IQ

• Linda Gottfredson (2000) has a few points of interest:

"Siblings differ among themselves by an average of 12 IQ points; strangers differ by an average of 17 IQ points."

There is no persuasive evidence that the IQ bell curves for different racial-ethnic groups are converging over time, and they differ as much when children leave high school as when they enter kindergarten.

A large, longitudinal study of ex-servicemen in Australia found that safe driving relates to IQ level. Rates of death from motor vehicle accidents doubled and then tripled across three ranges of normal IQ: In the 100-115 IQ range, there were 51 deaths per 10,000 men; at IQ 85-100, there were 92 deaths; and at IQ 80-85, there were 147 deaths.

The shared effects of a family environment on intelligence disappear with age. ...The heritability of intelligence fairly rapidly becomes the dominant influence, rising from 40 percent of the explanation of differences in IQ scores in the pre-school years, to 60 percent by adolescence, to 80 percent in old age.

Studies of children who have become siblings through adoption illustrate this counterintuitive discovery. They become less like one another as they get older, but more like their biological parents and biological siblings, whom they have never met. By adolescence, adopted siblings tend to be no more alike in IQ than complete strangers. On the other hand, identical twins reared apart are almost as similar as identical twins reared together and considerably more similar than fraternal twins reared together.

Special interventions to raise low IQ’s are somewhat successful with young children, but the effects of these interventions almost always fade out as children approach early adolescence.