Neural and hormonal mechanisms in aggression, including the roles of the limbic system, serotonin and testosterone. Genetic factors in aggression, including the MAOA gene.
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. 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.
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.
Stanley et al. (2000) 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.
Cremniter et al (1999) supported the idea that low serotonin reduces impulse control. Their research 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.
Popova, Voitenko, Kulikov & Avgustinovich (1991) found that Russian silver foxes bred for tameness had higher serotonin levels than wild foxes bred in captivity.
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.
Issues & Debates
Environment-neurotransmitter interaction: 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. Not all people with low levels of serotonin display aggressive behaviour, so other factors must play a part in the determination of aggressive behaviour. Booji et al. (2010) carried out a longitudinal study that followed individuals throughout childhood and into adulthood. They measured aggression through parental and self-reports and serotonins synthesis through PET scans. As expected, children with low levels of serotonin displayed higher levels of aggressive behaviour than children with normal serotonin levels. However, by the time these children reached adulthood, they showed no difference in aggression levels compared to normal controls, despite the fact that their serotonin levels remained low.
Reductionism: The link between neurotransmitters and aggression largely ignores the important role played by social factors in aggression. For example, research by Bandura et al. (1961) has shown that social learning can be a powerful influence on the aggressive behaviour of children, and Zimbardo et al. (1973) has described how deindividuation can increase aggressive behaviour in particular situations where personal responsibility is diminished. Reductionist explanations of human aggression underestimate the complexity of aggressive behaviour and are insufficient on their own to explain the many different aspects of human aggressive behaviour.
Animal research: The link between neural mechanisms and aggression is well established in non-human animals; however, the link is less clear in human beings. A major challenge to the belief that research on animals can easily be generalised to humans was provided by the Seville Statement on Violence (1986). In this statement, scientists from 12 different countries formally challenged a number of popular beliefs based on scientific findings with animals and humans that had been used to justify violent behaviour in humans. This included the notion that human aggression is inherited, or instinctual, or could be reduced to the action of neurochemicals as suggested by animal models of aggression.
Real world application: If aggressive behaviour is a product of decreased levels of serotonin in the brain, then it should be possible to treat aggression with SSRI antidepressant drug, which would increase the amount of serotonin available in the synapse. These drugs are being used effectively to control violent behaviour in selective groups, e.g. elderly patients with dementia living in residential nursing homes, and aggressive children. For example, a study by Ghaziuddin and Alessi (1992) found that administration of the antidepressant drug trazodone produced a noticeable improvement in about half of the children treated.[/message_box][/toggle_item][/toggle_box]
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.
The challenge hypothesis (Wingfield et al. 1990) proposes that, in monogamous species, testosterone levels should only rise above a baseline level in response to social challenges, such as male-male aggression or threats to status. As humans are essentially monogamous, we might therefore expect male testosterone levels to rise sharply in response to such challenges, particularly those which are considered threats to reproductive success. According to this explanation, therefore, competitive encounters between young men would lead to a surge in testosterone levels.[space=10]
Archer (1991) carried out a meta-analysis of five studies (involving 230 male adults) and found a low positive correlation between levels of testosterone and aggression.
Book et al. (2001) carried out a larger meta-analysis of 45 studies and found an average correlation of +0.14 between testosterone and aggression.
Edwards (1969) Strong evidence for the role of testosterone in influencing aggression comes from animal studies. This research has found that castration leads to a decrease in aggression in animals, whilst replacement of testosterone restores the behaviour.
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.
Issues & Debates
Gender bias: Most of what we know about the relationship between testosterone and aggression is based on studies of males. Currently there is little research that has explored the relationship between testosterone and aggression in females. Research suggests that the association between testosterone and aggression may be even higher among females (Archer et al. 2005). Other research (Baucom et al. 1985) has found that women with higher testosterone levels had higher occupational status, possibly as a result of being more assertive. Studies such as this also report that these women tend to respond to challenging situations with increased levels of testosterone, with an associated rise in traits such as aggressiveness and dominance.
Reductionism: 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. For example, research by Bandura et al. (1961) has shown that social learning can be a powerful influence on the aggressive behaviour of children, and Zimbardo et al. (1973) has described how deindividuation can increase aggressive behaviour in particular situations where personal responsibility is diminished. Reductionist explanations of human aggression underestimate the complexity of aggressive behaviour and are insufficient on their own to explain the many different aspects of human aggressive behaviour.
The positive influence of testosterone: Zitzmann (2006) argues that not only is the link between testosterone and aggression not proven, but that testosterone may also have many positive effects for males. Testosterone supplements have been shown to increase energy and vigour in older males and low levels of testosterone are linked with depression in older males (Barrett-Connor et al. 1999). Increases in positive mood and decreases in negative mood were confirmed following testosterone replacement therapy in a study of 208 men with testosterone deficiency (McNicholas et al. 2003). Testosterone has also been shown to have positive effects on women’s well-being, Davis (2000) found that testosterone was important in maintaining a woman’s energy level and sense of well-being, regardless of her age.
Pro-social behaviours: Eisenegger, Fehr and Naef (2009) found a link between rises in testosterone levels and fairer, more pro-social behaviour, rather than aggression. They 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.
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