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Aggression is any behaviour that intends to harm others psychologically or physically. Numerous mechanisms both enable and facilitate aggression in humans and animals, and we will outline and discuss the role of neural and hormonal mechanisms in aggression. Each topic will have its own dedicated article to evaluate the theories and provide deeper insights into each aspect of aggression in psychology.
For your exam, you will need to know about:
The limbic system
Serotonin research into aggression
Testosterone research into aggression
Cortisol research into aggression
Two figures ready to fight, flaticon.com
Proposed initially is as the Papez circuit in the 1930s, the limbic system was expanded upon and now includes:
The hypothalamus regulates emotional responses and the autonomic nervous system (ANS).
The hippocampus: This is involved in long term memories primarily.
The amygdala is considered the brain's emotional centre, mainly for fear and threatening stimuli.
The cingulate gyrus is important in regulating aggression, responses to pain, and communication.
The limbic system and nearby structures labelled, John Taylor, Wikimedia Commons.
The amygdala is essential in the limbic system and aggression, as it is one of the main components of one of the most well-known responses humans exhibit: the fight or flight response.
The fight or flight response starts in the amygdala and can involve aggression.
As the above components are involved in mood regulation and emotional responses, it stands to reason that they are important in the regulation of aggression. They help choose the appropriate behaviour to a threatening situation and begin the emotional process to react to it.
Multiple studies have been performed to identify how crucial the above components are in aggression:
Groves and Schelsinger (1982) removed the amygdala to reduce aggression initially but found it caused issues with all emotional responses.
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. His prefrontal cortex was damaged and could no longer inhibit the amygdala, which may be why he struggled with his temper and aggression.
Kluver and Bucy (1939): here, rhesus monkeys had the core parts of their limbic systems removed. These monkeys then went on to have issues and complete absences of responses with their emotions, motor functions when they were shown certain stimuli (specifically, stimuli meant to induce fear and anger), and vocal functions.
They also lost an understanding of their places in social hierarchies and would fight to gain dominance (an aggressive behaviour). This suggests just how important the limbic system is in regulating aggression, as well as other emotions.
Remember, the link is only correlational, not causal.
Serotonin is a neurotransmitter that is key in mood stabilisation. Feelings of happiness, calmness and the general ability to regulate moods are due to this neurotransmitter (alongside others, but serotonin is one of the major ones involved in this process).
When serotonin levels are abnormal or unusual due to an issue with the production, uptake, or even down to the genetic level of serotonin function, issues with mood may arise. Aggression seems to be one of the key aspects of this.
Serotonin, Wikimedia commons.
The orbitofrontal cortex is associated with self-control. Normal serotonin levels are why the OFC can inhibit impulsive or aggressive behaviours. It communicates with the amygdala for emotional processing.
A key theory of this is the serotonin deficiency hypothesis: If serotonin levels are low, for instance, impulses from the amygdala are not inhibited, and the OFC cannot correctly regulate aggressive behaviours coming from the limbic system.
Study tip: The Serotonin Deficiency Hypothesis is one of the key features in serotonin research on aggression; make sure you're familiar with the concept.
Some studies to consider:
Deneris et al. (2003): In this study, they found PET-1 genes were associated with the neurone development of serotonin. Mice lacking in this gene had serotonin production, reuptake, and storage issues. They were more aggressive as a result.
Brown et al. (1979): In this study, military men had their serotonin byproduct (what serotonin is broken down into after it is 'used') levels measured. These levels were negatively correlated with a history of aggressive behaviours in these men.
Testosterone is an androgen that is important in developing the body, especially in males. It is produced in the gonads and somewhat in the adrenal cortex. The hypothalamus regulates testosterone in the brain and the pituitary gland, the gonads.
Testosterone is said to be the key driving force behind: anger, verbal and physical aggression, and dominance, amongst other aggressive behaviours. It activates the amygdala, enhancing the resistance of the prefrontal cortex (PFC) to inhibition and increasing emotional reactions to stimuli.
High testosterone affects aggression, usually meaning people are more aggressive!
Without mediation, more aggressive behaviours are likely to occur.
There are two models:
The Basal Model: This suggests that testosterone changes a person's level of dominance.
The Reciprocal Model: This suggests testosterone levels are determined by a person's standing in their social hierarchy, which determines their dominance levels.
A study to consider:
Kreuz and Rose (1972): Prisoners had their testosterone levels measured, and those who had a history of aggressive behaviours had higher testosterone levels than those who didn't have this history.
Although cortisol has multiple effects on the body, cortisol is the 'stress hormone' (although it has multiple effects on the body) 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.
Man with stress indicators (line and lightning) over his head, freepik/flaticon.com.
It aids in:
Stress responses.
Regulating blood sugar levels and metabolism.
Heart rate and blood pressure
If the body is stressed, for instance, if you're under threat, cortisol will then prioritise certain functions over others to maximise chances of survival. If you're running away from something, it will reduce your bodily functions, increase your heart rate, increase blood glucose levels, amongst other things, to ensure you can 'run away' as efficiently as possible.
This isn't always ideal. Sometimes 'fight-or-flight' responses activate in inappropriate ways.
Cortisol itself is a modulator of aggression, much like testosterone. It reduces levels of aggression by inhibiting testosterone, so when cortisol levels are high, usually, a person is less aggressive. Low levels mean the opposite - testosterone isn't inhibited, and the ANS is not as aroused.
There is also the fearlessness theory:
This theory suggests that stress caused by cortisol can inhibit aggression through fear. Lower levels of cortisol mean behaviours are less inhibited, as mentioned above.
People will likely act more impulsively as a result.
A study to consider:
Neural mechanisms include the components of the limbic system. For instance, the hypothalamus, the hippocampus, the amygdala, and the cingulate gyrus.
The two most influential neural systems in the brain are the amygdala (the emotional control centre of the brain and fundamental in the fight or flight response) and the hypothalamus concerning aggression.
There may be multiple causes. It is subjective and influenced by the environment. Aggressive responses to the environment may be exaggerated by dysfunctional processes in the limbic system, serotonin functions, testosterone functions, and/or cortisol functions.
Depending on what type of aggression is occurring (are you fighting for your life? Or are you simply angry at something?) the amygdala, prefrontal cortex, and orbitofrontal cortex will work together and communicate to react appropriately (or inappropriately) with aggression. This is a modulated and regulated process, affected by serotonin, testosterone, and cortisol.
Three types of aggression can be physical aggression, emotional/mental aggression, and verbal aggression.
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