American partisans vastly under-estimate the diversity of other partisans' policy attitudes

A popular explanation for America's democratic ills is that Republicans and Democrats misperceive one another to hold extreme attitudes. However, Americans may also misperceive the diversity of partisans' attitudes to ill effect. This paper uses surveys and pre-registered experiments with representative and convenience samples (n = 9405) to validate a measure of perceived attitude extremity and diversity and compare it to canonical measures of perceived polarization. We find that American partisans vastly under-estimate the diversity of each party's attitudes. Yet, contrary to existing research, we see little evidence that partisans over-estimate how extreme the "average" Republican or Democrat is. Finally, perceptions of both the "average" partisan and within-party attitude diversity predict partisan animosity and perceptions of out-party threat.