Implicit biases change more slowly in larger cities.

Implicit biases remain a contributor to discrimination despite widespread endorsement of equality. Although there has been a large-scale reduction in sexuality, race, and skin tone biases in the past two decades, there is a current shortage of quantitative theories that describe how and why biases change. Here, I present a theory of implicit bias change, which focuses on changes in exposure due to cultural information and how the speed of these changes varies geographically. This model makes counterintuitive predictions when compared with previous findings, including 1) that biases change more slowly in larger cities and 2) that one-off interventions are less effective in larger cities. I confirm these predictions using 14 million tests of implicit biases from U.S. residents over 14 y and experimental data from 1,150 individuals. Documenting the typical timescales and patterns that govern implicit bias change is an important step toward designing interventions that can facilitate further reductions in biases and prevent biases from increasing.