Hormonal Factors

Androgens (male hormones) such as testosterone are considered important factors in aggressive behaviour. There is some evidence for this point of view. For example, castrated mice display lower levels of aggression than they did pre-castration, and normal levels of aggression are restored when testosterone is replaced intravenously. Nevertheless, there is a difficulty in generalising from studies on rodent aggression to human aggression; it could be argued that human social systems are more complex than those of mice and, therefore, social factors may be more relevant in explaining aggressive behaviour in humans.

Research on the effects of testosterone in humans has, nevertheless, suggested that it is a factor in aggression. Berman, Gladue and Taylor (2005) asked Male participants to take part in a reaction time competition where they were allowed to set the shock level given to an increasingly provocative competitor. The level of shock given was taken as a measure of aggression. The researchers found a relationship between salivary testosterone and aggression: the higher the levels of salivary testosterone, the greater the aggression. While this study does give some support to the idea that testosterone increases aggression, the measure of aggression used - shock level - may lack construct validity. Construct validity refers to whether or not a measure is actually measuring what it purports to measure. In this case, the shock level given may be a measure of competitiveness, rather than aggression.

More recent research has suggested that increases in testosterone levels can promote fairer, more pro-social behaviour rather than aggression. Eisenegger, Fehr and Naef (2009) injected participants with either a placebo or testosterone before asking them to take part in a game that involved negotiating about the distribution of amounts of real money. Test participants had to make an offer of how much money they would receive and how much their partner would receive. The partner then made a decision to either accept the offer or decline it. If they declined the offer neither partner would receive any money.

Participants who were given testosterone made fairer offers than those given a placebo. Another finding was that placebo participants who believed they had been given testosterone were the most aggressive in their strategy and, consequently, less successful in the task. This study suggests that testosterone may increase sensitivity to status and not aggression. Therefore, when the social situation requires negotiation to secure status, testosterone can promote fairer behaviour and not aggression.

The observation that there is an interaction between hormone levels and the social situation highlights the issue that viewing testosterone as a sole cause of aggression is reductionist; i.e., a complex phenomenon is being reduced to a overly simple explanation. Human aggression does not occur in isolation, it occurs in a variety of complex social situations. It is therefore important to acknowledge the importance of other biological factors and social factors.

Neural Factors in Aggression

Neural Factors in aggression include structures in the brain, as well as the action of neurotransmitters on those structures.

The Amygdala

One brain structure that has been considered important in aggression is the amygdala. Research in hamsters has found that stimulation of the corticomedial amygdala increases aggression and lesioning of this area reduces aggression in hamsters (Potegal et al., 1996).

In humans, Amygdalectomy (surgical removal of the amygdala) reduces aggression in previously violent individuals; however, a side effect of this type of surgery is the loss of emotion, initiative and enthusiasm (Groves and Schlesinger, 1982). It is possible that the amygdala does not directly cause aggression, but is involved in processing associated emotions, such as anger.

Medial Hypothalamus

Flynn (2006) found that stimulating the lateral area of the hypothalamus in cats led to predatory aggression. Predatory aggression is a hunting behaviour, whereby the cat will quickly and silently catch and kill its prey. Predatory Aggression is very different to rage aggression, which in cats involves arching of the back, teeth baring and hissing.

Flynn found that rage type aggression was elicited by stimulation of the Medial Hypothalamus. This suggests that different types of aggression are controlled by different brain areas. Nevertheless, in humans it is likely that higher brain areas play a part in aggression, since human aggression appears to be linked to social behaviour and cognition rather than simple responses to stimuli.

Frontal Lobes

The frontal lobes are likely candidates for a role in aggression, as they are known to be involved in many activities requiring decision making, such as social behaviour, motor functions, problem solving, judgement, impulse control and personality.

Evidence that aggression is related to frontal lobe function has been around since the 1800s. In 1848, Phineas Gage had an accident while packing gunpowder into a blasting hole while working on a railroad in Vermont, USA. The tamping iron he was using was propelled through his skull when he accidentally ignited the powder, causing it to explode. The damage to Gage's frontal lobes did not kill him, but it apparently changed his personality, making him more aggressive, obstinate, impatient and impulsive (Harlow, 1868).

The case of Phineas Gage suggests that the frontal lobes may be important in moderating aggressiveness. Nevertheless, it is impossible to find out exactly where Gage's brain was damaged, as it can only be deduced from examination of his skull and this only provides a rough estimate.

Nevertheless, further evidence that the frontal lobes are involved in aggression comes from some of the most violent people in history: serial killers and mass murderers. Many of these ultra-violent individuals, have a history of frontal lobe injury; for example:

• Albert Fish suffered frontal lobe damage when he fell out of a tree
• Fred West, was in a coma for 8 days after a motorbike accident and was unconscious for a further 24 hours two years later after falling off a fire escape
• Richard Speck suffered head injuries after being hit on the head with a claw hammer when he was 5 years old, fell out of a tree twice and ran headfirst into a steel girder when he was 15.

A difficulty with this type of evidence, however, is that brain damage is not usually located to one specific brain region; other areas may have been damaged and they may be the real reason for the violent behaviour. Also, serial killers have usually suffered very abusive childhoods and this may account for their violent behaviour or it may be that damage to a particular part of the brain combined with childhood abuse leads to excessive violence.

Nevertheless, there is further evidence that abnormal brain activity is linked to aggression; for example, Amen (1997), used SPECT (single photon emission computed tomography) analysis to image the functioning brains of 40 Aggressive Psychiatric Patients. Amen found that, compared with 40 non-aggressive psychiatric patients, the aggressive patients showed decreased activity of the pre-frontal cortex, increased activity on the left side of the basal ganglia (associated with anxiety) increased activity on the left side of the limbic system (associated with negative mood and a higher chance of violent behaviour), increased temporal lobe activity (associated with temper outbursts) and increased activity in the anterior cingulate area (associated with depression and negative thinking). This suggests that aggressive behaviour involves multiple brain areas; however it is unclear whether some, all or none of the differences in brain activity contribute to aggressive behaviour since the data is correlational which makes it difficult to determine cause and effect.

This view of aggression as being caused by differences in brain structure is deterministic; it claims that violent people are violent not through choice, but because of brain abnormality or injury. This is important because it has implications about how violent offenders, such as serial killers are treated. Are they responsible for their behaviour? Is it morally correct to punish someone who has no choice about how they behave?

Neurotransmitters

Much of the research on the role of neurotransmitters on aggression has focussed on serotonin activity. A number of research approaches have suggested that low activity of serotonin increases aggression.

One way of investigating serotonin activity is to compare levels of the serotonin metabolite 5-HIAA in participant's cerebrospinal fluid. A study by Lloyd, Farley, Deck & Horykiewicz (1974) found that suicide victims had reduced levels of 5-HIAA in their cerebrospinal fluid compared with controls. As low 5-HIAA indicates low serotonin activity, this suggests that low serotonin activity is associated with aggression. However, this assumes that suicide is inwardly directed aggression. Some researchers, such as Roggenbach (2002) have questioned whether this is a useful assumption, as suicide is complicated by other factors, such as depression and impulsivity.

A study by Stanley et al. (2000), however, compared the cerebrospinal fluid concentrations of 5-HIAA in aggressive and non-aggressive psychiatric patients, while excluding those with a history of suicide. They found that aggressive participants had lower levels of 5-HIAA than the non-aggressive participants, which again supports an association between low serotonin activity and aggression. An issue with this study is that the participants were all psychiatric patients, which makes it difficult to generalise the findings to aggressiveness in people without mental illness.

Linnoila et al (1983) compared impulsive and non impulsive violent offenders and found that there was low CSF 5-HIAA in impulsive violent offenders, but not in those whose violent crimes were premeditated. This means that serotonin's role may be to moderate aggressive impulses and low serotonin levels may reduce a person's control over these impulses.

The idea that low serotonin reduces impulse control has been supported by research by Cremniter et al (1999) who found that 5-HIAA levels were lower in suicide victims than controls, but this was only true for impulsive suicides. When impulsive suicides are not included in the data there is no significant difference in 5-HIAA levels.

Animal Breeding Studies have also been used to investigate serotonin's role in aggression. Popova, Voitenko, Kulikov & Avgustinovich (1991) found that Russian silver foxes bred for tameness had higher serotonin levels than wild foxes bred in captivity.

A problem with many of the serotonin studies above is that they merely show an association between serotonin activity and aggression, this means that it is not possible to say that low serotonin levels cause aggression. It is only possible to ascertain cause and effect if experimenters manipulate serotonin activity and observe a change in aggressiveness.

Dodman, Donelly, Shuster, Mertens, Rand & Miczek (1996) carried out a study where they increased the serotonin activity of dogs by administering fluoxetine (prozac), a drug which increases the activity of serotonin in the brain. They found that fluoxetine was effective in reducing the aggressiveness of dogs. This implies that increasing serotonin levels causes a reduction in aggression. Nevertheless, it is difficult to generalise from dogs to humans, as humans may react differently to fluoxetine. Nevertheless, Fuller (1996) in a review of studies on fluoxetine claims that the clinical data suggests that fluoxetine does reduce aggression in humans.

Several lines of research have all suggested that low serotonin is associated with aggressive behaviour. The research also indicates that action of serotonin may be to control aggressive impulses and that low serotonin, consequently, leads to lowered control. As there is also evidence that the frontal lobes are involved in the control of aggressive impulses it may be low serotonin in this area of the brain that increases the aggressiveness of some individuals.

The research above indicates that aggression is associated with testosterone levels, serotonin activity and brain structure. These factors probably interact to determine an individual's level of aggression. Nevertheless, it is a reductionist and deterministic approach to aggression. It is more likely that these factors also interact with social factors to lead to aggressive acts.