Brain

From road rage and bar fights to terror attacks and global confrontations, humans tend to be an aggressive species. On the average, members of the same species cause only 0.3 percent of deaths among mammals [1]. Astoundingly, in Homo sapiens the rate is around 2% (1 in 50), nearly 7 times higher! There are two crucial aspects that favor this kind of behavior: dwelling in social groups and being ferociously territorial. The chances are that struggle for various resources like suitable habitat, mates and food played a key role in shaping aggression in humans, favoring genetic variants that promote aggression and therefore increase changes of survival. Indeed, anthropologists who lived with exceptionally violent hunter-gatherers found that men who committed acts of homicide had more children, as they were more likely to survive and have more offspring [2]. This lethal legacy may be the reason we are here today.

You probably know some people that could be characterized as “having a short fuse”. Perhaps you have even pondered why they seem to have such a hard time to keep their temper in check? Indeed – while scientists have known for decades that aggression is hereditary, there is another crucial component to those angry flare-ups: self-control. In humans, the impulses to react violently stem from the ancient structures located deep within the brain. The part capable of controlling those impulses is evolutionally much younger and located just behind the forehead – the frontal lobes. Unfortunately, this “top-down” conscious control of aggressive impulses is slower to act compared to the circuits of eruptive violence deep in the brain.

People who are genetically predisposed toward aggression actually usually behave more violently than the average only when provoked. People not genetically susceptible to violent outbursts seem to be better able to remain calm and “brush it off”. The ones who are predisposed in fact try hard to control their anger, but have inefficient functioning in brain regions that control emotions – in the frontal lobes [2]. Several studies have found that men genetically susceptible to act aggressively are especially likely to engage in violence and other antisocial behavior if they were exposed to childhood abuse [3]. Again, we see that although genes may carry certain predispositions, there are essential environmental aspects that determine the final outcome.

Early physical aggression needs to be dealt with care. Long-term studies of physical aggression clearly indicate that most children, adolescent and even adults eventually learn to use alternatives to physical aggression [4]. Still, the importance of proper guidance and favorable environment cannot be understated. As mentioned before, Homo sapiens have been found to cause 2 percent of deaths among their fellows. However, this has fluctuated substantially throughout the history and in different cultures. During the medieval period, human-on-human violence was responsible for stunning 12 percent of recorded deaths. For the last century, people have been relatively peaceable compared to the Middle Ages, violence being the cause of death in just 1.33 percent of fatalities worldwide. In the least violent parts of the world today, the homicide rates are as low as 0.01 percent [1].

Our brains have evolved to monitor for danger and spark aggression in response to any perceived hazard as a defense mechanism. Aggression is part of the normal behavioral repertoire of most, if not all, species; however, when expressed in humans in the wrong context, aggression leads to social maladjustment and crime [5]. By identifying genes and brain mechanisms that predispose people to the risk of being violent – even if the risk is small – we may eventually be able to tailor prevention programs to those who need them most.

References

[1] Gómez, J. M., Verdú, M., González-Megías, A., Méndez, M. (2016). The phylogenetic roots of human lethal violence. Nature 538(7624), 233–237.

[2] Denson, T. F., Dobson-Stone, C., Ronay, R., von Hippel, W., Schira, M. M. (2014). A functional polymorphism of the MAOA gene is associated with neural responses to induced anger control. J Cogn Neurosci 26(7), 1418–1427.

[3] Cicchetti, D., Rogosch, F. A., Thibodeau, E. L. (2014). The effects of child maltreatment on early signs of antisocial behavior: Genetic moderation by Tryptophan Hydroxylase, Serotonin Transporter, and Monoamine Oxidase-A-Genes. Dev Psychopathol 24(3), 907–928.

[4] Lacourse, E., Boivin, M., Brendgen, M., Petitclerc, A., Girard, A., Vitaro, F., Paquin, S., Ouellet-Morin, I., Dionne, G., Tremblay, R. E. (2014). A longitudinal twin study of physical aggression during early childhood: Evidence for a developmentally dynamic genome. Psychol Med 44(12):2617–2627.

[5] Asherson, P., Cormand, B. (2016). The genetics of aggression: Where are we now? Am J Med Genet B Neuropsychiatr Genet 171(5), 559–561.

About the author:

Mariliis Vaht, PhD

Research Fellow of Neuropsychopharmacology at Institute of Psychology, University of Tartu, Estonia. Area of research: genetic and environmental factors in longitudinal health study designs.