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Neural and Hormonal Mechanisms in Aggression

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Neural and hormonal mechanisms in aggression include the roles of the limbic system, serotonin and testosterone.

Neural mechanisms in aggression:

The Limbic system-inside the brain lies a network of structures known as the limbic system. Within the limbic system, it involves many structures such as the amygdala, cingulate gyrus, fornix and the hypothalamus. The most significant structure is the amygdala because it plays a key role in humans and non-human animals in how an organism responds and assess to environmental challenges and threats therefore is believed to be the main factor in whether we respond aggressively or not. It also is responsible for attaching emotional significance to sensory information. Within the amygdala, its reactivity has proven to be the main indicator of aggressive behaviour. The prefrontal cortex is vital for regulating aggressive responses and social behaviour. However, damage to this prefrontal cortex would lower the inhibition of the amygdala, giving higher levels of aggression.

On the other hand, the hypothalamus is responsible for the regulation of the autonomic nervous system, which in turn helps to regulate responses to emotional circumstances. If this structure was to ever become damaged, it would result in an inappropriate aggressive response to a perceived threat.

Serotonin -serotonin has a key role in aggressive behaviour and is a neurotransmitter with widespread inhibitory effects throughout the brain; it dampens neuronal activity and slows it down. It is also involved in communication impulses between neurons. It has been associated that low levels of serotonin cause an increased risk of aggressive and impulsive behaviour. Serotonin usually prevents stimulation of the amygdala therefore if there are low levels of it within the brain, the amygdala will be more active once stimulated by external events occurring, resulting in the person to make aggression more likely and to act on their impulses. The role of serotonin illustrates itself by the use of drugs that raise the levels of it within the brain such as antidepressants. During trials, antidepressants were found to raise the serotonin levels and shown to reduce impulsive aggression and irritability. This therefore suggests that an increase in the level of serotonin can reduced aggression however this may not be the case always.

Hormonal mechanisms in aggression:

Testosterone - is a hormone from the androgen group responsible for the development of masculine features. It also plays a role in helping to regulate areas in the brain to do with social behaviour such as the amygdala and the hippocampus. It is evident that males are generally more aggressive compared to girls. Testosterone is thought to raise the levels of aggression from a young adulthood onwards. It peaks in young adolescent males before then declining with age.

Genetic factors in aggression:

Twin studies - many twin studies have suggested that heritability accounts for approximately 50% of the variance in aggressive behaviour. In a set of monozygotic twins, they share 100% of their genes however in a set of dizygotic twins; they only share on average 50% of their genes. Therefore, we would be expected to find that monozygotic twins would have greater similarities in aggressive behaviour if aggression is mostly influenced by genetic factors. This is because both monozygotic twins and dizygotic twins are raised together in the same environment however monozygotic twins do have a greater degree of genetic similarity than dizygotic factors. Researchers then found that 50% of concordance rates (proportion to which both twins exhibit the same behaviour) was for monozygotic twins and only 19% was for dizygotic twins. This illustrates that it is probably mostly genetically influenced.

Adoption studies - Adoption studies can help us to determine what the causes of aggression are either from the environment or heredity. Adoption studies compare the rates between adopted children who share behaviours with their adopted parents (who only share the environment with), and the rates between the adopted children who share behaviour with their biological parents (who share their genes but not environment with). If the results show that the concordance rate between the adopted child and their biological parents are high, then it illustrates that their behaviour is genetically influenced. However, if the concordance rates between the adopted child and their adopted parents are high then it suggests that the behaviour is more than likely environmentally influenced. A case to support that behaviour is genetically influenced is when a study was completed in Denmark of over 14,000 adoptions. They found that a huge number of adopted boys that had criminal convictions also had biological fathers with criminal convictions too.

The MAOA gene - the MAOA gene is a gene responsible for the activity of the enzyme monoamine oxidase A within the brain.  Monoamine oxidases A role is to 'mop up' neurotransmitters once a nerve impulse has been sent from one neuron to the other in the brain. This happens by the neurotransmitter, especially serotonin, being broken down into chemicals that can then be excreted or recycled. This is a process called catabolism. The production of this enzyme is determined by the MAOA gene. If the operation of this gene becomes dysfunctional it can result to abnormal activity of the MAOA enzyme which then further affects the levels of serotonin in the brain. It has been found in studies that violent criminals often have a defect in the gene that produces MAOA.

Gene-environment (GxE) interactions - Genes do not work on their own but they are crucial influences on aggressive behaviour. Low MAOA gene activity appears to only be related to adult aggression when combined with early traumatic life events.


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