IT IS not the “unknown unknowns” that catch people out, but the truths they hold to be self-evident that turn out to be completely wrong. On many issues, the gap between public perceptions and reality is very wide. The polling company Ipsos Mori found that Americans think 33% of the population are immigrants, for example, when the actual number is 14%. A 2013 poll found that Britons thought 24% of the population was Muslim—almost five times the correct figure of 5%.

Misperceptions about economic policy are common, too. Asked to name the top two or three areas of government spending, 26% of Britons cited foreign aid, more than picked pensions or education. In fact, aid spending is a small fraction of the other two and only 1% of the total.

Some of this is to do with innumeracy. Only a quarter of Britons could work out that the odds of throwing two consecutive heads in a coin toss was 25%. People are also heavily influenced by anecdotal evidence and by fears for themselves or their families—hence the tendency to overestimate the prevalence of crime or teenage pregnancy. (Asked how many teenage girls get pregnant each year, Americans…Continue reading