Is aggressive behavior contagious?

By Stephen Beech

Aggression can be contagious, suggests new research.

People who repeatedly observe aggressive behavior have a higher likelihood of engaging in violent behavior themselves, scientists say.

Researchers at Southern University of Illinois School of Medicine in the United States used mice to explore the environmental factors and neural mechanisms that lead to the aggression that witnesses later acquire.

In an experiment created by the research team, mice observed known peers or unfamiliar strangers attack intruder mice.

Only male witnesses later displayed increased aggression themselves, and that happened only after watching familiar peers attack intruders.

The researchers recorded activity from neurons in a part of the amygdala of the mice that is implicated in aggression priming.

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Study leader Dr. Jacob Nordman said: “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.

"For example, imagine getting in an argument with a coworker or family member.

"Afterward, your agitation and frustration make you more likely to have another outburst.”

The researchers theorized that the neurons might be active in male witnesses observing violent peers because the "familiarity" makes them mirror their friends’ own aggression priming.

The neurons were active in males as they saw familiar - but not unfamiliar - attacks during the experiment.

Dr. Nordman explained that artificially inhibiting those neurons suppressed later aggression after witnessing peers, and activating the neurons while males watched violent strangers promoted attacking behavior in observers later.

He says the findings, published in the journal JNeurosci, shed light on aggression learned via observation, suggesting that not only proximity, but also familiarity of attackers may be risk factors for behaving violently later, at least in males.

Dr. Nordman added: "This neural mechanism could inform the development of neural and behavioral treatment interventions for learned violence."