Disentangling Heredity and Environmental effects on IQ

No sensible person thinks intelligence is completely determined by heredity or by environment. Recent Swedish work [ Proc. Natl. Acad. Sci. vol. 112 pp.4612 – 4617 ’15 ] tries to control for heredity while measuring environmental effects on IQ, assuming that IQ measures intelligence, a position some find contentious. Every Swedish 18 year old man is conscripted into the military apparently. IQ tests are given to all. Amazingly the authors found 436 sibships where the brothers had been raised apart.

The intelligence of the biological and adoptive parents wasn’t measured. Rather the surrogate of educational level was used instead. It was divided into 5 classes.

What did they find? Adopted sibs had an IQ 4.41 points higher than the nonAdopted sib (recall that average IQ is stated to be 100 points although it’s been rising, and that IQ levels of the population fall on the Bell (Gaussian) curve, with a standard deviation of 15 points). These results are not surprising, as few willingly give children up for adoption, so the adopted environment was quite likely better. The educational level of the adoptive parents was an average of 2.6 points higher.

Next, the authors measured the effect of the surrogate marker for intelligence (educational level) on IQ. For each point in the 5 point scale that the adoptive parent was at a higher educational level than the biologic ones there was an increase in IQ of the adopted sib relative to the unadopted one. This is as unequivocal evidence as we have for the effect of environment and educational level on IQ.

We’ll never have perfect data, and many caveats about this work are possible, but it is an impressive effort. 436 sibs is a huge number compared to the twins who’ve been reared apart and studied this way.

Just how large an effect do you think it was? I’ve already told you everything you need to know.

Each additional unit of rearing parental education was associated with 2 IQ units. Are you surprised? I was, because I thought the effect would be much larger. So environment is important in determining intelligence, just not so much.

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  • John Wayne  On May 4, 2015 at 11:02 am

    My guess is that IQ isn’t a great measure of intelligence. I’ve found that a person’s perspective and handling of time, money and personal relationships to be a bigger indication of intelligence/maturity/success than raw processing power.

    It would be interesting to take this study and contrast the results to a person’s happiness, credit score/income, and the life trajectory of their children.

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