A violent history: how genes and environment influence aggression

Represented by a conscious propensity to harm others against their will, aggressiveness is a complex behavior depending on which environmental conditions we have been living in, and the kind of features we have inherited from our ancestors. Humans tend to be an aggressive species. Among mammals, members of the same species cause only 0.3 percent of deaths of their conspecifics [1]. Astonishingly, in Homo sapiens the rate is nearly 7 times higher, around 2% (1 in 50)! More than 1.3 million people worldwide die each year because of violence in all of its forms (self-directed, interpersonal and collective), accounting for 2.5% of global mortality. There are two critical conditions that endorse aggressive behavior: being fiercely territorial and living in social groups.  

From the evolutionary perspective, aggression is usually described as adaptive. Struggle for resources like habitat, mates and food have had a key role in forming aggressive behavior in humans. Genetic variants that promote aggression have been more likely to be passed on to the next generation because they have increased the chances of survival. Indeed, among tribes of extremely violent hunter-gatherers, 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.

Although there are several biological aspects related to aggression, their predictive value continues to be rather low. It is possible to inherit a predisposition to acting violently, but scientists also emphasize that modeling violence in the home environment is the most certain way of propagating aggressive behavior. Children learn to act violently through the simple observation of aggressive models. The way parents manage the inevitable conflicts that arise between themselves and their children is central to the learning of aggression. When parents are unable to stop the child from escalating the intensity of conflict, and when they at least intermittently reinforce the child’s coercive behavior, the child learns that escalation is a viable method of resolving conflict. When this conflict strategy is applied to interactions with siblings or peers, and if it is also reinforced in these contexts, this conflict escalation is likely to include acts of aggression [3].

In addition to being hereditary and learned through social modelling, there is one other crucial component to aggressive behavior: self-control. In humans, the urge to react aggressively stems from the ancient parts located deep in the brain. The structure capable of controlling those impulses is evolutionally much newer and located just behind the forehead – the frontal lobes. Unfortunately, this “top-down” conscious control of violent impulses is slower to act in contrast with the circuits of eruptive violence deep in the brain. People convicted of murder had been found to have reduced activity in the prefrontal cortex and increased activity in deeper regions [4]. Although there are plenty of examples of people with prefrontal cortex damage who do not commit violent acts, these findings clearly demonstrate that the damage to the prefrontal cortex impairs decision making and increases impulsive behavior.

Early physical aggression needs to be dealt with care. Long-term studies of physical aggression clearly indicate that most children, adolescents and even adults eventually learn to use alternatives to physical violence [5]. 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 [6]. By identifying 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.

/This post is adapted from an earlier blog on MiND the Gap/

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] Hodges, E.V.E., Card, N.A., Isaacs, J. (2003). Learning of Aggression in the Home and the Peer Group. In: Heitmeyer, W., Hagan, J. (eds) International Handbook of Violence Research. Springer, Dordrecht.

[4] Raine, A., Buchsbaum, M., LaCasse, L. (1997). Brain abnormalities in murders indicated by positron emission tomography, Biol Psychiatry 42(6), 495–508.

[5] 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.

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