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One of the reasons for the popularity of Terman’s test was that the scores were expressed as a catchy number — the intelligence quotient, or IQ. Psychologist William Stern had earlier proposed dividing a child’s “mental age” by the chronological age to get a “mental quotient” that would tell how smart the child is. Terman appropriated this idea, multiplying the ratio by 100 and calling it the intelligence quotient.

This scheme doesn’t work so well with adults. What would it mean to be thirty and have a mental age of fifty — that you hate house music and are starting to forget things? Terman solved the age problem simply by adjusting his test’sscoring so that 100 was average for a person of any age.

That was not the only adjustment he made. As Terman assembled more and more IQ test scores, he discovered some interesting patterns. One was that girls scored higher than boys. Another was that whites scored higher than blacks, Mexicans, and recent immigrants.

Terman decided that the first finding revealed a flaw in the test while the second finding represented a real fact about human beings. He went back and looked at what questions had the biggest gender gap. He tossed out questions that favored girls and/or added questions that favored boys until the gender difference vanished. There was nothing underhanded about this tweaking. It is part of creating any good psychological test.

The interethnic differences in IQ scores were several times larger than those between genders. Terman had no interest in adjusting the test to minimize these differences. He was a white male, and if the test said whites were smarter, then it just confirmed what most white males in 1916 America already assumed. That, at least, is one possible interpretation. Another is that Terman wanted to believe the ethnic differences were “real,” because otherwise they would be a humbling demonstration that it really isn’t so easy to measure intelligence. Intelligence testing is founded on the assumption that certain tasks or puzzles gauge “true” intelligence, independent of education, social station, or culture. That there were substantial intercultural differences in IQ scores could have been seen as evidence of the test’s inadequacy.

Terman didn’t see it that way. Nor did most of America The Stanford-Binet ushered in a national obsession with IQ testing that continues, in attenuated form, to the present day.

Excerpted from page numbers27-28 of ‘How Would You Move Mount Fuji?’ by William Poundstone.