From the article:

When it comes to estimating the size of demographic groups, Americans rarely get it right. In two recent YouGov polls, we asked respondents to guess the percentage (ranging from 0% to 100%) of American adults who are members of 43 different groups, including racial and religious groups, as well as other less frequently studied groups, such as pet owners and those who are left-handed.

[...]

Misperceptions of the size of minority groups have been identified in prior surveys, which observers have often attributed to social causes: fear of out-groups, lack of personal exposure, or portrayals in the media. Yet consistent with prior research, we find that the tendency to misestimate the size of demographic groups is actually one instance of a broader tendency to overestimate small proportions and underestimate large ones, regardless of the topic.

If exaggerated perceptions of minority groups’ share of the American population are due to fear, we would expect estimates of those groups’ share that are made by the groups’ members to be more accurate than those made by others. We tested this theory on minority groups that were represented by at least 100 respondents within our sample and found that they were no better (and often worse) than non-group members at guessing the relative size of the minority group they belong to.

[...]

Although there is some question-by-question variability, the results from our survey show that inaccurate perceptions of group size are not limited to the types of socially charged group divisions typically explored in similar studies: race, religion, sexuality, education, and income. Americans are equally likely to misestimate the size of less widely discussed groups, such as adults who are left-handed. While respondents estimated that 34% of U.S. adults are left-handed, the real estimate lies closer to 10-12%. Similar misperceptions are found regarding the proportion of American adults who own a pet, have read a book in the past year, or reside in various cities or states. This suggests that errors in judgment are not due to the specific context surrounding a certain group.

But note that they specifically asked people to guess:

Estimated proportions median weighted responses (ranging from 0% to 100%, rounded to the nearest whole percentage) to the question "If you had to guess, what percentage of American adults..." True proportions were drawn from a variety of sources, including the U.S. Census Bureau, the Bureau of Labor Statistics, and polls by YouGov and other polling firms.