Think about the last time you got really angry; what happened to your body? Your muscles likely tensed up, your heart rate and breathing became faster, preparing you to fight, your pupils dilated to increase your awareness of the threat, and you may have even clinched your fists. And that's not all that happens, there is so much more activity on the neural and hormonal levels behind this response.
Neural and hormonal mechanisms in aggression explore biological explanations for aggressive behaviours.
Fig. 1 - Aggression is associated with patterns of neural and hormonal activation.
Biological Factors of Aggression: Psychology
Biological factors of aggression explore biological explanations behind aggressive behaviours. To understand biological explanations for aggression, however, it is important we know what aggression is in the world of psychology.
Aggression refers to any behaviour that intends to harm others psychologically or physically.
Numerous neural and hormonal mechanisms facilitate aggression in humans and animals, and we will outline and discuss the role of these mechanisms in aggression. These include:
The role of the limbic system
Serotonin research into aggression
Testosterone research into aggression
Cortisol research into aggression
The Relationship Between Neural and Hormonal Mechanisms in Aggression
Decreased serotonin levels in the orbitofrontal cortex reduce self-control and lead to increased impulsive behaviour, including aggression. Normal levels of serotonin mean no aggressive behaviour.
Neural Mechanisms of Aggression in Psychology
The limbic system is a brain structure that correlates with aggression in humans and other animals.
The limbic system is a system of interconnected neural structures that include the hypothalamus, the hippocampus, the amygdala and the cingulate gyrus.
The limbic system is associated with primitive instincts and emotional aspects of behaviours and responses. Without this system, it's been shown that animals' emotional responses, including the fight and flight response, become impaired.
Kluver and Bucy (1939) removed the core parts of rhesus monkeys' limbic systems (bilateral temporal lobectomy). These monkeys then went on to have complete absences of emotional responses. For example, their motor and vocal responses were impaired when the monkeys were shown stimuli meant to induce fear and anger.
The monkeys also lost an understanding of their places in social hierarchies and would fight to gain dominance (aggressive behaviour). This suggests just how important the limbic system is in regulating aggression, other emotions and social cognition.
Structure of the Limbic System | Function |
The Hypothalamus. | Regulates homeostasis, involved in neuroendocrine and limbic systems, and the autonomic nervous system. |
The Hippocampus. | Storing memory, processing emotional responses. |
The Amygdala. | Detecting and responding to threats, processing fear. |
The Cingulate Gyrus. | Regulating aggression, response to pain and communication. |
Fig. 2 - Aggression is regulated by the limbic system in the brain.1
While emotional responses trigger the activity of the limbic system associated with emotional impulses, the prefrontal cortex regulates this activity and helps inhibit aggression.
Consider the case study of Phineas Gage. After suffering a severe injury at work, where a pipe went through his skull and destroyed much of his left frontal lobe, Phineas was reported to be more aggressive and short-tempered.
It's likely that Gage's prefrontal cortex was damaged and could no longer inhibit the amygdala, which may be why he struggled with his temper and aggression.
However, the link between neural activation and aggression is only correlational. It's difficult to tell whether one causes another or whether they just co-occur.
Serotonin
Serotonin is a neurotransmitter that is key to maintaining a stable mood. This neurotransmitter is associated with feelings of happiness, calmness, and the general ability to regulate moods.
Other than regulating mood, serotonin has also been associated with regulating physiological processes like sleep, digestion, and maintaining the body's temperature.
When serotonin levels are abnormal due to an issue with the production, uptake, or even down to the genetic level of serotonin function, issues with mood and emotional responses may arise. We can measure abnormalities in serotonin by examining serotonin turnover in people and animals.
Fig. 3 - Serotonin is a neurotransmitter, which is associated with regulating mood and physiological processes in the body.
The orbitofrontal cortex is associated with self-control and inhibiting aggressive instincts, as it is strongly linked to aspects of the limbic system involved in instinctive, emotional, and motivational drives. Serotonin is thought to play a key role in this process, as it facilitates communication between the orbitofrontal cortex and the limbic system.
Serotonin itself regulates the amygdala. A key theory explaining the role of serotonin in aggression is the serotonin deficiency hypothesis.
The serotonin deficiency theory in aggression proposes that low levels of serotonin increase levels of impulsive aggression, as serotonin cannot inhibit aggressive impulses triggered by amygdala activity.
Brown et al. (1979) studied the correlation between the concentration of amine metabolites produced as a result of the breakdown of neurotransmitters like serotonin and aggression in 26 military men.
The researchers found that the levels of serotonin metabolite in men correlated negatively (r = -0.78) with their history of aggressive behaviour.
This study suggests that lower levels of serotonin are associated with greater aggression.
Testosterone and Aggression: Psychology
Testosterone is an androgen that is important for development, especially in males. It is produced in the gonads and the adrenal cortex. Testosterone release is regulated by the hypothalamus and the pituitary gland.
Testosterone is said to be the key driving force behind anger, verbal and physical aggression, and dominance.
Testosterone activates the amygdala, increasing impulsive, emotional reactions to stimuli. High testosterone can increase reactive aggression.
Kreuz and Rose (1972) studied the correlation between testosterone levels, aggressive behaviours in prison and criminal history in 21 young white male prisoners.
There was no correlation between testosterone and behaviour in prison. However, the group with more violent criminal histories had significantly higher testosterone levels compared to the group without violent criminal histories.
Hormones and Aggression Psychology: Cortisol
Cortisol is the 'stress hormone' produced in the adrenal glands and regulated by the pituitary gland. Every cell in your body has a receptor for cortisol, so it has widespread effects when produced.
Cortisol is involved in:
Under threat, cortisol can help prioritise functions related to survival over others. It can increase your heart rate, increase blood glucose levels, among other things, to ensure you can fight or flight as efficiently as possible.
When this stress response is activated too often, it can have detrimental effects on our health.
Cortisol itself is a modulator of aggression, much like testosterone. It reduces levels of aggression by inhibiting testosterone, so when cortisol levels are high, aggression decreases.
Virkkunen (1985) conducted a study where male violent offenders had urinary cortisol levels measured.
Those who have antisocial with habitual violent offences had lower cortisol levels than other violent offenders, individuals with antisocial personalities but without habitual violent offences, and male hospital personnel.
Evaluation of Neural and Hormonal Mechanisms in Aggression
One weakness of neural and hormonal theories of aggression is that the association between these mechanisms and aggression is correlational. The evidence doesn't necessarily tell us whether aggression is caused by these mechanisms, just that they are linked.
Another problem with biological explanations of aggression is that they can promote biological determinism.
- According to these theories, biological abnormality, like lower serotonin levels, could result in a lack of control over one's impulses.
This suggests that some people might not have a choice but to be aggressive, which questions the idea of personal responsibility and the ethics of legal punishment of violence.
These explanations can also be criticised for reductionism.
- Explaining aggression in terms of neural and hormonal factors produces testable hypotheses, which means we can design experiments to test these explanations. However, these explanations might not show us the whole story of how environmental or individual factors contribute to and interact with our biology and behaviour.
Laboratory studies also have issues with having low ecological validity, and many of the above studies are conducted in these settings. We also cannot generalise, for instance, studies on prisoners to the general population.
Neural and hormonal mechanisms in aggression - Key takeaways
- Neural explanations of aggression focus on the role of the limbic system in aggression. The limbic system, particularly the amygdala, has been associated with aggression.
- The limbic system is a system of interconnected neural structures that include the hypothalamus, the hippocampus, the amygdala and the cingulate gyrus, amongst other structures.
- Serotonin is associated with aggression in that low serotonin levels correlate with higher levels of impulsive aggressive behaviours. According to the serotonin-deficiency hypothesis, it is because serotonin cannot inhibit aggressive impulses triggered by amygdala activity.
- Testosterone activates the amygdala, increasing emotional reactions to stimuli. High testosterone can increase reactive aggression. Cortisol is also linked to aggression, particularly social aggression. Cortisol modulates aggression through the inhibition of testosterone.
- Neural and hormonal explanations of aggression can be criticised for being deterministic and reductionist and for relying on correlational evidence. Research also tends to have low ecological validity, and studies on prisoners cannot be generalised to the public.
References
- Fig. 2 - Illustration from Anatomy & Physiology, Connexions Web site. http://cnx.org/content/col11496/1.6/, Jun 19, 2013 by OpenStax College is licensed by Creative Commons Attribution 3.0 Unported license.
- Brown, G. L., Goodwin, F. K., Ballenger, J. C., Goyer, P. F., & Major, L. F. (1979). Aggression in humans correlates with cerebrospinal fluid amine metabolites. Psychiatry research, 1(2), 131–139. https://doi.org/10.1016/0165-1781(79)90053-2
- Kreuz, L. E., & Rose, R. M. (1972). Assessment of aggressive behavior and plasma testosterone in a young criminal population. Psychosomatic Medicine, 34(4), 321–332. https://doi.org/10.1097/00006842-197207000-00006
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