Neural and Hormonal Mechanisms of Aggression

What is the limbic system?

Subcortical structures in the brain (including the hypothalamus and amygdala

What is serotonin?

A neurotransmitter with widespread inhibitory effects throughout the brain

What is testosterone?

A hormone from the androgen group that is produced mainly in the male testes (and a smaller amount in the female’s ovaries)

What did Brunner et al research in 1993?

• He studied an extended family in the Netherlands, where several male family members showed patterns of behaviour involving impulsive aggression, including violent crime

• They identified a fault in the MAOA gene of the individuals who showed impulsive aggression, which wasn’t present in other male family members. The individuals with this fault were deficient in MAOA

What did Cases et al research in 1995?

• He found that adult male mice who were missing the MAOA-producing gene had specific behaviour patterns, which included heightened aggression

• This is known as a knockout study- Animals are bred so that they are missing a specific gene (the gene is knocked out) and are then studied to observe the effect of the gene

What does the limbic system do?

The Limbic System (including the Hypothalamus and Amygdala) tends to act as an alarm system triggering aggressive responses to certain types of threats.

What does the limbic system include?

• Comprised of the cingulate gyrus, septal areas, hypothalamus, fornix, amygdala and parts of the hippocampus and thalamus. Involved in the processing of emotional responses and behaviour regulation.

• The most important influence on aggression is the amygdala.

Why is the limbic system linked to aggression?

1. Different areas of the brain, including the temporal lobe and the limbic system, have been linked to different forms of aggressive behaviour.

2. One part of the limbic system, the amygdala, has been found to have a particularly strong connection to aggression.

3. Animal studies have shown that electrical stimulation of different parts of the amygdala can either cause or reduce aggression. Lesions to the amygdala have been found to cause cats to attack, but caused dogs to become more submissive and less aggressive - they needed more stimulation to provoke a response.

4. There is some evidence for the role of the amygdala in human aggression too. Charles Whitman, a sniper who killed 14 innocent people and wounded 31 others, left a note that pleaded for his brain to be examined after death for possible dysfunction. An autopsy showed that he had a temporal lobe tumour, pressing on his amygdala.

• An amygdalotomy is a procedure which involves disconnecting the amygdala from the rest of the brain. Studies have shown that, following the procedure, many patients experience a loss of emotion and become a lot less aggressive.

What did Mpakopoulou et al research in 2008?

He conducted a review looking at studies investigating aggression in patients before and after an amygdalotomy. Of the 13 papers they looked at, they found that overall, aggressive behaviours in those who'd had an amygdalotomy had decreased between 33 and 100% with no impact on the patients' learning or intelligence.

What is the amygdala?

• The amygdala has been shown to be an area that causes aggression. Stimulation of the amygdala results in augmented aggressive behaviour, while lesions of this area greatly reduce one's competitive drive and aggression

• Plays a key role in how an organism assesses and responds to environmental threats and challenges

• The amygdala has connections with other body systems related to fear, including the

  the sympathetic nervous system, facial responses, the processing of smells, and the release of neurotransmitters related to stress and aggression.

What is the evaluation of the amygdala?

• Pardini et al (2014)

  • Longitudinal study of male ppts (childhood to adulthood)

• MRI at age 26

• Reduced amygdala volume can predict the development of severe and persistent aggression

• Suggests the Amygdala plays an important role in aggression.

What method did Pardini et al use in 2014?

• Participants were selected from a longitudinal study of 503 males initially recruited when they were in the 1st grade in 1986–1987. At age 26, a subsample of 56 men with varying histories of violence was recruited for a neuroimaging substudy. Automated segmentation was used to index individual differences in amygdala volume. Analyses examined the association between amygdala volume and the participants’ levels of aggression and psychopathic features measured in childhood and adolescence. Analyses also examined whether amygdala volume was associated with violence and psychopathic traits assessed at a 3-year follow-up.

What is Highley et al’s animal study which he did in 1996?

He studied a group of 49 rhesus monkeys over 4 years. They determined each individual's serotonin turnover. Monkeys with lower serotonin turnover were observed to be more aggressive. After 4 years, 11 monkeys had died or were missing.
All of the monkeys in the highest serotonin turnover group survived, and the 4 monkeys who had died as a result of aggressive encounters were all in the lowest serotonin group.

What is Valzelli and Bernasconi’s animal study which they did in 1979?

They bred mice with low serotonin turnover. They found that, when isolated, these mice were more aggressive than mice with normal serotonin turnover.

What research has been don on Rhesus Monkey’s?

• Early studies by Kluver and Bucy (1939) and Weiskrantz (1956) found that the amygdala is a critical structure in mediating fear, anxiety, and other defensive behaviours.

• These studies, performed in rhesus monkeys with large lesions of the temporal lobe, reported dramatic effects such that monkeys with feral behaviour became tame.

• These initial studies were a major impetus for numerous other investigators to explore amygdala functions in relation to emotion and behaviour.

What are the features of the Hippocampus?

• Involved in the formation of long term memories and allows an animal/person to compare the conditions of a current threat with similar past experiences and respond

  rapidly.

• Impaired Hippocampal function prevents the nervous system putting things into a

  relevant context and so may cause the amygdala to respond inappropriately to

  the situation, resulting in aggressive behaviour.

• Boccardi et al (2010) found that habitually violent offenders exhibited abnormalities of hippocampal functioning

What is the evaluation of the Hippocampus?

• Raine et al (2004)

  • Studied 2 groups of violent criminals “unsuccessful psychopaths” (impulsive) – got caught. “successful psychopaths” (cold, calculating criminals) – evaded the law.

• MRI showed hippocampal asymmetry in the “unsuccessful” group which suggests emotional information was not processed correctly.

Why might the turnover of serotonin affect aggression?

1. Serotonin is a neurotransmitter which is thought to inhibit aggressive behaviour —
   low serotonin in the brain (or reduced serotonin activity) is thought to be linked to aggression.

2. The level of serotonin activity in the brain is measured by turnover — this is how much serotonin is produced and then broken down.

What are the features of the neurotransmitter serotonin?

• Under normal circumstances, the neurotransmitter serotonin works on the frontal areas of the brain to inhibit the firing of the amygdala, the part of the limbic system in the brain that controls fear, anger and other emotional responses.

• Consequently, low levels of serotonin mean that people can’t control their impulsive and aggressive behaviour.

• Serotonin also regulates the pre-frontal cortex; therefore, lower levels of serotonin affect our response to external stimuli, meaning the person becomes aggressive easily and can’t control their responses in a ‘normal’ way.

What research has been done on serotonin?

• It suggests that when serotonin levels are low, it may be more difficult for the prefrontal cortex to control emotional responses to anger that are generated within the amygdala.

• In normal levels, Serotonin exerts a calming inhibitory effect on neuronal firing in the brain, inhibiting the amygdala's emotional responses. (Fear/Anger)

• A low level of serotonin removes this inhibitory effect so individuals are less able to control impulsive and aggressive behaviour (The Serotonin Deficiency Hypothesis)

• Low levels have also been associated with an increased susceptibility to

  impulsive behaviour, aggression and even violent suicide

What are the limitations of the link between serotonin and aggression?

• There's plenty of evidence that serotonin is related to aggression, but scientific support which shows how serotonin causes aggression in humans is lacking. There are several weaknesses in the evidence available:

  1. Animals studies show a strong link between low serotonin turnover and aggression, but this doesn't mean the link is causal. Also, findings from animal studies can't be generalised to humans.

  2. Studies using diet are better controlled, and usually involve healthy participants. However, they don't directly link serotonin to aggression — it's just thought that this is why a controlled diet can affect aggression.

  3. Laboratory studies have low ecological validity. Natural experiments (see page 93) have higher ecological validity, and these studies tend to support a more complicated relationship between serotonin and aggression. For example, lots of studies have used convictions for criminal offences as a measure of aggression

  4. However, results from studies of convicted criminals may not be generalisable to the population as a whole.

What did Virkkunen et al find out in 1987?

He studied serotonin turnover in 20 arsonists, 20 violent offenders and 10 healthy volunteers. Serotonin turnover was significantly lower in the arsonists than in the other groups. However, serotonin turnover didn't correlate with the severity of the arsonists' offences. Virkkunen et al proposed that this showed a link between serotonin and impulsive behaviour, rather than aggressive behaviour.

What is the evaluation of serotonin?

• Evidence from non-human studies:

  • Raleigh et al (1991) – vervet monkeys

    • Diets high in tryptophan (increases serotonin)

    • Decreased levels of aggression

  • Popova et al (1991) - Animals bred for domestication

    • Docile temperaments = corresponding increase in brain concentration of serotonin

    • Silver foxes selected for more than 30 years for tame behaviour and displaying no defensive reaction to human contact were shown to have a higher serotonin level in the midbrain and hypothalamus

• Evidence from antidepressants:

• Bond (2005)

  • Clinical studies of antidepressants that elevate serotonin levels

  • These drugs do tend to reduce irritability and impulsive aggression

How does Testosterone link to aggression?

1. Testosterone is an androgen (male sex hormone) — it's responsible for the development of male characteristics, and may also affect the brain.

2. Men produce much more testosterone than women, and men are statistically more likely to engage in violence than women. For example, a very high proportion of people arrested for violence against another person in the UK are men.

3. It's been argued that this indicates testosterone is related to aggression.

4. Studies involving people convicted of violence have supported this suggestion.

5. However, there's a problem with establishing cause and effect - this data is only correlational. Another factor could be causing aggressive behaviour, or it could be that being aggressive raises levels of testosterone.

6. Van Goozen et al (1994) studied the effects of testosterone directly. This avoided having to depend on correlational data, which made it easier to establish cause and effect.

What did Dabbs et al find out in 1987?

He measured testosterone levels in the saliva of 89 male prison inmates. Those with a higher testosterone concentration were more likely to have been convicted of violent crimes. Almost all of those with the highest levels had been convicted of violent crimes, and almost all of those with the lowest levels had been convicted of non-violent crimes.

What method did Van Goozen et al use in 1994?

In a repeated measures design, 50 transgender people transitioning to the gender that they identified as, using hormone therapy, completed questionnaires to assess proneness to aggression. They completed the questionnaires before and after receiving hormones. Those transitioning from female to male were given were given testosterone (an androgen) and those transitioning from male to female were given anti-androgens. Treatment lasted 3 months.

What results did Van Goozen et al get in 1994?

People transitioning from female to male reported an increase in aggression proneness, whereas those transitioning from male to female reported a decrease.

What conclusion did Van Goozen et al get in 1994?

Levels of testosterone determine the likelihood of displaying aggressive behaviours.

What is the evaluation of Van Goozen et al’s study from 1994?

By controlling levels of testosterone experimentally, the direction of cause and effect between testosterone and aggression can be established. However, self-report measures of aggression were used, which are subjective and so may not be valid. The participants may have been conforming to stereotypes of their new gender roles by expressing an increase or decrease in aggression.

How does Testosterone link to aggression?

• The male sex hormone testosterone is thought to increase levels of aggression from young adulthood onwards.

• It is thought that testosterone may influence areas of the brain that control behavioural reactions, such as the amygdala and the hypothalamus.

• Testosterone also influences the levels of other hormones that are thought to be involved in aggression, such as vasopressin, which is linked to defensive aggression in animals.

What does testosterone cause?

• Testosterone causes a change in a person's level of dominance.

• The more testosterone that person produces the more competitive and dominant they will become.

• Therefore it follows that a man with a high-level testosterone will take part in antisocial behaviour such as fighting as a way of expressing this dominance.

What research did Connor and Levine do in 1969?

• Sample: castrated rats

• IV: C1: castrated when young / C2: castrated when fully developed

• Findings:

  • C1 – lower levels of testosterone and lower levels of aggression as an adult. Testosterone injections had no effect.

  • C2 – castration reduced aggression levels. Testosterone injection returned rats to pre-castration level.

• Conclusion: Sensitivity to testosterone is part of the developmental process

What research did Dabbs et al do in 1987?

• Sample: 692 male prison inmates from Lee Arrendale Correctional Institution in Georgia

  • IV violent and non-violent offenders.

  • DV: Measured salivary testosterone.

• Findings:

  • Those with the highest testosterone levels had a history of primarily violent crimes (crimes of sex and violence).

  • Those with the lowest testosterone levels had committed only non-violent crimes (burglary, theft and drugs).

  • Also, those with the highest levels of testosterone broke more prison rules and were more ‘confrontational’.

What is the evaluation of testosterone?

• Inconsistent evidence

• Albert et al (1993):

  • Although some studies show a correlation between testosterone and aggression, other studies have not found the same relationship.

  • Most research that has found a correlation has involved a small sample of men within prisons and has used self-report methods of aggression.

How are biological mechanisms reductionist?

• Links between serotonin and testosterone are well-established in non-human animals

• However, the position is not so clear in humans

• The complexity of human behaviour means that a biological explanation on its own is insufficient to explain all the aspects of violent, aggressive or dominant behaviour we observe.

What is gender bias of neural and hormonal mechanism?

• Most studies in this area have involved male participants but research suggests that the link is even stronger for females than males.

• Baucom et al (1985) found that high testosterone and higher occupational status were linked in women. This implies that these women used dominance and aggression in a similar way to men when faced with career challenges.

• However, other research found that testosterone could make women act “nicer” depending on the situation.

• This supports the idea that rather than causing an increase in aggression testosterone encourages our “status seeking” behaviour and that aggression is just one type of this behaviour (most often used by males?)