Speaking of Andrew Gelman, Statistical Modeling, Causal Inference, and Social Science (the blog he runs with Samantha Cook) has an intereting post and follow ups about estimations of minority populations by ordinary people.
Across the western world, the average estimates by people on the street of the size of minorities tend to be way off from the actual share of the population minorities make up. For example, when Americans are asked what percentage of the population is black, the average response is usually around 25%; the actual share of blacks and African-Americans in the population is about 12%. This tendency to overestimate the (immigrant) minority population is also found in Europe, but interestingly, the overestimate becomes larger as the size of the immigrant minority population grows smaller.
The misperceptions are clearly biased; the means of the errors of individual estimates don’t equal zero.
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See the posts and commenst for possible causes behind the misperception. Some possible reasons suggested by the cognitive psychologist David Budescu:
This overestimation is, probably, due to a combination of several factors: (a) different definions of the target event (the judges may generalize and assume, for example, that all the children of foreign born residents are also born abroad), (b) vividness (members of of the target population stand out — looks, accent, language, clothing), (c) clustering (often they are concentrated in certain areas), (d) typically, these surveys don’t employ incentives for truthful responding (i.e. proper scoring rules), and some people may respond “strategically” by inflating their estimates to make a political point.