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Neural and Hormonal Mechanisms in Aggression

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Neural and Hormonal Mechanisms in Aggression

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:

  1. The limbic system

  2. Serotonin research into aggression

  3. Testosterone research into aggression

  4. Cortisol research into aggression

Fighting Neural Hormonal Mechanisms Aggression StudySmarterTwo figures ready to fight, flaticon.com

The limbic system

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.

Neural and Hormonal Mechanisms in Aggression The limbic system and nearby structures StudySmarterThe 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 research

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).

  • It aids in sleep, digestion, and other bodily functions.

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 Aggression StudySmarterSerotonin, 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 research

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:

  1. The Basal Model: This suggests that testosterone changes a person's level of dominance.

  2. 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.

Cortisol research

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.

Stressed Hormonal Mechanisms Aggression StudySmarterMan 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:

  • Virkkunen (1985): Male violent offenders had urinary cortisol levels measured. Those who were antisocial with habitual violent offences had low levels of cortisol.

Neural and hormonal mechanisms in aggression - Key takeaways

  • There are multiple neural and hormonal mechanisms involved in aggression, through modulating it, inciting it, or inhibiting it. Dysfunctions of these mechanisms can affect levels of aggression.
  • The limbic system involves the hypothalamus, the hippocampus, the amygdala and the cingulate gyrus. The amygdala is particularly important in the limbic system and aggression. One of the main components of one of the most well-known responses humans exhibit is the fight or flight response.
  • Serotonin is a mood-regulating neurotransmitter. The serotonin-deficiency hypothesis suggests those deficient in serotonin will have issues with their mood. Low levels of serotonin suggest higher levels of aggression.
  • Testosterone is an androgen and is one of the main instigators of aggressive behaviours. The basal model suggests that testosterone and subsequent aggression levels are innate, and the reciprocal model suggests testosterone levels result from social hierarchy.
  • Cortisol is the stress hormone. Low levels of cortisol usually mean higher levels of aggression. The fearlessness theory suggests that stress caused by cortisol can inhibit aggression through fear.

Frequently Asked Questions about Neural and Hormonal Mechanisms in Aggression

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.

Final Neural and Hormonal Mechanisms in Aggression Quiz

Question

Define aggression.

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Answer

Aggression is any behaviour that intends to harm others psychologically or physically.

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What was the limbic system originally referred to as?

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Answer

 The Papez circuit.

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What does the limbic system comprise of?


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The hypothalamus, the hippocampus, the amygdala, and the cingulate gyrus. (There are more components, however we care about these areas for the exam!)

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What is the amygdala considered to be?

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The amygdala is considered the emotional centre of the brain, particularly for fear and threatening stimuli.

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What does the hypothalamus do?


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The hypothalamus regulates emotional responses and the autonomic nervous system (ANS).

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What does the hippocampus do?


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This is primarily involved in long term memories.

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What response begins in the amygdala and is important in aggression?

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The fight or flight response starts in the amygdala and can involve aggression. As these components are involved in mood regulation and emotional responses, it stands to reason that they are essential in the regulation of aggression.

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What is serotonin?


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It is a neurotransmitter.

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 How do levels of serotonin affect aggression?

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Serotonin is a mood regulator, affecting aggression if it is too low. Low levels of serotonin are correlated with aggressive behaviours.

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What is testosterone?


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Answer

Testosterone is an androgen that is important in the development of the body, especially in males.

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Is testosterone the key driving force behind aggression?

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Some consider this so, as it is directly related to aggressive and dominant behaviours, particularly in males.

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What is the basal model of testosterone?


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The Basal Model: This suggests that testosterone changes a person’s level of dominance.

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What is the reciprocal model of testosterone?


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Answer

The Reciprocal Model: This suggests testosterone levels are determined by a person’s standing in their social hierarchy, which determines their dominance levels.

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What is cortisol?


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Answer

It is said to be the ‘stress hormone’.

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What is the fearlessness theory in aggression?


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Answer

This theory suggests that stress caused by cortisol can inhibit aggression through fear. Lower levels of cortisol mean behaviours are less inhibited.

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Who first proposed the limbic circuit?

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Papez, in 1937.

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How does the limbic system process information?

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It processes it hierarchically.

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What are the main components of the limbic system?

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The amygdala, the hippocampus, the hypothalamus, and the cingulate gyrus. (There are more components, but these are important for the exam!)

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What are the functions of the hippocampus?

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It is involved in the formation of long-term memories and learning. It is also involved in spatial awareness and navigation.

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What are the functions of the hypothalamus?

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It is crucial in regulating the autonomic nervous system (ANS), which regulates emotional responses. Damage here can cause inappropriate responses to perceived threats.

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What are the functions of the amygdala?

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It is the emotional centre of the brain, processing fear-inducing and threatening stimuli and how they are linked/associated with memory. It integrates emotions with motivational behaviours.

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What are the functions of the cingulate gyrus?

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Regulating aggression, emotional responses to pain, communication and maternal bonding, amongst other functions.

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What response starts in the amygdala?

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The fight or flight response.

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What is the amygdala, if stimulated, a good predictor of?

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Aggressive behaviours.

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What did Groves and Schlesinger find in their study?

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In this study, the amygdala was surgically removed to reduce aggression in violent individuals. However, it affects emotion overall and suggests the amygdala is linked to aggression but is not the cause.

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What did Gospic et al. find in their study?

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In the ultimatum game, there was a heightened response by the amygdala in response to rejected, unfair offers, more noticeably in males than females.

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How is the link between the limbic system and aggression only correlational?

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Answer

The research suggests there’s an association between the two. However, the studies only show a correlational link between aggression and the limbic system. It is not a direct cause.

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Abnormalities in the limbic system are caused by aggression, true or false?

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False. It cannot be proven that the abnormalities cause or result from aggression.

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What problems did Wong et al. have in their study?

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Issues with beta bias. Although there were structural differences in the aggressive, reoffending inmates, they could not apply it to females, as the study was a small sample size of males.

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How does the prefrontal cortex affect the amygdala?

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It inhibits it to help regulate aggression.

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What is serotonin?

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It is a neurotransmitter in the central nervous system and a hormone in the peripheral nervous system.

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What is serotonin key in?

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Mood regulation and feeling relaxed, calm, and happy.

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What widespread effect does serotonin have on the brain?

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An inhibitory effect.

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What else can serotonin aid in?


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Sleeping, digestion, eating, healing wounds.

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What amino acid is the building block of serotonin?


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Tryptophan.

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How is the orbitofrontal cortex involved in aggression?


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When serotonin levels are normal, it helps inhibit impulsive, aggressive behaviours received from the amygdala.

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What happens if serotonin levels are low in the OFC?


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It cannot control or regulate impulsive, aggressive behaviours properly.

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What is the serotonin-deficiency hypothesis?


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Decreased levels of serotonin cause a reduced inhibitory effect, affecting the OFC and causing more impulsive and aggressive behaviours.

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What did Crockett et al. (2012) find in their study?

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Lowering serotonin levels resulted in weakened communication between the amygdala and frontal cortex. Their studies suggest that low serotonin levels are the reason why the frontal cortex struggles inhibiting and controlling the amygdala's aggressive impulses.

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What did Coccaro et al. (2007) find?


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Answer

Those with IED had exaggerated amygdala reactivity and diminished OFC activation to angry faces, compared to the controls. The findings show an amygdala-OFC dysfunction in response to processing angry faces, supporting the connection between the OFC and the amygdala.

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What did Brown et al. (1979) find in their study?


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Answer

Low levels of 5HIAA were found in the spinal fluid of military men with aggressive behaviour tendencies. 5HIAA levels had a significantly negative correlation with a history of aggressive behaviours in these men. To put it simply, they had lower levels of this byproduct of serotonin being broken down.


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How is serotonin associated with melatonin?


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Serotonin is associated with melatonin synthesis.

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What can low levels of serotonin do to your sleeping patterns? 


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Answer

It can cause insomnia.

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What happens if you have normal levels of serotonin?


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Answer

You have little or no aggressive behaviours and can feel relaxed and in control of your emotions.

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Do serotonin and aggression have a causal relationship?


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No, context matters. Both the individual and their social environment affect the aggressive behaviours, even with abnormal serotonin levels.

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What is testosterone?

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It is an androgen and an anabolic steroid. It is a hormone.

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Where is testosterone produced?

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In the gonads and the adrenal cortex.

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What does testosterone enable?


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Testosterone enables the development of male characteristics, and secondary ‘male’ characteristics such as muscle and skeletal growth, as well as body hair and facial hair.

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What happens if testosterone levels are low during puberty in men?

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When there is not enough testosterone in the body, especially for males undergoing puberty, the development of male characteristics is affected, such as hair growth on the body and face, as well as affecting healthy sexual development.

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What are examples of aggressive behaviours?


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Anger, verbal aggression, dominance, competitiveness.

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