Neural And Hormonal Mechanisms In Aggression

What did Papez and Maclean link the limbic system to?

Emotional behaviours, including aggression.

What did Papez and Maclean define the limbic system as?

The hypothalamus, amygdala and parts of the hippocampus.

What is the reactivity of the amygdala in humans an important predictor of?

Aggressive behaviour (ie the more responsive the amygdala, the more aggressive a person is).

What does the amygdala have a key role in?

How mammals assess and respond to environmental stress.

How is amygdala activity illustrated?

In a study by Gospic et al.

What was the procedure of Gospic et al.?

Some participants were subjected to mild provocation.

In Gospic et al.’s study, when participants reacted aggressively, what did fMRI scans show?

A fast and heightened response by the the amygdala.

In Gospic et al.’s study, what did participants taking a benzodiazepine drug (reduces arousal of the autonomic nervous system) lead to?

Two effects:

• It decreased the activity of the amygdala.

• It reduced aggression (halved the number of rejections).

What is serotonin?

A neurotransmitter with widespread inhibitory effects in the brain.

What are normal levels of serotonin in the orbitofrontal cortex (OFC) linked with?

Reduced firing of neurons, which is in turn associated with greater behavioural self-control.

What does decreased serotonin in the OFC do?

It disrupts the mechanism of better behavioural self-control, leading to an increase in impulsive behaviour including aggression.

What is testosterone?

A hormone from the androgen groups that is produced mainly in the male testes (and in smaller amounts in the female ovaries).

Why is testosterone thought to be linked to aggressive behaviour?

For several reasons:

• Many people (not just researchers) have observed that men are generally more aggressive than women,

• Men become more aggressive towards other men at a time in development when testosterone levels are highest.

• Testosterone has a role in regulating social behaviour via its influence on certain areas of the brain imlicated in aggression.

What do castration studies of animals show?

That removing the testes (the source of testosterone) reduced aggression in the males of many species. Giving injections of testosterone in the same animals restores aggressive behaviour.

Where does other evidence for testosterone having implications in aggression come from?

Studies of prison populations. Dolan et al. found a positive correlation between testosterone levels and aggressive behaviours in a sample of 60 male offenders. These men mostly had personality disorders and histories of impulsively violent behaviour.

What evidence is there surrounding progesterone?

There is some evidence that progesterone (a female ovarian hormone) plays an important role in aggression in women.

When do progesterone levels vary?

They vary during the ovulation cycle and are lowest during and just after menstruation.

What did Ziomkiewicz find?

A negative correlation between progesterone levels and self-reported aggression, suggesting that low levels of progesterone are linked to increased aggression in women.

What is one limitation of the limbic explanation?

Other brain structures.

How is other brain structures a limitation of the limbic explanation?

There is more recent research showing that non-limbic brain structures are also involved in aggression.

Limbic structures (e.g. the amygdala) function together with the orbitofrontal cortex (OFC), which is not part of the limbic system. The OFC is involved in impulse regulation and inhibition of aggressive behaviour. According to Coccaro et al, OFC activity is reduced in those psychiatric disorders that feature aggression. This reduced activity disrupts the OFC's impulse-control function, which in tum causes aggressive behaviour.

What does the other brain structures show?

That the neural regulation of aggression is more complex than theories focusing on the amygdala suggest.

What is the strength of the serotonin explanation?

Drugs and serotonin.

How is drugs and serotonin a strength of the serotonin explanation?

One strength of the serotonin explanation is research into the effects of drugs: Drugs (e.g. paroxetine) that increase serotonin have been found to also reduce levels of aggressive behaviour. Berman et al. gave participants either a placebo or a dose of paroxetine. The participants then took part in a lab-based game that involved giving and receiving electric shocks in response to provocation (e.g. insults). The paroxetine group consistently gave fewer and less intense shocks than the placebo group.

What does the study of drugs and serotonin show?

This study is evidence of a causal link between serotonin function and aggression.

What is the strength of hormonal explanations?

Animal research.

How is animal research a strength of hormonal explanations?

Giammanco et al's review of studies confirms the role of testosterone. For example, in male rhesus macaque monkeys there is an increase in both testosterone levels and aggressive behaviour during the mating season. In rats, castration of males reduces testosterone and also mouse-killing behaviour. Injecting female rats with testosterone increases mouse-killing.

What does the animal research show?

These findings show the role of testosterone in a range of animal species.

What is the limitation of the hormonal explanation?

Dual-hormone hypothesis.

How is the dual-hormone hypothesis a limitation of the hormonal explanation?

Mixed evidence of the link between testosterone and aggression in humans. Carré and Mehta developed a dual-hormone hypothesis to explain why. They claim that high levels of testosterone lead to aggressive behaviour but only when levels of cortisol are low. When cortisol is high, testosterone's influence on aggression is blocked. The hormone cortisol plays a central role in the body's response to chronic stress.

What does the dual-hormone hypothesis show?

Therefore the combined activity of testosterone and cortisol may be a better predictor of aggression than either hormone alone.