If the average IQ score is 100, who does everyone self-report having an IQ of 130+?

Everyone on reddit seems to think they have an IQ well over 120. But as wikipedia shows you, only 1% of people have an IQ of 120. [...] So either either Reddit is a meeting place of Mensa or we don't understand statistics at all.

— TheCleverestUsername on /r/AskReddit on 4 December 2011

If you have even been in a discussion about IQ on the internet, you have probably noticed that everyone claims to have an extremely high IQ. This is an reliable observation. To give the high level view, look at this XKCD comic from 2010:

One example is the blog Slate Star Codex which runs an annual survey of its readership that asks them to report IQ scores from legitimately administered IQ tests. The average score reported in 2017 was 138. It would make sense for readers of this blog to be better at puzzles and tests than an average individual, but this is unbeleiveable. The SSC data can be downloaded so we can work throught it. In it 18 of 1,419 people claimed to have an IQ in the average range, between 90 and 100. This is 1.2% when you would expect 49.5%, so the relaive frequency of people in this range reading SlateStarCodex is 0.025. 1,167 reported an IQ of 130+, when you would expect 2.2%, so the relative frequency of people in that range reading SSC would then be 37.38. Then compare those two and we get the result that people with IQs are 1,458 times more likely than people with average IQs to be readers of SlateStarCodex.

We can compare the finding the high IQ people are 1,458 times more likely to become SSC readers than normal IQ people to other findings to see if it is reasonable. The best studies would be longitudinal ones that measured IQ, then later measured outcomes so these are the comparisons that will be made.

The National Longitudinal Study of Adolescent to Adult Health contained the Peabody Picture Vocabulary Test, 3.8x Master's Degree, 18x Doctoral degree, 1.7x atheist/agnostic, 2.5x income over $150,000+
The National Longitudinal Survey of Youth 1979 contained the Armed Services Vocational Aptitude Battery

Most people have only taken on-line IQ tests which almost allways give very high scores

The most common way people get an “IQ score” is by taking an on-line test. And most on-line tests are completely bogus and give everyone a high score.

The tests people find are usually though Google, and Google shows you the tests that are popular and tests become popular by giving out high scores. Someone is much more likely to share a website if that website told them that they were smart than if they were dumb.

An example would be an IQ test I took on arealme.com on October 9, 2019: I gave random answers and they gave me an IQ score of 147. Implying that I am smarter than 99.9% of the population. This test had 190,000 facebook likes, so there are a lot of people walking around feeling good about how they got high scores like mine. Sure, this is an unusually bad test, but this effect pushes scores higher across the board.

People only report their higest score

The correlation between these two scores is r=0.67, which is typical of the correlation between two different IQ tests. A review of correlations between the Wechsler Adult Intelligence Scale and Raven's Progressive Matrices found a weighted average of r=0.67.