The amygdala as an explanation for aggressive behaviour

The amygdala as an explanation for aggressive behaviour As mentioned earlier, Charles Whitman, who was responsible for killing around 16 people, was subsequently, after his death, found to have a brain tumour affecting the amygdala. It was not shown that the tumour accounted for his behaviour, but it could have done. The amygdala is involved in fear and aggression, as well as social interactions, and it is thought to be involved in crime. The role of the amygdala in criminal and anti-social behaviour is covered in this section

Yang et al. (2009): link between smaller volume of the amygdala and anti-social behaviour

Yang et al. (2009) looked at 27 psychopathic people and 32 controls and used structural MRI to measure amygdala differences. 

The aim of Yang et al. (2009) was to look for regions in the amygdala that are problematic in psychopaths. The volume of the amygdala of each person was measured. Yang et al. (2009) found that people with psychopathy had volume reductions on both amygdalae (bilateral) compared with the controls. They had less volume and deformations in the surface of the amygdala. There were significant correlations between the reduced volumes and the psychopathy scores and the strongest correlations related to emotions and interpersonal abilities and the amygdala volume. Yang et al. (2009) point out that psychopathy relates to manipulative personality and anti-social behaviour, such as poor behavioural control, so their findings support the idea that the amygdala is involved in criminal or anti-social behaviour or in the control of such behaviour, as a smaller volume in the amygdala seemed to mean more anti-social behaviour and less control.

An important issue discussed by Yang et al. is the link between the amygdala and conditioning, including conditioning related to fear. If there is an impairment in the amygdala this might affect conditioning related to fear and as psychopathy can link to lack of fear Yang et al.’s findings make sense. The amygdala also relates to rewards and moral reasoning so again if there is amygdala impairment that links to psychopathic behaviour

Definition Psychopathy is defined as ‘a personality disorder characterised by a severe affective deficit, lack of respect for the rights of others and the norms of society’ (Woodworth and Porter, 2007, cited in Mossière and Dalby, 2008).

Pardini et al. (2013): lower amygdala volume and links to violence and aggression 

Pardini et al. (2013) carried out a study using scanning. Adrian Raine was a member of the team. They found that men with lower volumes of the amygdala were a lot more likely to show aggressive and violent behaviour, as well as psychopathic traits showing up three years later, than a control group of men with normal sized amygdala. This finding supports the finding of Yang et al. (2009). This was a longitudinal study and 56 men with histories of violence were recruited. They looked at violence and psychopathic traits in the three-year follow-up and also looked at the association between level of aggression and psychopathic tendencies of the participants in childhood and adolescence. As the researchers predicted more aggression and psychopathic traits after three years, this suggested to them that the volume of the amygdala might be a biomarker for ‘severe and persistent aggression’.

Pardini et al. (2013) use a longitudinal design because they follow their participants and take data again after three years to look at their behaviour in terms of violence and aggression. You can use the study as an example of a longitudinal design. Yang et al. (2009) used a cross-sectional design.  

Study Participants  Methods Results

Elst et al. (2000) 48 patients – 24 with temporal lobe epilepsy and 24 matched health controls MRI (magnetic resonance imaging) 44% of patients who were aggressive showed problems with the amygdala.

Amen et al. (1996) 40 psychiatric patients showing aggression and 40 matched controls who were not aggressive SPECT (single-photon emission computed tomography) Less activity in the prefrontal cortex in aggressive patients as well as more activity in left basal ganglia, left temporal lobe and anteromedial frontal cortex.

Volkow and Tancredi (1987) Four psychiatric violent patients and four normal matched controls PET (positron emission tomography) 50% of the violent patients had reduced functioning in the frontal lobes and there were left temporal lobe problems in all the violent patients.

Raine et al. (1997) 41 murderers pleading NGRI (not guilty by reason of insanity) and 41 matched controls PET Less activity in the lateral prefrontal cortex of the murderers as well as the medial prefrontal cortex and the posterior parietal cortex. Murderers also showed less activity in the corpus callosum and reductions in the left amygdala, thalamus and medial temporal lobes. 

STUDY HINT Raine et al.’s (1997) study was in Bufkin and Luttrell’s review and you can use it when discussing the amygdala and brain regions relating to aggression. 

Tonkonogy (1991) 14 violent and 9 non-violent patients with organic mental disorders MRI Just 1 of the 14 patients showed orbitofrontal lesions. Violent patients were more likely to have less tissue in the temporal lobe, particularly in the amygda

Evaluation points relating to the amygdala explaining anti-social behaviour

Though Bufkin and Luttrell (2005) list studies that include the amygdala or the temporal lobe as a region affected in people that show violence and aggression, their list also shows studies that mention frontal lobe functioning, which relates to brain injury as an explanation of criminal and anti-social behaviour. It seems that there is evidence for the amygdala being involved in psychopathy, and related to that behaviour can be anti-social or criminal behaviour. However, other areas of the brain can be damaged or dysfunctional and can lead to aggression, violence or anti-social behaviour. Possibly there is not one single explanation for criminal and anti-social behaviour. The observed behaviour might be explained in different people in different ways.

When discussing these studies, you can use evaluation points about the use of neuroimaging/brain scanning as a research method, since a number of these studies adopt that method. 

Gender and the amygdala as an explanation for criminal and anti-social behaviour

It is not generally emphasised that male and female brains differ with regard to the functions of brain regions like the amygdala. Blakemore and Choudhury (2006) looked at white and grey matter differences between children aged 11 years and found some differences. However, they suggested that differences in male and female brains with regard to the frontal cortex needed to be studied further, and they did not mention specific regions such as the amygdala. Studies looking at the role of the amygdala in anti-social and aggressive behaviour seem not to highlight gender differences. However, it might just be that gender has not featured as a variable of study.

Shirtcliff et al. (2009): gender differences in neurobiology relating to amygdala functioning

Shirtcliff et al. (2009) reviewed the neurobiology of empathy and callousness and considered implications regarding gender for anti-social behaviour. Although their review does not emphasise gender differences, they do include a passage on gender differences towards the end of the paper. They say that there are differences between the genders, citing Zahn-Waxler et al. (2005). They state that girls show more empathy than boys (Zahn-Waxler, 2000, cited in Shirtcliff et al., 2009) and there are more instances in conduct disorder and anti-social behaviour in boys than in girls (e.g. Moffitt, 1993a, cited in Shirtcliff et al., 2009). Cale and Lilenfeld (2002, cited in Shirtcliff et al., 2009) show that gender differences carry on into adulthood and relate to psychopathy. Importantly, with regard to biological explanations of crime and anti-social behaviour, they point out that the neurobiology involved in empathy and callousness is different in males and females, with more empathy-related neurocircuitry being active in females (e.g. Hein and Singer, 2008, cited in Shirtcliff et al., 2009). They also show that stress response is different according to gender (e.g. Wang et al., 2007, cited in Shirtcliff et al., 2009). They conclude that ‘the neurobiology of anti-social behaviour may be fundamentally different in males and females’. They do not mention the amygdala specifically; however, they do earlier state that the amygdala is critical for emotion responses and might be the most important limbic area with reference to emotion, so there may indeed be gender differences in the amygdala which can help to explain crime and anti-social behaviour.

Hyde et al. (2014): the amygdala, emotions, antisocial personality and psychopathy

Hyde et al. (2014) used both men and women in their study looking at anti-social personality disorder and psychopathy, relating these to negative emotionality and amygdala reactivity to perceived threat and to distress. They found higher scores for psychopathy to be associated with lower negative emotionality and lower amygdala activity. They found higher anti-social personality scores associated with higher negative emotionality and higher amygdala activity. This suggests that the amygdala has a role in anti-social behaviour, at least in some way. What is interesting is that, although men and women were used in the sample, gender was not used as a variable in the findings. This suggests that it might not be that there are no gender differences in amygdala functioning and negative emotionality, but that gender is not a variable of interest for some researchers.

Summary of gender differences in the amygdala affecting crime and anti-social behaviour

In general, it seems that there are no gender differences in amygdala functioning and neurobiology related to anti-social behaviour. Blakemore and Choudhury (2006) suggest that there are not many differences but more research is needed. Hyde et al. (2014) did not report gender differences in their study, but this does not mean that there weren’t any, just that they did not look at them. Shirtcliff et al. (2009) give gender differences such as in empathy (more in girls) and conduct disorders (more in boys). They confirm that the amygdala is important in emotions and so it can be assumed that differences in emotionality between the genders might come from differences in the amygdala. Shirtcliff et al. (2009) give some evidence for biological differences in brain functioning between males and females when they point out that the neurobiology involved in relation to empathy and callousness is different in males and females, with more empathy-related neurocircuitry active in females (e.g. Hein and Singer, 2008, cited in Shirtcliff et al., 2009). 

Evaluation for the amygdala explanation for criminal behaviour and/or anti-social behaviour

Strengths

● Just some of the evidence is presented here. However, it is clear that there is a lot of evidence relating to amygdala size and possibly functioning to suggest that lower volume or less activity relates to problems with moral reasoning and control over behaviour, as well as problems in learning fear responses to stimuli. Though the focus tends to be on psychopathy, psychopathy relates to aggression and anti-social behaviour so the evidence for the amygdala as an explanation for criminal and/or anti-social behaviour is strong

. ● Studies tend to use neuroimaging, which is a research method that can show reliable findings because more than one person can analyse the scan pictures. There is validity in what is being measured too, if the amygdala is highlighted and then measured using the scan pictures.

Weaknesses

● Scanning is seen as scientific, objective, reliable and, to an extent valid, which are strengths. However, when relating what is found in the scans to behaviour, it is harder to achieve those qualities. For example, psychopathic tendencies are hard to relate to crime and violence directly and there are many factors that relate to crime and violence, including age, gender and early life experiences. A scan might show reduced amygdala volume, but relating this to aggression and criminal behaviour does not have the same objectivity and reliability.

● As some studies show, it is not just the amygdala that is implicated in aggression. Studies have shown links between aggression and prefrontal lobe functioning, forexample. Also some studies use the size of the amygdala and others the level of activity, so it is not certain what it is about the amygdala or how its relationship to other brain areas relates to criminal and anti-social behaviour.