Levels of neurotransmitters in the brain can influence behaviour, so it’s plausible to think that in the criminal brain there may be some abnormalities in neurotransmitter levels.
Numerous research studies have shown that violent criminals do in fact tend to have low levels of serotonin (e.g. Moi and Jessel, 1995; Scerbo and Raine, 1993). Studies have also shown that serotonin is associated with controlling impulsive behaviour (Krakowski, 2003)
One method of investigating biological correlates of behaviour is to use animals in the laboratory, like rats and mice. There have been numerous studies that show a correlation between serotonin levels in rats and aggression. That is to say, low levels of serotonin are correlated with high levels of aggression. (Chiara, et al, 1971) But because of the complex nature of the serotonin system in the brain and the difficulty of manipulating aggression in a lab, it’s been difficult to explain how serotonin affects violence.
With modern technology, researchers can now investigate the relationship between areas of the brain and neurotransmitter levels in ways they couldn’t before. The following study provides one possible answer for explaining the relationship between neurotransmission (serotonin levels) and violent behaviour.
Passamonti et al, 2012 (Full Study Link)
Healthy volunteer samples serotonin levels were manipulated by altering their diet. A repeated measures design was used where on one day they were given a diet that lacked tryptophan, which is an important amino acid that helps build serotonin. A lack of tryptophan in the diet will reduce levels of serotonin available in the brain. In the control condition they were given a placebo diet, which was the same mixture but had normal amounts of tryptophan.
The participants were put in fMRIs and their brain activity was measured while they were seeing images of happy, angry or neutral faces. The researchers could see the activation of different areas of the brain as the different faces were shown.
The results showed that there was reduced activity in the frontal lobe during the low serotonin conditions when the participant was viewing the angry face. Moreover, communication between the amygdala and the frontal lobe was weaker in this condition.
By applying what we know about the frontal lobe and its role in regulating impulsive behaviour, as well as the amygdala in emotion and the stress response, this provides plausible clues as to why low serotonin might lead to acts of aggression or violence. If an individual is threatened and they have low levels of serotonin, they may not be able to perform top-down control; that is to say, the lack of activity in the PFC may affect their ability to regulate the stress response as triggered by the amygala’s reactivity towards the threat. This might increase their emotional level and increase chances of a highly emotion reaction to the threat. The reduced activity in their PFC as a result of the low serotonin may also affect their ability to inhibit impulsive reactions and think through their actions, so if someone has a tendency towards violence they may not be able to reduce an impulsive reaction towards an individual who is threatening them.
This study might not make heaps of sense to you unless you have first learned about the role of the PFC in impulse control and the amygdala in emotion, social threat and the stress response. In the themantic unit on Criminology, these concepts are covered first before going into the role of serotonin on aggression and violence.
- How might levels of serotonin influence behaviour?
Dr Molly Crockett was a co-author of this study and you can watch her TED Talk below. In this talk she addresses the requirement for people to beware of the media’s tendency to oversimplify scientific findings.[youtube https://www.youtube.com/watch?v=b64qvG2Jgro]
Travis Dixon is an IB Psychology teacher, author, workshop leader, examiner and IA moderator.