Neural Mechanisms in Aggression (Neurotransmitters
Two neurotransmitters are particularly important in the control of aggressive behaviour:
- Serotonin - Low levels
- Dopamine - High levels
Serotonin and aggression
Low levels of serotonin increases aggressive tendencies. Studies have shown that serotonin has a calming, inhibitory effect on neuronal firing in the brain (Cases 1995). Low levels of serotonin removes this effect, making the individual less able to control their impulses and aggressive responses. The waste product of serotonin tends to be low in the spinal fluid of people who are aggressive.
Dopamine and aggression
There is some evidence to suggest that increases in dopamine are associated with increase in aggression (e.g. through use of amphetamines) (Lavine 1997).
Dopamine is the 'happy transmitter'. Dopamine is produced in response to rewarding stimuli such as food etc. Some people get a dopamine rush when they engage in aggressive acts or even just observe the acts such as crowd violence.
Evaluation of Neural Mechanisms in Aggression
- Davidson, Putnam and Larson (2000) suggested that serotonin may provide an inhibitory function so that when comparing violent criminals to non-violent ones, the levels of serotonin found in violent criminals were markedly lower.
- Studies of domestic pets which have been bred for reduced aggression show that they seem to have higher levels of serotonin
- Badawy (2006) claimed the influence of serotonin on aggression may be important in explaining the well established relationship between alcohol and aggression. He found that alcohol consumption caused major disturbances in the metabolisms of brain serotonin; in particular acute alcohol intake depleted serotonin levels in individuals. In susceptible individuals, this depletion may induce aggressive behaviour.
- Couppis et al (2008) Studied with mice. The turning out of dopamine in the animals brain also makes it difficult for the animal to move because of dopamines role in coordination of movement . Its therefore difficult to explain a drop in aggressive behaviour as their inability to move makes it difficult to act aggressively.
- The study of the relationship between neurotransmitters and aggression is based upon correlational research.
Hormonal mechanisms in aggression
A hormone is a chemical messenger produced by endocrine glands. One example is the pituatry gland releasing ACTH which stimulates the adrenal cortex.
The endocrine system produces several hundred hormones which interact with eachother and with the nervous system to regulate short-term, such as the fight of flight, and long-term, such as sex differentiation, maturation and reproduction.
Testosterone and aggression
Males produce testosterone in the testes. Women also produce testosterone but in smaller amounts in the adrenal glands. Testosterone levels reach a peak in young adult males and typically levels of testosterone decline with age.
Wagner et al (1996) found that castrated mice exhibit lower levels of aggression than non-castrated mice. The nature of the link between testosterone and aggressive behaviour however is not a simple biological cause and effect mechanism.
Contradicting evidence for testosterone
Wagner et al's research is correlational so you cannot establish cause and effects. It cannot be stated that hormones cause aggressive behaviour. Testosterone has been implicated in other behaviours which are more positive e.g. sportsmen have higher levels of testosterone.
There are huge individual differences in testosterone levels with high level testosterone females having high levels than males with the lowest levels. Several studies have consistently shown that in psychological research the P with the highest level of testosterone is not necessarily the most aggressive. Aggression may cause higher testosterone levels.
- Bernhardt et al (1998) who measured testosterone in saliva samples of football fans watching the 1994 world cup final. After the match testosterone levels had decreased in fans of the losing team and increased in fans of the winning team.
Many studies are animal studies. Behaviourists believe that it is possible to apply theories and evidence gathered from animal studies to humans however many other groups e.g. cognitive psychologists believer that humans are much more complex and that the results of animal studies only provide part of the story when it comes to humans.
The role of genetic factors in aggression
There is evidence to suggest that all behaviour is influenced by genetic factors. They do not work in isolation but alongside environmental factors. Sapolsky (1997) said that genes are the 'hand behind the scenes' directing testosterone's actions.
Genes determine how much testosterone is produced, the synthesis of testosterone receptors and how many and sensitive such receptors are. One method used to study aggression genetically is through heritability studies of animals. Such mice exhibit normal behaviour in most respects but appear to be twice as aggressive. This research shows the importance of genetic processes to our understanding of human behaviour.
Twin studies are used to determine if there is a genetic cause in various disorders. Conditions such as schizophrena, depression, ADHD and anoreixa have all been investigated using twin studies. The method involves looking at the likelihood that a person will have a condition if their twin has it. The degree of similarity in a pair of twins with respect to a trait such as aggression is called a concordance rate.
Twin studies investigating aggressive behaviour
McGuffin and Gottesman (1985) investigated the concordance rates for aggressive and antisocial behaviour for MZ twins and DZ twins. MZ twins had an 87% concordance rate and DZ twins had a 72% rate. From these results we can tell that genes play a factor in aggression but aren't solely responsible for aggressive behavior because 13% did not share the charecteristics.
Within this study, while the figure for MZ twins is higher, the figure for DZ twins at 72% indicates the importance of shared family environment. Researchers have the task of assessing the relative contribution of both genes and environment to aggressive behaviour.
The gene for MAOA (Monoamine Oxidase A)
In 1995 scientists discovered that mice that lacked the gene for MAOA suffered serious anger management problems. The enzyme made by the gene mops up excess neurotransmitters, so mice lacking the gene had unusually high levels of serotonin, noradrenaline and dopamine.
The same gene had previously been implicated in human aggression when it was found that members of a Dutch family whose men suffered from excessive bouts of aggression carried a rare MAOA gene mutation.
Key Research Study: Brunner et al (1993)
In the 1970's, scientists discovered an extremely violent family. When they analysed the x chromosomes of 28 members of the family, they found a marker on the x chromosome which was present on all the violent men. One particular gene for MAOA lay in the vicinity of this marker. They found, when they tested their urine, excess levels of neurotransmitters, and low levels of the substance left over after MAOA. This led them to believe that excess neurotransmitter may predispose the men to violence.
It is unlikely that there was a single causal relationship between a single gene and a specific behaviour. The behaviour found may be more widespread than in 'normal' families because of shared environmental factors, such as bad parenting and innappropriate role modeling.
Brain anatomy and aggression
Two parts of the brain involved:
- Amygdala (the emotional centre) - has innate impulses that don't change until we are 3 years old when we start to learn, before this change we are unable to control our aggression.
- The prefrontal cortex - developed in early childhood and how it develops is dependant on how we are socialised by adults and what we are taught.
If you have a car crash the prefrontal cortex can bworkerecome damaged. After a car crash people have been known to have a complete change in 'persona' and often become violent and aggressive, this supports the theory that the prefrontal cortex has an important role in aggression levels. Phineas Gage was a railway worker who suffered a blow to the head which blew a tamping iron through his head in 1848. He lost his job due to aggressiveness and confusion. He became impatient, and continued to devise radical plans which he woud later abandon and continue with the next plan.
Potegal (1991) showed that hamsters have more active neurons in and around the medial nucleaus of the amygdala during acts of aggression. No other increases in neural activity were recorded in any other areas. Such research does argue that generalisation between animals and humans should be more viable - human and animal differences are qualitative. The underlying neural circuitry related to emotional expression seems more similar, allowing generalisation.
Evaluation of MAOA gene
Although the Brunners study is interesting, a direct causal relationship between MAOA and aggressive behaviour has not been established.
Caspi et al (2002) conducted a longitudinal study on boys in New Zealand. Men maltreated as children but who had the genotype for high MAOA activity were far less likely to show antisocial behaviour as adults.
In contrast, men maltreated as children as children without the genotype for high MAOA who accounted for only 12% of the P's accounted for 44% of the groups convictions for violent crimes.
So the MAOA genes play a more important role on our aggression than our background. However, maltreatment as a child in combination with a faulty gene can cause extreme aggression.
Newman et al (2005) investigate the role of the MAOA gene in 45 unrelated male macaque monkeys. They concluded that aggressiveness is influenced by a variation in MAOA activity in itself is sensitive to social experiences early in development. In other words the effect of the gene depends on the social context.
This is an example of an interaction between a gene and the environment and studying either in isolation might not give the complete picture.
Evaluation of twin studies
- You cannot generalise results from twin studies as they are often small sample sizes meaning they wont represent the target population, this is because twins are atypical.
- Being a twin is a unique experience that most people wont encounter. This means that you might not be able to generalise these studies to the wider population.
- Twins do share genes but they also share the same family environment, this means that if they grew up with a violent family they are more likely to become violent.
- Epigentics is the study of how the environment can affect genes. Twin researchers ignore environmental factors as they assume that genes are the only factor that determine behaviour i.e. the MAOA gene.