G was known in the late 90s. But scientists were afraid to publish, even then. Very well-known cognitive and neuro psychologists noticed the correlations and they were real. In neuropsychology there are specific tests given to detect deficits.  They measure simple abilities like short term memory,  processing speed, attention, "multi-tasking", etc. Most of the tests are artificial.  But they measure abilities everyone would want to have in the real world as opposed to a science fiction novel. And if you don't have them at some basic level, with the deficits someone with a traumatic brain injury often has, you have difficulty doing activities of daily living.<br><br>None of the above is intended to ignore problems with factor analysis and psychometrics. It's strange that Lex isn't familiar with factor analysis, discriminant analysis, and latent variable analysis, since the basic statistical techniques are related to many of the principles in deep learning. Intelligence can be seen as a latent (meaning hidden or non-direct) variable. Younger people who have taken computer SATs are using the technique. You ask what  is the square root of 9. People who get it wrong are asked what is 3 X 3. The idea is to discriminate. Does the test-taker not know what a square root is or not how to multiply? This approach allows fewer questions,  but hones in on the latent variable.  The questions need to be ones that smart people get, less smart people miss, and those in the middle are basically answering at chance. At the level being tested.