Suzana Herculano-Houzel

Brain and violence: where genetics and society meet


Some place all the blame on society, which neglects them and doesn’t give them opportunities. There are those who disagree and argue that the problem is essentially bad genes, “bad blood”. What causes the violent behavior of these individuals who bring fear and suffering to so many families? Neuroscience has another opinion: an unfortunate combination of genetic predisposition and abuse, which modifies the way the brain responds to threats.

Both nature and society alter behavior, and today we understand why: both affect the brain. On the one hand, each person’s genetics affect both the structure of the brain and its chemistry, and for example the hereditary excess of serotonin in the brain regions that respond to threats is associated with uncontrollable and exaggerated, violent responses. On the other hand, stress, abuse and physical and moral violence in childhood, the antithesis of affection and all that it represents and changes in the nervous system, also leave their mark on the same alarm and threat response brain systems and, for the same reasons, encourage outbursts of unbridled violence.

In evolutionary terms, it makes sense. If you live in a dog-eat-dog world, where the norm is to bully, shout and hit, then bullying, shouting and hitting back becomes a reasonable strategy, implemented directly and automatically by the brain – that is, without anyone having to choose to be violent. Interestingly, the opposite strategy, of becoming apathetic and invisible, is also “interesting”, and perhaps explains why depression is such a common response to social abuse.

However, neither genetics nor society are 100% determinant. Genetic risk manifests itself as violence or depression if it occurs alongside a history of abuse and violence. Likewise, abuse leads to violence and depression, especially in those individuals with a genetic risk.

And therein lies the biggest problem. Victims of an unfortunate conjunction, these people will not only pass on their genetic risk to their children, but they will also be very likely to treat them with violence, creating the same unfavorable conjunction for them, which thus propagates itself. Violence begets violence.

But this is not inevitable. Social support, peace and affection act on the brain and break the vicious circle – even if genetics are against it. The combination is explosive, but genetics and society alone are no guarantee of anything. Just as well.

Originally published in Folha de São Paulo on June 29th, 2006.

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