If you've ever felt unusually irritated after witnessing a heated argument even if it wasn't directed at you, science may have an explanation. Aggression can spread socially, not just through direct experience, but simply by watching someone you know lash out.

Exploring this issue scientifically, the findings of a recent study by researchers at Southern University of Illinois (SUI) School of Medicine suggest that in male mice, this vicarious aggression turns out to be wired into the brain.

The new research provides some evidence for the idea that simply watching someone you know behave aggressively can make you more likely to act out later. In mice it all comes down to a small part of the brain known as the amygdala.

Vicarious aggression turns out to be wired into the brains of mice and can ripple through communities.

Researchers set out to understand how aggression might be socially transmitted. Their test subjects? Male lab mice.

Using a carefully designed behavioral test, neuroscientist Jacob Nordman and his colleagues observed what happened when male mice watched other mice behave aggressively. The observer mice were separated into two groups: those who saw familiar peers (cage mates) attack an intruder mouse, and those who watched unfamiliar stranger mice doing the same.

What the researchers found was striking. Only the mice that witnessed peers they were familiar with attacking others became more aggressive themselves later. Those that watched strangers didn't seem affected.

The team then looked in on the brain to understand why.

As the mice watched these attacks, researchers recorded brain activity in a region of the amygdala, a part of our brain long linked to emotions like fear and aggression. They identified specific neurons that lit up only when the observer mice saw familiar peers being violent.

These neurons weren't just spectators in the process, they were instigators. When researchers inhibited the activity of the neurons that were activated by viewing aggression, the learned aggression disappeared. On the flip side, when they activated the neurons in mice watching strangers attack, those mice started acting more aggressively — just as if they'd witnessed familiar peers instead.

“We previously found that these neurons are involved in an ‘aggression priming' effect, meaning that being a perpetrator of an attack increases the likelihood of attacking again,” Nordman, an Assistant Professor of Neuroscience at Southern University of Illinois (SIU) School of Medicine, explained in a media release. “For example, imagine getting in an argument with a coworker or family member. Afterwards, your agitation and frustration make you more likely to have another outburst.”

The neurons weren't just spectators in the process — they were instigators.

The same neurons may be firing in observers because they're emotionally “mirroring” their peers' responses.

One of the most intriguing findings from the research is that the aggression was contagious only when the observer knew the attacker. In humans, this suggests a possible explanation for why aggression may seem more contagious in certain environments such as among close-knit peer groups, siblings or within tightly bonded social circles.

Like mice, humans are more likely to take emotional cues from friends and family.

While the study was conducted in mice, it offers a powerful window into the way social context and brain chemistry may combine to spread violent behavior. It may also help explain why children who grow up in aggressive households, or teens surrounded by hostile peers, may themselves become more prone to acting out.

Most importantly, the findings open the door to targeted interventions, both behavioral and neurological. By identifying the brain circuits involved, researchers could potentially develop treatments to prevent the cycle of learned aggression.

By uncovering the brain's role in aggression-priming, especially the role of familiarity, researchers hope to one day develop therapies that help those at risk break free from violence — whether it's on the streets, at home or in our schools.

The study is published in the Journal of Neuroscience.