How do people infer what counts as "normal" behavior? We propose that the diversity of those enacting a behavior serves as a powerful, yet underexplored, signal of societal diffusion. If people observe a wide range of individuals engaging in a practice, they infer that the behavior is more common, accepted, and expected. Across five preregistered vignette studies in pro-environmental and pro-social domains, including charitable donations, green pensions, and food-waste reduction, we test this "diversity-as-diffusion" hypothesis. Our samples (total N > 1,400 across the UK and the Netherlands) consistently show that diverse (vs. homogeneous) representations of actors increase perceived normativity. Mediation analyses reveal that norm perceptions fully account for the positive effect of diversity on behavioral intentions. Importantly, we demonstrate that these effects are not reducible to identification or homophily. A replication study with an explicit identity manipulation confirmed that diversity cues influenced norm perceptions independently of whether participants identified with the actors. This is one of the first empirical demonstrations that diversity can enhance prosocial behavioural through informational rather than identity-based pathways.
We further probe boundary conditions. Effects are strongest in domains with low prior knowledge and weaker in contexts with entrenched priors (e.g. environmental protests). Moderation analyses suggest that diversity signals are effective when they provide novel, independent information and people represent a wider part of the population (representation). While direct effect sizes on observed behavior are small, diversity reliably strengthens perceived norms, which in turn predict behaviours.
Taken together, these studies advance norm psychology by integrating diversity into models of belief updating, and they offer practical insights for campaigns seeking to foster sustainable and prosocial behavior through subtle representational choices.