Neural & Hormonal Mechanisms in Aggression (AQA A Level Psychology): Revision Note

Neural mechanisms in aggression

• Aggression is behaviour intended to harm another person who is motivated to avoid that harm

• The limbic system is the key structure/region in the brain that is associated with aggression

• The first attempt to link limbic structures to emotional behaviours such as aggression was by Papez (1937), later revised by MacLean (1952), who identified the limbic system as comprising the:

  • cingulate gyrus

  • septal area

  • hypothalamus

  • fornix

  • amygdala

  • parts of the hippocampus

• The most important structure in aggression is the amygdala, which has a key role in how humans and non-human animals assess and respond to environmental threats and challenges

  • The reactivity of the amygdala has proven to be an important predictor of aggressive behaviour

Neurotransmitters and brain structures in aggression

• Gospic et al. (2011) scanned participants' brains using fMRI while they played the Ultimatum Game, and found that when Responders rejected unfair offers (an aggressive reaction to a social provocation) this was accompanied by a fast and heightened response by the amygdala

  • When participants were given a benzodiazepine drug before the game, which reduces arousal of the autonomic nervous system, this halved the number of rejections and decreased the activity of the amygdala

    • This provides strong evidence of an association between reactive aggression and amygdala activity

• Serotonin has widespread inhibitory effects on the brain, slowing down and dampening neuronal activity, with normal levels in the orbitofrontal cortex linked to reduced firing of neurons and associated with greater behavioural self-control

  • Decreased serotonin may disturb this mechanism, reducing self-control and leading to an increase in impulsive behaviour including aggression (Denson et al., 2012)

• Virkkunen et al. (1994) compared levels of a serotonin breakdown product, 5-HIAA, in the cerebrospinal fluid of violent impulsive and violent non-impulsive offenders, and found levels were significantly lower in the impulsive offenders

  • They also had more sleep irregularities, which is significant because serotonin regulates sleep patterns, so disturbance of this pattern strongly implies disruption of serotonin functioning

Research which investigates neural mechanisms in aggression

• Coccaro et al. (2007) found that in people with psychiatric disorders that prominently feature aggression, activity in the orbitofrontal cortex is reduced, disrupting its impulsive-control function and leading to increased aggression

• Brunner et al. (1993) – males from one family with a dysfunctional MAOA gene (which regulates serotonin production) showed high levels of aggression

Hormonal mechanisms in aggression

• Testosterone is the main hormone associated with aggression

  • Males typically secrete more testosterone than females

• Males also tend to display more aggression than females, and testosterone is an androgen responsible for the development of masculine features, which also has a role in regulating social behaviour via its influence on certain areas of the brain implicated in aggression

• Giammanco et al. (2005) demonstrated experimental increases in testosterone related to greater aggressive behaviour in several animal species, with the converse also true - decreases in aggression following castration

• Dolan et al. (2001) found a positive correlation between testosterone levels and aggressive behaviour in a sample of 60 male offenders in UK maximum security hospitals, who mostly suffered from personality disorders such as psychopathy and had histories of impulsively violent behaviour

Research which investigates hormonal mechanisms in aggression

• Mehta & Josephs (2006) measured changes in male participants' testosterone levels before and after a competitive game which they all lost, then gave them a choice of either challenging their victorious opponent to another competition or completing an unrelated task

  • Of the losers whose testosterone levels rose after losing, 73% rechallenged, but of those whose testosterone dropped, only 22% rechallenged, confirming Mazur's model and increasing its validity as a plausible explanation of how testosterone may exert its effects on aggression

• Klinesmith et al. (2006) – male college students who interacted with a gun rather than a children's toy added more hot sauce to a cup of water they believed another student would have to drink;

  • Interaction with a gun showed significantly greater increases in their testosterone, seen via saliva samples, and their aggression (operationalised as the addition of the hot sauce)

Evaluation of neural & hormonal mechanisms in aggression

Strengths

• Berman et al. (2009) gave participants either a placebo or a dose of paroxetine, a drug which enhances serotonin levels, then had them take part in a laboratory game in which electric shocks of varying intensity were given and received in response to provocation

  • The paroxetine participants consistently gave fewer and less intense shocks than the placebo group, and this was only true of participants who had a prior history of aggressive behaviour

  • This provides useful evidence of a link between serotonin function and aggression that goes far beyond the usual correlational findings

• Prisoners who had committed violent crimes showed higher testosterone levels than those who had committed theft; plus, these violent criminals also violated more rules, particularly rules involving overt confrontation (Dabbs et al. 1995)

Limitations

• Most research into both neural and hormonal influences on aggression is correlational, as there are good ethical reasons why brain structures and hormones cannot be experimentally manipulated in humans

  • When two variables are correlated it is impossible to establish which one is the cause of the other, or whether a third variable is involved

• Both neural and hormonal explanations of aggression ignore key social/environmental factors as being of relevance to aggressive behaviour e.g. upbringing, education, substance abuse

Issues & Debates

• Both neural and hormonal explanations are reductionist, as they reduce aggression to brain structures and chemical imbalances, such as low serotonin or high testosterone

  • This ignores complex social, environmental, and cognitive influences like upbringing, peer pressure, or learned behaviour

  • As a result, the explanations may oversimplify a highly multifaceted behaviour

• These theories suggest that aggressive behaviour is biologically determined by brain function or hormone levels

  • This raises questions about individual responsibility, especially in legal or moral contexts — can someone be held accountable if their biology ‘made them do it’?

  • It challenges the notion of free will in aggressive acts