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Extremely violent behaviour due to low serotonin levels

06 October 2009

Not everyone copes with social conflicts in the same way. Most people, for example, do not choose violence. They talk, avoid someone or quarrel without having to resort to a physical fight. However, a small group of people display extremely violent behaviour in such situations. This is something that has a major impact on our society. Doretta Caramaschi has discovered, with the help of laboratory experiments with mice, that too great an inhibition of serotonin release in the brain plays a major role in aggressive behaviour. Caramaschi: ‘This is an important discovery with an eye to the possible treatment of extremely violent people, or preventing them becoming violent.’ Caramaschi will be awarded a PhD by the University of Groningen on 9 October 2009.

Caramaschi: ‘Worldwide, interpersonal violence is one of the most important causes of death of people aged between 15 and 44. Disproportional aggression is thus a serious problem. My research was designed to reveal the mechanism in the brain that lies behind extreme violence. What makes someone an aggressive person and what triggers aggression to turn into violence? Once we know exactly how that works, we may be able to intervene – and maybe even prevent extreme violence.’


‘Studies with human subjects in this field are difficult’, admits Caramaschi. ‘It goes without saying that we can’t trigger physically violent conflicts simply for the sake of an experiment.' That’s why she conducted her laboratory research with wild mice.
‘The similarities between mice and men are massive when we’re dealing with aggression’, says Caramaschi, ‘not only in terms of genetic predisposition, such as self-control, inhibitions and social skills, but also the inclination towards routine formation and reduced adaptability.’

Aggressive mice

Caramaschi divided the mice up into different groups. ‘Mice with normal levels of aggression use highly controlled forms of aggression only when marking their territory and to protect it against intruders. However, they never go so far as to wound or mortally bite the intruders.’ This does happen with a group of mice genetically selected for their high levels of aggression. With them, functional aggression usually escalated into violence. ‘They no longer reacted to the subordination signals of their opponents, they even attacked drugged mice and even females and young mice.’

Malfunctioning nervous system

Caramaschi investigated the physical characteristics that are involved in extreme aggression. ‘In a stressful situation, all kinds of organ systems are activated by your nervous system – for example your heart rate increases and your blood pressure rises. In extremely aggressive mice, this system is clearly not functioning properly. Among other things, their heart rate remains low, even in an aggressive confrontation.’ Previous research has revealed that the same applies to violent criminals. ‘They remain “cold-blooded” and feel nothing. It is very probable that this can be attributed to a malfunctioning emotional brain circuit, just as in mice.’


Previous research has revealed that the neurotransmitter serotonin plays an important role in violent behaviour. Very aggressive animals turn out to differ from non-aggressive animals in that they have a much stronger inhibition of serotonin release via the serotonin 1A autoreceptors. ‘Discovering what is behind this stronger inhibition is an important starting point for future research,’ thinks Caramaschi. ‘There are various indications that children become more aggressive due to a combination of genes involved in the serotonin system and upbringing conditions in early childhood. In addition, a lot of medication, such as antidepressants and drugs like Ecstasy, can influence serotonin levels. The consequences for the development of violent behaviour definitely need to be researched further.’

Curriculum vitae

Doretta Caramaschi (Italy, 1979) studied biology at the University of Parma and conducted her PhD research at the Department of Behavioural Physiology of the Faculty of Mathematics and Natural Sciences of the University of Groningen. Her supervisors were Prof. Jaap Koolhaas and Dr Sietse de Boer. Her thesis is entitled The physiology of aggression: towards understanding violence. Caramaschi is currently a researcher at McGill University in Canada.

Note for the press

For more information: Doretta Caramaschi, Canada, e-mail:
Further information, if necessary, is available from: Dr Sietse de Boer, University of Groningen, tel. (050) 363 2355, e-mail: 

Last modified:15 September 2017 3.27 p.m.
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