If you have spent any time on social media, you have a different kind of bystander effect in action. Psychologists say if many people are around, the bystander effect is why everyone is less likely to help. They believe someone else will be more competent or know something you don't. If you walk by a person laying unconscious in New York City, based on experience they did not have a heart attack and are in peril. They are on drugs or alcohol.

On the opposite end, there is an aggression bystander effect, where if your exposure to aggression is frequent you are more likely to behave that way also. A new paper sought to see how it happened in mice. Because it is mice, it is only EXPLORATORY, like claims about the harms of ultraprocessed foods or the benefits of quinoa. Mice are not little people.

In the experiment, mice observed mice they knew or unfamiliar strangers attack intruder mice. Only male witnesses later displayed increased aggression themselves, and this happened only after watching familiar peers attack intruders. The researchers recorded activity from neurons in a part of the amygdala that is implicated in aggression priming.

The researchers speculate that these 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. Notably, artificially inhibiting these neurons suppressed later aggression after witnessing peers, and activating these neurons while males watched violent strangers promoted attacking behavior in observers later. 


Image: Storyblocks

“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,"  says Dr. Jacob Nordman from Southern University of Illinois School of Medicine. "Afterwards, your agitation and frustration make you more likely to have another outburst.”

These findings 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. According to the authors, this neural mechanism could inform the development of neural and behavioral treatment interventions for learned violence.