AGGRESSION: Biological

  • Created by: ava.scott
  • Created on: 02-10-14 17:10


Neural Mechanisms:

The role of neural transmitters in causing aggressive acts such as low levels of serotonin or high levels of dopamine.

Hormonal Mechanisms:

The role of the endocrine system, which secretes hormones, causing aggressive acts, such as high levels of testosterone and cortisol.

Genetic Factors:

Aggression may be inherited; aggressive people inherit genes that leads them to be aggressive e.g. mutation that causes a deficiency on monoamine oxidase (the warrior gene)

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Neural Mechanisms

Neurotransmitters are chemicals that send messages across the synapse of two neurons.

  • A key chemical in research into aggression is serotonin. When levels are normal, serotonin has a calm inhibitory effect on a individual.
  • However, when they get low, people can act more impulsively when under stress.
  • Serotonin is mainly found in the pre-frontal cortex of the brain and has a inhibitory effect on amygdala, which controls emotional responses.
  • Serotonin inhibits the firing of neurons in the amygdala, and if there is too little serotonin, the amygdala fires more, causing us to act impulsively.
  • Some people have permanently lower serotonin levels, and are treated with MAOI's and SSRI's.
  • However, it has been found that serotonin fluctuates during the day. Foods which are high in the amino acid tryptophan increase serotonin levels, because it is a key component in the neurotransmitter.


  • People with very high dopamine levels can have schizophrenia, and this results in aggressive impulsive behaviour too.
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Neural Mechanisms Research: Crockett

Crockett- The Ultimatum Game

Participants played a game where they had to transfer money between themselves, and the computer (though particpants thought it was another participant.) Those who had not consumed the tryptophan drink responded more aggressively than those who did.

Grounding:  As tryptophan is a key amino acid in serotonin, we can deduct that a decrease in serotonin increases aggressive behaviour, because the amygdala is less inhibited.

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Neural Mechanisms Research: Mann +MRI

Mann- Found that in suicide, those with increased numbers of serotonin receptors in the pre frontal cortex, chose more violent methods to die.

Grounding: The pre frontal cortex grew more receptors to try and pick up more serotonin, because of the low numbers of the neurotransmitter. Serotonin is linked to violence in suicide.


Further research using MRI scanning showed higher activity in the amygdala when a participant was low in serotonin/ starved of tryptophan.

Grounding: Shows that the amygdala and serotonin are linked, and when the neurotransmitter is low, the ammydala is more active, potentially leading to emotionally impulsive behaviour.

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Neural Mechanisms research: Davidson


found that violent criminals had lower levels of serotonin than non-violent criminals, but this did not effect behaviour within prison.


Serotonin within criminals seemed to be linked to the degree of violence of their crime. This could be due to their lack of control over the amygdala resulting in impulsive actions.

HOWEVER- the serotonin levels may be decreased after being in prison, as they may be stigmatuised for their violent crime.

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Neural Mechanisms Research Evaluation

  • GOOD:The research into neural mechanisms and the role of neurotransmitters is very extensive. It is laboratory based, making it highly repeatable and controlled.
  • Internal validity is high, as the participants were carefully monitored and controlled e.g. Crockett's fasted overnight.


  • However, they Crockett did use questionaires to establish feelings of aggression and hostility, which lack validity due to demand characteristics and social desirability bias.
  • Artificial conditions and stimulations make the results less externally valid.
  • Tryptophan may be apart of another chemical in the human body that effects mood- in which case it would not support the serotonin theory directly.
  • In Mann's research, the suicide victims are very extreme cases, and lack population validity.
  • The amygdala isn't just involved in angry responses; the high activity shown in MRI scanning could encourage other emotional responses too.

Grounding: The resaerch is consistent with the theory, and due to its empirical nature, can be seen as quite compelling. There is slightly low population and ecological validity, so more repeats would be good.

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Neural Mechanisms: IDA



  • Allows complex and thorough understanding of biological mechanisms.
  • This allows many practical applications in a medical sense.


  • The theory looks just at the role of neurotransmitters in aggression, and ignores many other factors and even the way in which external influences trigger neural mechanisms.
  • Medicalsiation isn't always a good thing, and doesn't cure the cognitive side of aggression,
  • This medicalsiation can make people feel helpless, and give them an excuse to justify aggression.
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Hormonal Mechanisms

Testosterone is present in both men and women, but men have 8x more.

Theory 1: High testosterone leads to more aggression.

Theory 2:

  • Testosterone is linked to dominance rather than aggression.
  • It becomes related to aggression when high levels is paired with low serotonin levels.
  • Here, frustration of dominance may occur, and the low levels of serotonin allows frustration to appear as aggression, because the amygdala isnt inhibited.

Theory 3:The mismatch theory.

  • The higher your testosterone, the more dominance you want.
  • You can become aggressive when there is a mismatch in status and aggression.
  • For example, businessmen and athletes have high testosterone levels, but are not necessarily aggressive.
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Hormonal: Supporting Evidence- Kreuz and Rose

Kreuz and Rose

Studied testosterone levels in 21 young adult male prisoners.

  • Their testosterone levels did not relate to how much they fought with others in prison,
  • but it did relate to the nature of the crime they had committed before.
  • 10 prisoners with history if more violent crim such as assault and armed robbery had high levels of testsoernone that those who had committed non violent crimes.

This supports the testosterone theory, as the higher the testosterone, the more violent and aggressive the crimes were.

However, the testosterone did not have an effect on the aggression inside the prison. This may be due to changes in their status; perhaps outside, those with higher testosterone had lower status, resuting in more furstration and aggrerssion. In the prison, their status may be higher, or unchangeable, so it does not lead to aggression.

However, the research did not look at status, so cannot provide evidence for the mismatch theory. The aggression levels remained high, so why did they not cause aggression inide prison?

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Hormonal Evidence: Josephs


  • Found that men and women with high testosterone reacted negatively after a loss of high status.
  • They became stressed, confused, and anxious, and these could lead to frustration and aggression.
  • However, women with low levels of testosterone were put into a hi9gh status, and shwoed teh same symptoms of stress.

Supports the MISMATCH theory.

Those with high levels of testosterone are fuelled to find a high status position. This makes them more comfortable and in control. Those with low levels are intimidated by the role and pressure, rsulting is frustration. Therefore, the mismatch of status and testosterone can lead to aggression.

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Hormonal Evidence evaluation


  • Uses empirical emthods to measure testosterone levels e.g. blood tests
  • Josephs study is recent- 2006.


  • Kreuz and Rose has low population validity- just young men in prison. Different relationship may be found in older people or women.
  • Temporal- klreuz and Rose was in 1970-- long time ago, and now testosterone level;s and criem may be different.
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Hormonal Mechanisms: IDA

Socially Sensitive:

  • As women nearluy always ahve lower levels of testosterone than men, this theory could argue that they won;t be as comfortable/relaxed in a high status job. The theory suggets women should stay in low status jobs to avoid aggressive feelings and frustration.
  • It also says that those with very high testosterone levls, and showing agression, should be given a high status job. This is almost rewarding aggression.
  • The evidence perhaps isn't strong enough to fully justify the mismatch theory, as this could lead to different policies and treatment o agressiopn and high testosterone levels.


  • The theory says if you have high testosterone levels, you will struggle to not be aggressive.
  • This medicalises aggressive behaviour, and reduces the treatment to chemical.
  • Cognitive approaches to treatemnet are ignored.
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Genetic Factors: Inheritance

Inherited aggression

  • Genes can lead to aggression.
  • The more closely related two people are, the stronger the inherited tendency will be.
  • The more forebears you have who were aggressive, the aggressive you are likely to be.
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Inheritance: Research McGuffin and Gottesman

McGuffin and Gottesman

  • Found that aggression was concodrant between MZ twins 87% of the time.
  • Found that aggression was concordant between DZ twins 72% of the time.


Genetics is a huge factor in aggression, as MZ twins had higehr concordance rate than the DZ twins. However, environmental factors play a big role too, as teh rates of DZ twins was still very high; at 72%

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Inheritance Research: Hutchings and Mednick

1145 adoptees.

Found a strong significant correlation between the number of convictions of fathers and their biological sons (who had been adopted else where) number of convictions. No correlation existed between those adopted and their adoptive fathers.


This supports the genetic theory because it controlled for social learning theory, as the adopted children would not learn the aggressive behaviour from their fathe, and so it must hav a genegtic underlying cuase.

However, the act of being adopted may increase chance of being agressive and convicted, and therefore, the genes are notthe underlying cause.

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Inheritance Research evaluation

Hutchings and Mednick has a very large sample size, but for a very specific target population- adopted sons of convicted parents. This makes it less populationally valid.

It is also a correlational study and this means we cannot define causality. Many other factors may play a role.

The twin studies are natural studies, and so there is a little control over variables. This reduces internal validity. Also, the participants are all twins, so the results cannot be generalised to the rest of the population.

The evidence is not strong enough to create a compelling conclusion that inheritence causes aggression.

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Genetic Factors: Warrior gene

  • The warrior gene has been linked to increased aggression
  • People inherit a mutation on the X chromosome which causes a deficiency in monoamine oxidase (MAO-A)
  • MAO-A is a enzyme that breaks down excess monoamine neurotransmitters, such as noradrenaline and dopamine.
  • This means there is an excess of these neurochemicals in the brain disposing the individual to aggresison when angry scared or frustrated..
  • Effects predominantly men, as they only have one allele of this gene.
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Warrior evidence: Dutch Family

4 generations of males in a Dutch family had inherited the fault MAO-A gene (the warrior gene.)

The men with the dfecetive gene (not all men inherited it) showed aggressive and violet behaviour including arson and attempted ****. They could not regulate their aggression, especially when provoked.

The urine of 5 indiividuals with the gene had a urine test, which showed high levels of the neurotransmitter chemicals. This shows the neurotransmitters coudl not be broken down, and so there was an excess in urine.


This is a very strong piece of evidnece, it displays the biological mechanism of a excess neurotransmitter, and shows how this may have cause the aggressive behaviour of the men.

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Warrior evidence: tg8 mice

Vishnivetskaya et al studied mice that lack the MAO-A gene all together (known as Tg8 mice), comapring them to a control group of normal mice.

The TG8 mice showed higher levels of aggression towards intruders, teritorial aggression, predatory aggression and isolation induced aggression.

However, there was no increase in aggression towards anaethetised or juvenile mice.


There is very strong support, as the gene has been shown to directly effect the behaviour of  mice, increasing aggression in many aspects. However, its hows that the gene cannot explain all types of aggression, as some went unaffected by the absense of the gene.

The research shows that perhaps this gene is directly linked to status and how men maintain their territory and deal with threats. It does not explain more malicious behavuour towards juveniles etc.

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Warrior research evaluation

  • Low generlaisability. The dutch family are just one family, and only 5 men were tested for urine. There will bemany families where aggression seems to be inherited but theyw ont have this gene. The aggression may just appear in this family due to social learning theory.
  • Vishnivetskaya's mice study was very empirical and controlled. This increases its internal validity.
  • However, it used mice, not humans, who lack the same social interecation and comprehensiveness, so the results cannot be entirely generalsied to humans. It also failed to explain soem aspects of aggression that appear in humans and mice e.g. unprovoked aggression to juveniles and asleep.

Studies are very empircal and highly repeatable, so this maikes its more reliable. The evidence is compelling for the small samples it invetsigated, but the theory can not be used to explain all aggression as only very few people have th warrior gene.

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Genetic factors IDA and applications

Reductionist and determinstic

  • The theory doesnt look at any social influences, making to reductionist. However, this means it gives no practical applictions, as is aggresison is caused by genes and genes only, there will be no cure to those with genes.
  • This could be very damaging to the individual, who will perhaps geel stigmatised, or even worse, use it an an excuse to be more aggresisve.
  • It takes away responsibility from the individual and so could cause an increase in aggression, as you can't punish someone for their genes.

It is similar to social learning theory as it suggests that aggression can run in families. However, social learning tgeory give smore practical applications, and could considered more usuefl. Medical intevrention would probably result in dependency, addiction or stigma.

The theory is compelling, as it is easily investigated and demonstarted using empriical methods. However its implications are negative and lack practical applications. 

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