Rule breaking by just a few people can be contagious

On the long drive to my kid’s sporting event this weekend, most drivers, myself included, cruised a few miles over the speed limit. A few drivers, though, blazed past in the left lane. Mindful of safety — and police cruisers — I didn’t follow suit.

Left unchecked though, a few bad actors can trigger social breakdown, researchers report October 21 in Proceedings of the Royal Society B. The truly deviant among us “basically take off the guardrails of the group,” says Lucas Molleman, a psychologist at the University of Amsterdam.

Molleman and his colleagues designed a thought experiment to show what it takes to sway people to bend or break the rules. They picked a contentious time: A week before the 2024 U.S. presidential election. The team recruited 750 U.S. participants, with roughly half identifying as Democrats and half as Republicans. The online participants watched an animated shooting star streak across a row of 100 squares and then selected which square shone the brightest.

Though the brightest square always appeared around the row’s middle, participants received more money for selecting squares farther to the right (the farthest-right square netted participants roughly the equivalent of $1.30). Dedicated rule followers, or those who always followed the rules, and rule breakers, existed in roughly equal numbers, with each comprising about a fifth of the larger group, initial tests showed.

The researchers then stirred the pot. Participants repeated the simple experiment several times while receiving information about a previous respondent, including their political affiliation and level of adherence to the rule. Rule followers rarely begot more rule followers, even when respondents belonged to the same political party, the team found.

The reverse held true for rule breakers, though. Receiving information that someone with the same political leanings had just disregarded the rules shifted selections rightward on average. Absent social cues, participants selected the farthest-right square roughly 23 percent of the time. That increased to almost 40 percent of the time after participants learned that a political comrade had made the most lucrative choice. Bad behavior, in other words, was contagious. 

Most individuals’ rule-breaking levels mirrored or stayed below that of others. But just over 4 percent of individuals escalated deviant behavior beyond that of the previous respondent.

The researchers then created a computer simulation to see how those rule-breaking behaviors might play out across time. They designed a digital twin for each of the 750 participants and had the doppelgängers interact in groups of 100. In some scenarios, those few escalation-prone individuals could push everyone in the group toward wild rule breaking. But if the 100-twin selection had the same proportion of dedicated rule followers as the human experiment, overall deviance settled at a more intermediate level.

This study provides some keen insights into how humans might behave absent consequences, says behavioral economist Eugen Dimant of the University of Pennsylvania. “But that’s not how the world works.” His work suggests that to truly understand social contagion and the emergence of mobs, researchers must consider societal forces, such as punishments and social norms, that tend to keep behavior in check.

To Molleman, though, the study’s insights into individual behaviors are key. This pared-down experiment shows how just a few bad actors can cause entire groups to spiral out of control. Such findings could underpin everything from collective speeding to peaceful protests that turn violent, he says. “This is fundamental science … Obviously reality is way more complex.”