This study investigates the persistence of group differences associated with mutable identities, using religious affiliation as a case study. Existing theory suggests such differences can persist via self-selection, where higher-performing individuals are more likely to join groups with higher average performance. Analyzing NLSY79 and GSS data, I test this mechanism by comparing movers who switch religious affiliations with stayers who do not. Findings confirm self-selection patterns: move- uppers who switch to affiliations with higher average education or wages, tend to have higher education and wages than stayers from the same origin group, while move-downers tend to have lower outcomes. However, other findings contradict the theory's mechanism: fixedeffects models show no significant direct wage gain from switching affiliations, and the frequency of moving down is comparable to moving up. These results suggest that while self-selection occurs, it may be driven more by factors like homophily rather than direct differential economic returns.