Selfish and prosocial individuals both distort their actions' impact, but selfish distortions can be more contagious.

People often act without knowing exactly how their actions impact others. Here, across nine preregistered online experiments (N = 8,769 adults, predominantly US residents), we tested whether people exploit this ambiguity to make themselves look good. On average, selfish individuals inflated the social benefit of their choice while downplaying that of the alternative, reporting the two options as more similar than prosocial individuals did (experiment 1a: b = 0.24, 95% confidence interval 0.14-0.34). Prosocial individuals portrayed their choice as comparatively more socially beneficial; they also inflated their implied relative sacrifice, although less consistently. We observed distortions even before people made a choice and when choices were forced, consistent with distortions reflecting social preferences rather than mere justification. While people differed in whether and how they distorted, distortions favouring selfishness showed evidence of contagion: others adopted them and became more selfish (experiment 6, selfish-serving versus no-distortion: b = 0.49, 95% confidence interval 0.10-0.88). These findings offer a psychological foundation for why polarized positions about the social impact of one's actions may emerge. When social consequences are open to interpretation, people may predictably form divergent interpretations.