Every human has implicit biases, or involuntary associations, about members of different social groups. But these biases are not fixed. Rather, they’re malleable reflections of our everyday experiences.
It’s harder to change the biases of people living in big cities, with millions of people, however, than it is to change the biases of people living in small cities, with around 150,000 residents, a new study shows. This is “the first strong proof of how contextual biases are,” says Andrew Stier, a Complexity Postdoctoral Fellow at the Santa Fe Institute.
Stier analyzed the results of an online test measuring biases for millions of study participants from cities across the United States. He also gave 1,150 study participants from cities across the United States the same test. After, they read a short story in which a member of a group they were biased against (or an “outgroup”) helped them out of a stressful situation, like facing a terminal illness or homelessness. Then they took the test again.
While the story reduced participants’ biases in every city, regardless of population size, the length of the impact varied. Stier asked participants to re-take the implicit bias test in the days following the intervention. In small cities, the intervention lasted until the end of the study, nine days later. In large cities, on the other hand, the effect faded within twenty-four hours.
Stier attributes this difference to two qualities of big city life: First, residents are likely to interact with outgroup members frequently, which means one additional experience doesn’t necessarily make a big difference. In small cities, on the other hand, the intervention provided in the study might be the first experience a study participant has ever had with a member of the outgroup, so it could shift their opinion quite a lot.
Second, big-city dwellers have a constant stream of information about other groups coming at them in the forms of interpersonal interactions, advertisements, TVs playing in restaurants, people listening to music, “let alone what we see on our phones,” Stier says. Small cities, on the other hand, tend to be quieter. So it makes sense that it will take a heftier intervention to shift the biases of people in big cities. In a big city, the intervention will get diluted by all the other information people get in a day.
Companies use implicit bias interventions to avoid making biased hiring decisions. For this approach to be successful, they should consider the size of the city they’re based in, Stier says. Big-city companies might be wise to host the intervention directly before making the decision because the effect will wane quickly. “That’s a pretty big reframing of how we think about the design of these interventions,” he says.
This material is based upon work partially supported by the National Science Foundation under grant 2438783.
Read the paper "Implicit biases change more slowly in larger cities" in PNAS (August 6, 2026). DOI: 10.1073/pnas.2514189123