We tend to considerably overestimate the extent to which party supporters belong to party-stereotypical groups, like 32% of Democrats are LGBT (6% in reality) and 38% of Republicans earn over $250,000 per year (2%)

The Parties in our Heads: Misperceptions About Party Composition and Their Consequences. Douglas J. Ahler, Gaurav Sood. Aug 2017, http://gsood.com/research/papers/partisanComposition.pdf

Abstract: We document a large and consequential bias in how Americans perceive the major political parties: people tend to considerably overestimate the extent to which party supporters belong to party-stereotypical groups. For instance, people think that 32% of Democrats are LGBT (vs. 6% in reality) and 38% of Republicans earn over $250,000 per year (vs. 2% in reality). Experimental data suggest that these misperceptions are genuine and party-specific, not artifacts of expressive responding, innumeracy, or ignorance of base rates.  These misperceptions are widely shared, though bias in out-party perceptions is larger. Using observational and experimental data, we document the consequences of this perceptual bias. Misperceptions about out-party composition are associated with partisan affect, beliefs about out-party extremity, and allegiance to one’s own party. When provided information about the out-party’s actual composition, partisans come to see its supporters as less extreme and feel less socially distant from them.

Keywords: groups, parties, partisanship, perception, polarization