American IQ scores have rapidly dropped, proving the 'Reverse Flynn effect'

Can someone actually explain how IQ tests work? By work, I mean how are the tests engineered, and the results computed.

Long time ago someone explained to me that the engineering of IQ tests was actually drafted from a very large pool of (regularly updated) questions, where statistical significance was extracted to form a _core symposium_ of questions to sample from. Also, the IQ score itself was normalized to be normally distributed centered at 100.

With this understanding, I was under the impression that IQ was a relative measure, at a specific point in time, of one's placement in the distribution.

Which meant to me that IQ cannot "drop" across a population, the mean will always be 100. And IQ scores cannot be compared on a time series basis, since they are only cross sectional measures.

Is that all wrong? Is there some truth to it?

An IQ test is a relative measure of where you are relative to the population at the point of time where the test was normalized.

That point of time is somewhere in the past. And when tests are renormalized, there is a conversion of "this score on the old test is that score on the new test". This allows for comparisons of IQ over time, across different versions of the same test. This is how the Flynn effect was first discovered.

Tests usually have solid conversions between them, you can can compare across different tests as well. Such conversions allow more verification of the Flynn effect.

If you go back far enough, you will find tests for children measuring mental age vs physical age, taking a ratio, and multiplying by 100. They fell on a distribution that was close enough to normal centered at 100 with a standard deviation of 15 or 16 that adult tests were developed to match them.

Since we don't know all the factors behind the Flynn effect in the first place, we also don't know all the factors behind why it might be reversing now.