The proportion dominance effect (PDE) refers to a higher motivation to help when the victims are part of a small (you can help 56 out of 60) rather than a large (you can help 56 out of 560) reference group. In two studies using different experimental paradigms, we investigated possible mediators of the PDE. Study 1 (N = 168) was conducted in three separate steps in order to test each link of the mediator model independently. Students read six vignettes where it was possible to help a fixed number of victims but where the size of the reference group was either small or large. When the reference group was small, helping motivation and perceived utility were higher, whereas sympathy toward the victims and perceived rights were not. A within‐subject mediation analysis showed that perceived utility mediated the PDE. Study 2 (N = 36) presented four versions of a single helping situation in a joint evaluation mode where the size of the reference group became gradually smaller in each version. All participants compared and responded to each version. Helping motivation increased as the reference group became smaller, and this effect was mediated by perceived utility rather than by distress, sympathy, or perceived responsibilities. Our results suggest that unlike, for example, the identifiability and singularity effects, which have been suggested to be mediated by emotional reactions, the PDE is mediated by perceived utility. Copyright © 2013 John Wiley & Sons, Ltd.