IQ Test Items
A final argument brings together the discussion of how we select IQ test items, and how we measure degrees of heritability. If we had a large selection of items which measured IQ with varying amounts of success - some "good" items, some "mediocre", others "poor" - then, if IQ is largely determined by genetic factors, the "good" items should show most heritability, the "poor" ones the least, and the "mediocre" ones intermediate. This is precisely what happens.
I have tried to describe all these experiments and their results in words, but in reality they are normally expressed in mathematical form, which makes all the arguments much more precise. Readers wishing to go further might like to consult a textbook like Nathan Brody's Intelligence. Suffice it to say that when we bring together all these different methods of estimating the heritability of intelligence, they converge on a value of about 70%, leaving environment to contribute 30%. There is yet more proof for the correctness of this general approach: when we measure indices of environmental determination of IQ (socioeconomic status of parents; type of schooling; number of books and journals in the house; cars, telephones and other appliances, etc.) the overall effect falls well short of 30%. In the formulae used by geneticists, measurement errors are counted as part of the environmental effects, so that 70% id probably an underestimate of the genetic determination of IQ in our society. So the environment matters, but much less so than the heredity.
It is necessary to qualify this estimate of IQ heritability because heritability is not an absolute quality, like the speed of light; it is a population statistic which describes a particular population at a particular time. A good comparison would be height. the average height of Englishmen at the moment is about 5 feet 10 inches. That of Japanese men is more like 5 feet 8 inches. But Englishmen 300 years ago were, if anything, smaller than Japanese men now, and Japanese in the USA are as tall as Englishmen now. Yet height is very hertiable, much more so than IQ! What is heritable is the cause of variation in a given population; what determines differences between populations may be something entirely different, like nutrition in the case of height.
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