Whether you’re a lover or a fighter depends on a whole host of factors, ranging from life experiences to genetic makeup. While scientists may not yet be able to explain why some people have a shorter fuse than others, they have finally identified the region of the brain that appears to fuel premeditated acts of aggression.
To be clear, this does not relate to retaliatory violence, whereby a person is provoked to defend themselves or aggravated to the point of attacking another. Rather, it refers to entirely malicious, unprovoked assaults, and as such could explain why some people have a tendency to pick fights or bully other people without any apparent justification.
Publishing a study in the journal Nature Neuroscience, the researchers behind the project explain how they implanted electrodes into the brains of male mice that were then repeatedly given the opportunity to attack a weaker male.
The mice were introduced into the same chamber as one another, but separated by a partition. These barriers contained two small holes through which the stronger males could poke their noses in order to sniff out their adversary. Depending on which hole they chose, the researchers would either remove the barrier or leave it in place, so that over time the mice were able to learn which hole would enable them to attack the weaker mouse.
Once this learning process was completed, some of the mice started showing a clear preference for the "fight" hole, while others did not, enabling the researchers to identify which mice were naturally motivated to fight and which were not.
Observing the aggressive mice, the study authors found that activity in a brain region called the ventromedial hypothalamus (VMHvl) increased while they were deciding which hole to choose, ultimately prompting them to seek out the "fight" hole. As such, the researchers suspected that the VMHvl may play a key role in motivating unprovoked acts of aggression.
Increased neuronal activity in the ventromedial hypothalamus appears to be a precursor to violent behavior in mice. Romanova Natali/Shutterstock
To test this theory, they decided to examine how the activity of this brain region was affected by the extinction of the learned association between a particular nose-poke hole and the chance to attack a weaker adversary. Accordingly, they stopped allowing the mice to fight, regardless of which hole they chose, until this conditioned association was lost.
Once this was achieved, the previously observed increased VMHvl activity during the hole-choosing period was extinguished, adding weight to the study authors’ hypothesis.
They then used mice that had been engineered to carry certain light-activated genes that could control the way neurons in the VMHvl fired. Using a flashing light, the researchers were able to either stimulate or inhibit these neurons, and found that stimulation caused an increased propensity to fight, while inhibition had the opposite effect.
These findings appear to indicate that the VMHvl does indeed play a major role in mediating unprovoked acts of violence, although exactly how the activity of this brain region is controlled remains a question for future research. For instance, the authors suggest that inputs to the VMHvl from other brain areas involved in linking aggression to reward – such as the nucleus accumbens – may be enhanced in overly aggressive mice. Alternatively, since the VMHvl contains a high concentration of androgen receptors, it is possible that this effect may be caused by high testosterone levels.
Commenting on these findings, lead researcher Dayu Lin explained that the information provided by the study could help to guide future efforts to understand aggressive impulses. “The ventrolateral part of the ventromedial hypothalamus should be studied further as part of future efforts seeking to correct behaviors from bullying to sexual predation,” says Lin.