Getting better and more equal

The Rational Optimist by Matt Ridley. Page 19.

In another respect, too, inequality has been retreating. The spread of IQ scores has been shrinking steadily - because the low scores have been catching up with the high ones. This explains the steady, progressive and ubiquitous improvement in the average IQ scores people achieve at a given age - at a rate of 3 per cent per decade. In two Spanish studies, IQ proved to be 9.7 points higher after thirty years, most of it among the least intelligent half of the group. Known as the Flynn effect, after James Flynn who first drew attention to it, this phenomenon was at first dismissed as an artefact of changes in tests, or a simple reflection of longer or better schooling. But the facts do not fit such explanations because the effect is consistently weakest in the cleverest children and in the tests that relate most to educational content. It is a levelling-up caused by an equalisation of nutrition, stimulation or diversity of childhood experience. You can, of course, argue that IQ may not be truly representative of intelligence, but you cannot argue that something is getting better - and more equal at the same time.