On page 522 of Arthur Jensen’s book The g Factor, he discusses a study of identical twins raised apart (Newman et al., 1937). One pair of raised apart twins known as Gladys and Helen differed 1.5 standard deviations in IQ. Converting to a scale where white Americans average 100 (SD = 15), their adult IQs were 92 and 115 (a 23 point gap).
If IQ is extremely heritable, how do we explain this huge gap? Well it’s just one data point, and the two twins were raised in extremely different environments and differed greatly in health history, and even differed in finger prints (suggesting different prenatal effects), but what really struck me, is that one twin had only a third grade education and the other had graduated from college to become a school teacher. That sounds like a difference of 13 years of schooling!
About a year ago I wrote:
In a fascinating 1968 Swedish study, a scholar named K. Harnqvist compared the IQ’s of males who were tested at both age 18 and age 13. He found that if two boys, for example, were both of the same social class, and both had the same IQ at age 13, but one boy completed all four years of high school and the other completed none of them, the latter boy scored nearly 8 points lower at age 18. In other words, each of year of missed schooling causes IQ to drop by 1.8 points. Herrnstein and Murray independently found a similar effect on page 615 of The Bell Curve (1.65 IQ points per school year).
So if schooling adds 1.8 IQ points per year, then a 13 year difference in schooling between identical twins should produce a 13(1.8) = 23 point difference in IQ, and that’s exactly what we see.
This is an example of the much maligned Phenotype = Genotype + Environment model working beautifully. Somethings really are as simple as they seem.
Of course as I also noted a year ago:
It’s very unlikely that school makes you smarter, but it is possible that school makes you more test savvy and more motivated to do well on mental tests. Many IQ tests require complex focused thinking. Someone who dropped out of school at 13 and has been working in an outdoors fun type job is likely to find such mental effort excruciatingly boring, annoying, and intimidating, while someone who stayed in school her whole life and works in academia is likely to be used to sitting still and focusing and may enjoy the challenge and have the confidence and intellectual ego to stay motivated.
So one twin was probably educated beyond her ability and over-performed on the IQ test. The other twin was educated beneath her ability and under-performed on the IQ test. In this case it would make sense to subtract IQ points from the educated twin and given them to the uneducated twin.
Does that mean we should subtract IQ points from all college grads and give them to grade school dropouts? No, because unlike adoption studies where environments are relatively random, for most of us, our environment reflects our genes. That is, if you have low IQ parents who raise you to flunk out of school, you probably have low IQ genes, while someone who had high IQ parents and was raised to graduate college, probably has high IQ genes. So controlling for schooling will reduce the absolute difference in test scores, but it will not much change the rank order because the latter is set by genes, and IQ is a measure of the latter.
However if everyone were adopted, then controlling the environment (including schooling) would indeed affect the rank order of people on cognitive tests.