause aggressive behaviour, further supporting the explanation. Mark and Ervin (1970) describe the case of a female patient who during electrical stimulation of the amygdala became very angry and flung herself at the wall, beating it with her fists. On another occasion, she picked up her guitar and threw it towards her psychiatrist. This is supported by observations of people experiencing temporal lobe epilepsy becoming aggressive towards people, objects and furniture near them. (Ashford 1980) This evidence could suggest that manipulation of the amygdala could resolve aggressive related problems in the world. This could provide a treatment for aggressive behaviour. However, this could be considered unethical as it may not be justified to remove or alter a amygdala when the outcomes are wholly known.
  • There have been a lot of animal studies and research in the aggression field. For example, Kluver and Bucy (1930s) removed part of the temporal lobes of rhesus monkeys to display aggressive behaviour. Le Doux (1996) similarly found that the stimulation of the amygdala in lab animals usually results in fear and rage. However, despite the supporting findings, animal research may not be entirely applicable to humans. This means that the animal research findings cannot be generalised and cannot be used to form a basis in human related research.
  • Hormonal
  • Testosterone
  • Observations across species show that males are more aggressive than females. This can be attributed to the hormone testosterone. Not only is it important in biological functions but it is also important in influencing a range of behaviours, including aggression.
  • This is supported of the observation of pets before and after castration, they become more docile and less aggressive. This was confirmed by Beeman (1947) in an experiment with the removal of mice testes. They also injected mice with testosterone and observed increased levels of aggression.
  • Vom Saal (1983) showed the influences of testosterone in aggression by looking at the development of rats in the womb. It was found that those female rats that lay closer to male rats in the womb were the most aggressive female rats in the litter. This was because they were exposed t more testosterone from their brothers.
  • According to Vom Saal, exposure to testosterone during the womb serves to organise the neural networks in the brain involved in aggressive behaviour. i.e. testosterone or lack of it permanently changes brain development. Therefore castration after puberty have less effect on aggression.
  • Evaluation:
  • As well as a relationship between testosterone and aggression being evident, there is evidence of one in humans. For example Dabbs et al (1987) measured salivary testosterone in criminals of a violent and non-violent nature. Those with the high levels had a history of primarily violent crimes whereas those with low levels had committed non-violent crimes. Archer (1991) analysed the results of 230 males over 5 studies in a meta-analysis. They found a low positive correlation, however the study could be questionable as the type of participant, form and measurement differed substantially between studies. Albert et al (1993) claim that despite many studies showing a positive correlation, other studies find no such relationship, particularly studies that have compared testosterone levels of aggressive and less aggressive individuals. Also, most studies showing a positive correlation have involved small samples of men within prisons using either self-report measures ore judgements based solely on the severity of the crime committed. Therefore meaning they may lack some validity.
  • The theory of the link between testosterone and aggression is gender biased as the majority of the studies involve male participants. Its possible that testosterone could also have a similar effect on women, as research suggests that there is a stronger association (Archer et al 2005). A study showed that women with higher testosterone levels had higher occupational status, possibly due to the assertiveness (Baucom et al 1985). These studies suggest that women may also respond to challenging situations with increased testosterone displaying aggressiveness and dominance (Eisenegger et al 2011). Therefore these studies suggest that testosterone in women can have positive effects as it doesn’t just increase aggression but it increases status-seeking behaviour.
  • The theory may be reductionist. The links between biological mechanisms and aggression are strong in animals but not so strong in humans. This could therefore mean that a biological explanation isn’t sufficient as an explanation for human aggression due to the complex and complicated nature of human behaviour which the theory seems to ignore when depending on animal research.
  • There may be a real world application of the link between testosterone and aggression. Due to the statistical increase in gun related crime in the UK, it raises the question for why guns lead to aggression. Its possible that the presence of a stimulus triggers and increase in testosterone levels which in-turn increases aggressive behaviour, triggering a negative chain of events. To test this, Kleinsmith et al (2006) had male college students provide saliva samples measuring testosterone before and after playing with either a gun or a child’s toy for 15 minutes. Males who interacted with the gun showed significant increases in testosterone levels and acted more aggressively towards other participants than the other group. This could therefore provide an explanation for the aggression associated with weapons such as guns or knives.
  • Genetic Factors
  • The Role of MAOA
  • A gene responsible for producing a protein called monoamine oxidase A (MAOA) has been associated with aggressive behaviour. MAOA regulates the metabolism of serotonin in the brain and low levels are associated with impulsivity and aggression.
  • In the 1980s, a study of a Dutch family found that a large number of its male members were violent and aggressive and a large number have been involved in a serious crime (e.g. rape or arson). These men were found to have abnormally low levels of MAOA and a defect in the gene. (Brunner et al 1993)
  • Different forms of the MAOA gene have been identified. One form is a low activity form (MAOA-L) which produces less of the monoamine oxidase A enzyme which removes excess neurotransmitters. There is also a high-activity form which produces more of this enzyme (MAOA-H).
  • Mc Dermott et al (2008) showed that their participants with the L gene displayed higher levels of aggression in response to provocation than those with the H variant. This appears to be evidence for a direct link between genetic variation and willingness to engage in acts of aggression.
  • Evaluation:
  • There is evidence from Twin and Adoption studies to support genetic inheritance of aggression. For example, Miles and Carey (1997) carried out a meta-analysis of 24 twin and adoption studies that supported the genetic basis of aggression. They found evidence of a strong genetic influence in the development of aggressive behaviour. In younger individuals, both genetic and environmental influences contributed to aggression but in older individuals, environmental factors were less important and genetic influences were more important in determining aggressive behaviour. However, in both these studies, several variables contributed to the strength of the genetic influence on aggression. These included the age of participant and the method used to assess aggression. This suggests that genetic factors are important in the development of aggression, but other factors determine the expression of aggression.
  • There are difficulties determining the role of genetic factors. The connection between genetic factors and aggression isn’t straightforward due to problems determining what is and isn’t due to genetic inheritance. This is because more than one gene usually contributes as well as other non-genetic factors which may all affect each other. Caspi et al (2002) demonstrates this. They linked MAOA to aggression using 500 male children. They found that those with low levels were significantly more likely to grow up to behave antisocially only if they had been maltreated as a child. Therefore aggression cannot be solely attributed to genetic influences.
  • There may be real world applications of the research into aggression. Research findings on the topic are far too uncertain to be valuable in understanding the causal factors affecting aggressive behaviour. Despite this, there are suggestions that the public should be aware of the findings about treatments of those who behave aggressively. This may lead to activities such as genetic engineering or other unpleasant methods. Therefore, this may raise ethical issues from allowing treatments or social treatment of those who behave aggressively based on such uncertain research.

Biological Explanations of Aggression- Genetic Factors

Essay Plan (8 + 16)


  • Gene MAOA has been associated with aggressive behaviour
  • It regulates the metabolism of serotonin in the brain, low levels associated with aggression
  • 1980s study, Brunner et al (1993): Dutch family studied, males were aggressive, many involved in a serious crime, found to have very low levels and a defect later identified.
  • Different forms: low activity: MAOA-L, high activity: MAOA-H
  • Mc Dermott et al (2008) showed p’s with L gene, higher levels of aggression than H variant p’s.


P: Evidence support from Twin and Adoption studies

E: Miles & Carey (1997): meta-analysis of 24 T & A studies. Strong genetic influence found. In the younger individuals both genetic and environmental factors influenced but in older, mostly genetic.

E: However, several variables contributed to the strength of the genetic influence, age and method.

E: suggests that genetic factors are important but other factors also determine the expression of aggression.

P: Problems of sampling

E: Many studied are focused on convicted criminals; many crimes never result in conviction. Only representative of minority.

E: Possible some of the convicted aren’t the most violent, could just be one violent incident rather than due to them being a violent person

E: could explain, little to no heritability for violence.

P: Difficulties determining the role of genetic factors

E: Not a straightforward connection, difficult to test what is and isn’t due to genetics. One gene influences another; non-genetic factors and each of the factors may affect the others.

E: Caspi et al (2002) demonstrates this: linked MAOA to 500 male children. Those with low levels, aggressive but only if they had been maltreated.

E: Therefore, aggression cannot be solely attributed to genetic influences.

P: Real world applications (IDA)

E: Findings are too uncertain however, there are suggestions that the information should be shared with the public.

E: Could lead to supposed treatments or genetic engineering, etc.

E: Could raise ethical issues, should those who behave aggressively be ‘treated’ or socially mistreated based on uncertain evidence?