I saw a post on r/iamverysmart about an IQ test, which is obviously not reliable, but I also recently learned IQ tests are supposed to be your “mental age” divided by your physical age times 100 (so a kid as smart as a 6 yr old who’s 4 would have an IQ of 150, for example). But the difference between those for an adult would get much smaller, as two years or so is a much smaller difference percentage-wise and an adult’s IQ would always approach 100. So how do IQ tests for adults work?
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>but I also recently learned IQ tests are supposed to be your “mental age” divided by your physical age times 100
That is not how it is done anymore, and it is never how it was done for adults.
There are actually several different IQ tests in use, and they are scored differently. But generally, they are based on how you rank among everyone else who took the test. So if you scored better than 50% of the population, your IQ is 100 by definition. Then they fit the rest to a bell curve, with some fixed standard deviation, for example 15. So if you score better than 84.1% of the population, that’s a 115, better than 97.7% is 130, etc.
Speaking as a clinical psychology student who administers adult (and child) IQ tests pretty regularly, these tests assess abilities in a range of areas encompassed by the general concept of “intelligence”. The score that you receive in the end is standardized and based on how your performance compares to a normative sample of people your same age. An IQ score of 100 is average, and the standard deviation for IQ scores is 15. This way of scoring IQ tests is the same for both children and adults and is used by all reputable modern IQ tests (which are valid and reliable). The “mental age vs. chronological age” method you described is no longer used.
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