Thomas Sowell has another one of his admirably balanced articles here -- where he broaches the heavily forbidden topic of the heritability of IQ. He is not a psychometrician, however, so he misses a few things.
He points out the long-standing evidence that IQ is strongly hereditary but then goes on to question that evidence on the basis of research by James Flynn -- who has documented a large rise in IQ scores throughout the 20th century. Since genetics do not change over such a short time, one might conclude that the causes of IQ are therefore non-genetic and presumably environmental. He seems to miss the point that psychometricians have always said that the environment has SOME influence on IQ. He is right, however, in saying that environmental improvement is universally seen as what lies behind the "Flynn effect". Exactly what environmental factors lie behind the "Flynn effect" is a decades-old topic of research and discussion among psychometricians but it is a controversy largely unknown to the general public and Sowell has done us all a service in bringing it to the attention of a wider audience.
Sowell would seem to be of the view that the famed black/white difference in average IQ is called into question by the Flynn findings. It is therefore regrettable that he does not mention that the changes of the 20th century seem to have affected most groups equally. Blacks now score more highly than they once did but so do whites. The black/white gap, in other words, has NOT been narrowed by the general rise. That the gap remains the same DESPITE obviously favourable environmental circumstances would therefore seem to STRENGTHEN the evidence in favour of the black/white gap being wholly genetic.
The broadest explanation for the rise in IQ scores over the 20th century is that it reflects the advantages of modernity in general: The elimination of hunger in Western countries, improved perinatal care (particularly less damaging obstetric practices), improved medical care generally, greater access to mental stimulation via movies and TV etc. But the leading cause is thought to be greater test sophistication. The many more years that kids now spend in education makes them good at test-taking strategies generally (e.g. the wisdom of guessing when you don't actually know the answer).
All these influences are essentially one-off however. Once the major limits on maximum brain development are removed, no further improvement in IQ scores can be expected. And in some countries that does already seem to have happened. Test scores there have ceased to increase. Environmental influences do have SOME effect on IQ but once the environment is removed from the equation, the still-large remaining differences in scores between individuals can only be genetic. So Flynn's findings in fact STRENGTHEN the evidence in favour of genetics being the overwhelming influence on IQ.
And the twin studies showed that long ago, anyway. Twins usually have extremely similar environments but identical twins have almost exactly the same IQs whereas fraternal (genetically different) twins may or may not. I personally know of one set of fraternal twins where one flunked High School and the other got a University degree.
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