YouGov neatly demonstrate how Americans see small subgroups as much larger than they actually are, while large subgroups are systematically underestimated.
For example on average people thought that muslims made up 27% of Americans when the true proportion is 1% or that gays and lesbians made up 30%, when the true number is 3%.
On the other hand “we find that people underestimate the proportion of American adults who are Christian (estimate: 58%, true: 70%) and the proportion who have at least a high school degree (estimate: 65%, true: 89%).”
