Biological explanations of aggression

what does the MAOA gene do

producing an enzyme called monoamine oxidase which regulates the metabolism of monoamines such as serotonin in the brain.  

what it the MAOA-L gene

low activity MAOA

Known as the warrior gene

What does the MAOA-L gene cause

abnormal levels of serotonin in the brain. Abnormal levels of serotonin are associated with impulsive and aggression behaviour.

1980s Dutch family study

many of its male members behaved in a particularly violent and aggressive manner, and a large proportion had been involved in serious crimes of violence including rape and arson.  These men were found to have abnormal levels of MAOA enzyme and consequently abnormal levels of serotonin due to having the MAOA–L gene.  The investigation indicated that the defective MAOA gene was passed onto problem men from the X chromosome of their mothers.

why don’t women display the MAOA-L gene

only affects men since they have only one X chromosome, and so show symptoms whenever they inherit the gene. Even though women can also inherit the gene, they don't show aggressive symptoms because they have a second X chromosome carrying a good copy of the gene.

Evaluation of the MAOA gene summary

• supporting evidence from twin studies

• reductionist

• biological determinism

• methods of measuring aggression differ between studies

Evaluation of the MAOA gene - supporting evidence from twin studies

A strength is that there is supporting evidence for the role of genes in aggression. Twin studies compare monozygotic (MZ) and dizygotic twins (DZ). MZ twins share 100% of their genes but DZ twins share only 50% (on average). If the MZ twins are more alike in their aggressive behaviour, then this should be due to genes rather than environment (both MZ and DZ twin pairs share the same environment, but monozygotic twins are more genetically alike). O’Connor (1980) found that the concordance rates for aggressiveness were 72% for MZ twins reared together compared to 42% for DZ twins reared together.  He also found for twins reared apart the concordance rates were 64% for MZ twins and 34% for DZ twins. This suggests a role for genes because the MZ %’s are higher than the DZ. However, although the MZ twin concordance rate is higher than the DZ concordance rate, which suggests the role of genes in aggression, the concordance rate is never 100%. This shows that aggression cannot be purely genetic.

Evaluation of the MAOA gene - reductionist

A criticism is that this theory is reductionist as it argues aggression is only caused by genetic influences thus focusing on the role of nature. However, it is argued that genetic influences alone are not enough to cause aggression and that environmental factors must interact with genetic ones in order to produce aggression.  

For example, in a large longitudinal study of 1,037 children followed over 25 years, Caspi et al (2002) found that males who had been severely maltreated as boys were more likely to engage in anti-social behaviour, including violence as adults if they also possessed the MAOA-L gene. 

 Maltreated males with the MAOA-L gene were more than twice as likely to have been with a conduct disorder diagnosed in adolescence compared with those with the MAOA-H gene, and three times more likely to have been convicted of violent crime by 26 years of age. The results indicate the importance of an interaction between genes and environmental influence. Neither having the MAOA-L gene nor the experience of maltreatment alone predicted later aggressive behaviour, but having them both did.  This gives evidence that it is the interaction between genes and environment that determines behaviours such as aggression. Therefore the Diathesis-stress model may be a better explanation for aggressive behaviour.  

Evaluation of the MAOA gene - biological determinism

A criticism of research into the role of genes in aggression is the issue of biological determinism, which suggests we do not have freewill over our behaviour. This leads to implications for crime and punishment. In 2007, Bayout admitted to stabbing and killing a man and received a sentenced of 9 years and 2 months. However, an appeal court judge in Italy, cut Bayout’s sentence by a year after finding out he has gene variants linked to aggression. We must question whether criminals should have their sentences cut for possessing such genes or should they be locked away forever if we can’t correct their faulty biology? Therefore, this research is socially sensitive and could lead to us questioning should unborn children be tested for aggression? If they are found to have the aggressive gene should the parents opt not to have the child? Should researchers try to find a cure for aggression? These types of ethical questions illustrate the controversy that behavioural genetics brings to the world. 

Evaluation of the MAOA gene - methods of measuring aggression differ between studies

A limitation with research into aggression is that the methods of measuring aggression differ significantly between studies. For example, criminal convictions, self-report, teacher/parent report, direct observations have all been used. It has been found that results differ depending on the type of measurement used e.g., a meta-analysis of 51 twin and adoption studies found that self-report studies found higher genetic influence than studies which used parent/teacher reports. Therefore, if research findings vary depending on how aggression is measured, then it is difficult to draw valid conclusions about the role of genetic factors.

what is testosterone

Testosterone is one of a class of male hormones called androgens which are important in producing sperm and developing secondary sexual characteristics.

what happens to aggression during puberty

Research has shown that aggression increases with increased levels of testosterone in puberty,

mice study on testosterone

animal research has shown that when animals such as mice are castrated aggression decreases.

why can’t androgens be given to humans

The role of testosterone in aggression is clear in the behaviour of lower order animals, but there is strong evidence that it contributes to human aggression too. For ethical reasons, androgens cannot be given to humans in order to see if changes in aggression occur, as they would need to be administered over a period of time which carries a health risk.

Dabbs et al. - testosterone

investigated the relationship between testosterone, crime and prison behaviour. They measured the testosterone in the saliva of 692 adult male prisoners and found a positive relationship between the level of violence in a crime and levels of Testosterone. For example, those who had committed crimes involving sex and violence had higher testosterone levels than inmates who had committed crimes like burglary and theft. The high testosterone males also broke more prison rules involving confrontation.  

evaluation of hormonal influence on aggression summary

• androcentric

• bi-directional ambiguity

• deterministic

• reductionist

evaluation of hormonal influence on aggression - androcentric

This research has been criticised for being androcentric as most of Dabbs’ work is carried out on males. Whilst women have less testosterone than men, its effects are considered to be just the same.  

However, Dabbs et al (1988) looked at the criminal history and the testosterone levels of 84 female prison inmates. They found testosterone levels were related to criminal violence, although this relationship was not straightforward. Testosterone was highest in cases of unprovoked violence but lowest where violence was defensive (for example, abused wives who had retaliated).  

evaluation of hormonal influence on aggression - bi-directional ambiguity

Another criticism is that research is correlational and therefore there is a problem with bi-directional ambiguity. It could be that rather than testosterone causing aggression as the theory suggests, it could be that being aggressive causes testosterone to increase. There is some evidence that this is the case as research has shown that during status conflicts testosterone rises in winners and declines in losers. Therefore, testosterone not only affects behaviour but also responds to it. 

evaluation of hormonal influence on aggression - deterministic

The research is deterministic as it argues aggression is determined by levels of hormones. This is socially sensitive as it has led some to suggest the use of chemical castration to reduce aggression. As high levels of testosterone are linked to aggression castrating males should reduce aggression. However, we must remember that research is correlational & there could be other factors causing the aggression

evaluation of hormonal influence on aggression - reductionist

A problem is that the research is reductionist as the hormones explanation reduces complex human behaviour of aggression down to high levels of T. The theory therefore emphasises the role of nature. However, it is likely that there are many other reasons for aggression such as social learning theory. SLT states that aggression can be learnt due to our environment thus arguing for the role of nurture. Therefore, aggression is likely to be due to combination of biological vulnerability & environmental trigger (diathesis-stress). 

what is the limbic system comprised of

many different structures including the hypothalamus, amygdala and the hippocampus.  

the role of the amygdala

a key role in how organisms assess and respond to environmental threats and challenges. Over-activity (high reactivity/sensitivity) of the amygdala in humans has proven to be an important predictor of aggressive behaviour. 

the role of the prefrontal cortex

controls the outward expression of aggression. Research suggests that this structure (which is involved in planning and moderating behaviour) is directly connected to the limbic system. It regulates the amygdala-driven emotional responses, and in this way exercises some form of emotional control by acting as a mediator, taking impulses from the amygdala and deciding whether to act on them. 

Damage to the prefrontal cortex results in a range of responses which include loss of control, impulsivity, immaturity and altered emotionality. All of these responses are associated with aggressive acts (Damasio et al, 1994).  

the role of serotonin

a neurotransmitter which has widespread inhibitory effects on the brain is also thought to be involved. Normal levels of serotonin are associated with a higher degree of self-control as serotonin inhibits the firing of the amygdala. Decreased levels of serotonin means an individual is less able to control aggressive impulses stimulated by external events. As a result when the amygdala is stimulated by external events, it becomes active, causing the person to act on their impulses, making aggression more likely. This may lead to an increase in impulsive behaviour, including aggression. This is also known as the serotonin deficiency hypothesis.  

evaluation research into the role of the amygdala in human aggression

The case study of Charles Whitman also supports the role of the amygdala. He was a student at the University of Texas. In the early hours of August 1, 1966 he murdered his wife and his mother in their homes. Later that day, he brought a number of guns to the campus of the University of Texas at Austin and climbed the clock tower. Over an approximate 90- to 95-minute period, he killed 14 people and wounded 32 others in a mass shooting in and around the Tower. Whitman was shot and killed by a police officer. He left a suicide note requesting that an autopsy be conducted to determine if there was a biological reason for his actions and increasing headaches. It was discovered that he had a tumor pressing on his amygdala and some neurologists have speculated that this may have in some way contributed to his violent acts

evaluation research into the role of the prefrontal cortex in human aggression summary

• Phineas Gage

• neuroimaging

evaluation research into the role of the prefrontal cortex in human aggression - phineas gage

The case study of Phineas Gage supports the role of the PFC in aggression. Phineas suffered severe brain injury after a metal rod was launched through his skull as part of working on a railroad. While he survived the injury, his personality changed notably after his accident, and he had issues with impulse control. He became more verbally aggressive, swearing an acting inappropriately in social situations. This shows how losing the PFC impacted on Phineas ability to control the impulses generated by his amygdala. 

evaluation research into the role of the prefrontal cortex in human aggression - neuroimaging

Neuroimaging studies have consistently found reductions in prefrontal brain tissue in people with anti-social and aggressive tendencies. Particular subgroups of aggressive individuals have been found to have impaired functioning of the prefrontal cortex. For example, Raine et al (1997) investigated the brain activity of 41 murderers using positron emission tomography (or PET) scans. PET scans are used to assess brain activity by measuring glucose uptake by brain cells. Glucose is used as energy by brain cells so the more glucose they take up (or metabolise) the more active they are. They found reduced glucose metabolism in the prefrontal cortex. This suggests therefore that damage to the PFC may lead to increased aggression.  

evaluation research into the role of serotonin in human aggression

Duke et al (2013) provided support for the serotonin deficiency hypothesis. They conducted a meta analysis of 175 studies involving 6500 Ps. They found a negative correlation between serotonin levels and aggression. The lower the serotonin level, the more aggressive an individual was found to be.  

However, they also found that the strength of the relationship varied depending on the method used to assess aggression. Self-report aggression gave different results compared to other people reporting the aggression. Therefore, research into the role of serotonin in aggression lacks validity

general evaluation of the limbic system summary

• socially sensitive

• focuses on nature and ignores nurture

general evaluation of the limbic system - socially sensitive

A criticism is that this research suggests people with damage to amygdalae & PFC will be violent at they are not able to control their aggressive urges. This suggests there is no free will due to biological determination of behaviour. This research is therefore socially sensitive because we have to ask if people with brain damage are responsible for their violent acts. If violent criminals have faulty brain circuitry that affects their moral reasoning, then should they be held responsible for their criminal behaviour? Furthermore, it raises implications for the criminal justice system E.g. should they receive a lighter/harsher punishment. But we must be aware of the limitations of the research before changing the law e.g. much research is based on case studies/correlational data.  

general evaluation of the limbic system - focuses on nature and ignores nurture

A criticism of this research is that it reduces aggression to the functioning of brain structures and therefore focuses on nature & ignores nurture. For example, it has been found that situational factors & social learning is likely to be important in aggression. Therefore, aggression is likely to be due to combination of biological vulnerability & environmental trigger (diathesis-stress).