‘IQ tests just measure how good you are at doing IQ tests.’ This is the argument that is almost always made when intelligence-testing is mentioned. It’s often promoted by people who are, otherwise, highly scientifically literate. You wouldn’t catch them arguing that climate change is a myth or that vaccines might cause autism. But saying that IQ tests are useless is just as wrong as these notions: in fact, decades of well-replicated research point to IQ tests as some of the most reliable and valid instruments in all of psychological science.
So what does an IQ test – which might consist of, for example, shape-based puzzles, timings of how quickly you can check through lists of meaningless symbols, memory tests, and vocabulary measures – actually tell you? The strongest correlation is perhaps unsurprising: an IQ score is highly predictive of how people will do in school. One large study found that IQ scores at age 11 correlated 0.8 (on a scale of -1 to 1) with school grades at age 16. Surely this gives us some basis for calling these measures ‘intelligence tests’. But that’s just the beginning: higher IQ scores are predictive of more occupational success, higher income, and better physical and mental health. Perhaps the most arresting finding is that IQ scores taken in childhood are predictive of mortality. Smarter people live longer, and this association is still there after controlling for social class.