Neural and hormonal influences of aggression
Aggression is an umbrella for behaviours that are intended to inflict harm. This complex social behaviour is regulated by numerous social and biological factors. Nevertheless, however may these factors be likely to work together as part of an integrated system, many human studies on aggression to date have revealed, confirmed and investigated the effects of neural and hormonal influences of aggression.
Neural mechanisms are structures such as neurons, neural circuits, and regions of the brain. They are also substances such as neurotransmitters and hormones.
Additionally, neural mechanisms regulate aggression, and the limbic system, which is a system of structures lying beneath the cortex including the amygdala, hippocampus, and hypothalamus. This region is associated with emotional behaviour.
As mentioned, the limbic system is an area of the brain in which helps to coordinate behaviours that would satisfy one’s motivational and emotional urges such as aggression and fear. Key structures in this system associated with aggression are where feelings are monitored and interpreted which then triggers a physical response.
1) The amygdala
Located in the medial temporal lobe in the lower half of the brain, it is responsible for quickly evaluating the emotional importance of sensory information and prompting an appropriate response. Because of the hierarchical method of signal passing from lower regions to higher regions in the prefrontal cortex in the limbic system; the prefrontal cortex is crucial for regulating social behaviour and aggressive responses, hence damage to the prefrontal cortex would reduce the inhibition of the amygdala resulting in higher levels of aggression. Moreover, studies have proven and shown that once certain areas of the amygdala are stimulated electrically, an animal would respond with aggression, with imposing actions such as snarling and adopting an aggressive stance or posture, likewise if the amygdala is taken away surgically, the animal would no longer respond to the same stimuli that would have previously led to rage.
Evidence for the role of amygdala in aggression
· In 1937, Kluver and Bucy (German psychologist and American neurosurgeon) discovered that the destruction of the amygdala in a monkey who was dominant in a social group caused it to lose its dominant place in a group
· Pardini et al (2014) also found that reduced amygdala volume can predict the development of severe and persistent aggression. This is proven through a longitudinal study of 503 male participants from childhood to adulthood. At 26 years old, a subsample of 56 men with varying violence histories were given an MRI. Results from such showed that participants with lower amygdala volumes exhibited higher levels of aggression and violence despite when other confounding variables were controlled. This suggests that the amygdala plays an important role in evaluating the emotional importance of sensory information, and that lower amygdala volume compromises this ability, making a violent response more likely.
2) The hippocampus
Located behind the amygdala, this brain structure is involved with the formation of long-term memories, therefore allows an animal to compare conditions of a current threat with similar past experiences. For example, if an animal was attacked by another, the next time they encountered it, they may respond with aggression or fear, whichever one is more appropriate. Impaired hippocampus function prevents the brain from putting things into relevant or meaningful context, which will prompt the amygdala to respond inappropriately to the sensory stimuli, resulting in aggressive behaviour.
Evidence for the role of hippocampus in aggression
· Boccardi et al 2015 found that habitually violent offenders exhibited abnormalities of hippocampus functioning
· Raine et al 2014 studied two groups of violent criminals. Whom some have faced conviction (unsuccessful psychopaths since they have been caught), and those who have evaded the law (successful psychopaths). The latter group were considered “cold, calculating” criminals and the former as impulsive, contributing to the reason why they got caught. MRI scans revealed asymmetries in the hippocampus of the impulsive group; because of the differed size on either hemisphere of the brain, an imbalance is to be presumed in brain development. This proves that asymmetry in the hippocampus may have impaired the ability for both the amygdala and hippocampus to work together, causing emotional information to be processed incorrectly, leading to inappropriate physical and verbal responses.
More neural mechanisms:
Serotonin in normal levels would inhibits a calming and inhibitory effect on neuronal firing (typically in the amygdala which controls fear, anger, and other emotional responses) in the brain, therefore lower levels of serotonin will remove this effect, hence resulting in the consequences of one being unable to control impulsive or aggressive behaviour. Lower serotonin levels is suspected to be associated with increases susceptibility to impulsive behaviour, aggression and even possibly violent suicide.
Evidence for the role of serotonin in aggression
· Mann et al in 1990 studied 35 healthy human participants in which some were given a drug (dexfenfluramine) which is used to deplete serotonin levels. He concluded that those received the drug to be on higher levels of hostility and aggression based on a questionnaire assessment. (However, the drug does not affect females as severely, as opposed to men, thus hinting at a potential gender difference that is not accounted for by the model of serotonin.)
2) Neurotransmitter dopamine
The link with aggression is however not as well established, since it is believed to be associated with a reward pathway in the brain that is engaged in responses to aggressive events. Dopamine hyperactivity in the brain regions are linked to reward related motivations, such as the nucleus accumbens (NCC) and prefrontal cortex, this leads to increased aggressive and impulsive behaviours.
Evidence of role of dopamine in aggression
· Kennedy’s study in mice revealed a reward pathway being engaged when showing dopamine to be a positive reinforcer. Nevertheless, this may suggest that dopamine is more of a consequence of aggression and not a cause instead.
The hormone testosterone (which produces male characteristics) is believed to influence aggression from young adulthood and onwards due to its action in the brain areas involved in controlling aggression. The more testosterone a person has, the more aggressive they are believed to be; hence females also exhibit an increase in aggression during puberty, very similar to the peak in aggression exhibited by male teenagers as a result of testosterone (which also proves that testosterone should not be a gender biased research topic). Additionally, this can also be demonstrated by animal research which revealed castrated mice to be less aggressive, but if injected with testosterone, aggression undoubtedly increases.
Note: testosterone levels peak in young males, being highest from ages 21-35 and then decline