I understand that by definition, the mean IQ is 100. And that over time IQ tests are updated to ensure that mean remains 100.
But if IQ scores were not updated, what would mean IQ be now and how has IQ changed over time?
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$\begingroup$ The distribution of IQ in the population is well-described by a normal distribution. Since normal distributions are symmetric, the mean and the median are identical. Since median IQ is 100 by definition, the mean IQ is also 100. $\endgroup$– Nuclear HoagieMar 12, 2021 at 18:17
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$\begingroup$ I've edited this question to make it clear that you are asking specifically about historical changes in intelligence test scores. $\endgroup$– Jeromy AnglimMar 15, 2021 at 1:06
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Until recently, when people took older tests, they tended to score higher.
https://en.wikipedia.org/wiki/Flynn_effect
The Flynn effect is the substantial and long-sustained increase in both fluid and crystallized intelligence test scores that were measured in many parts of the world over the 20th century.[1] When intelligence quotient (IQ) tests are initially standardized using a sample of test-takers, by convention the average of the test results is set to 100 and their standard deviation is set to 15 or 16 IQ points. When IQ tests are revised, they are again standardized using a new sample of test-takers, usually born more recently than the first. Again, the average result is set to 100. However, when the new test subjects take the older tests, in almost every case their average scores are significantly above 100.
The recent years saw a reversal of the Flynn effect.
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$\begingroup$ A complete answer should also mention the Reverse Flynn Effect, as this trend has slowed down and even reversed in some sub-populations. $\endgroup$– Arnon Weinberg ♦Mar 15, 2021 at 2:17
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$\begingroup$ @ArnonWeinberg Interesting. I didn't know about this. Will add a link. $\endgroup$– user16005Mar 15, 2021 at 3:28