Does one person make a difference? Many–one effects in judgments of prosocial action.

With prosocial action, there are both benefits to society and personal costs to those who act. We document a "many-one effect" in how people weigh these costs and benefits when assessing others' actions. For example, people might believe the expected air-quality benefits outweigh the personal inconvenience to 10,000 people walking to work instead of driving, but not one person. More generally, people more often judge the combined prosocial benefits to outweigh the combined personal costs when there are many actors than when there is just one (or a few). We find support for this hypothesis across 13 experiments (N = 8,402), including samples of adults and children, individualistic and collectivistic sample populations, elected policymakers, and lawyers and judges, in both between-subjects and within-subjects designs and in both judgments of hypothetical scenarios and real decisions about how other participants should spend bonus payments. The many-one effect persists in scenarios designed to limit rational justifications, for example, when each additional actor has a smaller marginal impact (Experiments 2A-2E). We propose that this effect arises from "differential scope sensitivity." Compared to personal costs, the societal benefits can more easily be compared to the size of the societal problem (Experiment 2F), which will make people more scope sensitive to them. (PsycInfo Database Record (c) 2026 APA, all rights reserved).