From The Genius Famine - by Ed Dutton and Bruce G Charlton
Objective measures show that intelligence has declined rapidly and substantially over the past century or two; but it is also true that the so-called ‘Flynn Effect’ has been evident.
This name refers to the fact that IQ raw scores (i.e. the results on IQ tests, the proportion of correct answers) have been rising throughout the 20th century in Western countries. So, performance in IQ tests has been increasing at the same time as real, underlying general intelligence has been decreasing.
This can happen because IQ score is a relative, not an absolute, measure of intelligence – and because it is essentially the result of a timed examination involving answering questions. There are likely to be many reasons for increasing IQ scores, indeed any reason for increased exam scores might be contributory – for example improved health, cultural change, educational expansion, socialization of testing procedures, test question and format familiarity, teaching of test strategies, increased use of multiple choice formats (where guessing is encouraged), probably also increased levels of cheating – all may contribute variously to IQ test scores rising even as intelligence declined.
But even the Flynn effect has now plateaued or gone into reverse in a number of countries, and the rise in scores have been shown to be occurring most on the least g-loaded parts of the tests. So, general intelligence has been declining substantially and rapidly even though IQ test scores used to be increasing.
Furthermore, it seems likely that while underlying intelligence was much higher in the past, the measurable intellectual performance – for example in examinations, intelligence tests, and in real life situations – of most people was severely damaged by lack of education, harsh physical conditions such as cold and damp, starvation, disease, exhaustion and endemic severe infectious disease, pain and disabilities and so on. Such factors would be expected substantially to reduce (or abolish) many aspects of intellectual performance in difficult tasks by (for example) impairing concentration and motivation.
Imagine doing an IQ test, an examination, or attempting any challenging intellectual activity such as reading a difficult book or performing calculations; while suffering with a fever or chronic pain or gnawed by hunger: imagine suffering fevers, pain, or hunger continuously for most of your life… but this was the normal situation for most of the population in earlier times. No matter what their underlying level of intelligence might be, their performance was significantly impaired for much of the time.