The Intelligence Quotient

What it is

Peter Watson's judgement, which is his own and is unfootnoted, is that the round whole number is as much as anything what made the quotient catch on. That is a plausible claim about popularisation rather than a finding, and it is worth holding: a figure of a hundred invites comparison in a way that a ratio of one does not.

Binet's objection travels with the number and is usually left out. It was never his intention that the measure be used for normal children or adults, and he was worried by any attempt to do so. The instrument was designed to find children who needed help. The quotient turned it into a universal ranking, and every later controversy sits downstream of that change of purpose.

The uses the tests were put to in the first half of the twentieth century, in immigration restriction and in compulsory sterilisation programmes justified by test scores, are rejected. They were rejected both in law and in the historical and methodological literature that reconstructed how the testing had been conducted, of which the best known are Leon Kamin's The Science and Politics of IQ in 1974 and Stephen Jay Gould's The Mismeasure of Man in 1981. Neither is held in this vault and both are named here from the general record rather than from a reading. No eugenic claim is restated on this page in any form.

In effect

The group-difference question is the most charged material in this area and is set out here neutrally, with positions attributed to published work rather than to persons.

What is measured is not seriously disputed. A gap of roughly one standard deviation between black and white average scores on standard cognitive tests has been observed repeatedly in United States samples. Everything after that is disputed.

The hereditarian case at its strongest, as argued by J. Philippe Rushton and Arthur Jensen in 2005 and again in 2012: within-group heritability of test scores in adults is substantial, the gap is stable across decades in some datasets, it appears on tests of differing content, and it has not closed despite large changes in schooling and income.

The environmental case at its strongest, and this is the majority position. Within-group heritability says nothing about between-group differences, which is the single most-cited flaw in the hereditarian argument. William Dickens and James Flynn reported the gap narrowing over the three decades to 2002, which a fixed genetic difference does not predict, and Rushton and Jensen questioned that narrowing in 2012 without the exchange being resolved. The Flynn effect shows population means can move a full standard deviation within a generation on environment alone. Richard Nisbett, Joshua Aronson, Clancy Blair, Dickens, Flynn, Diane Halpern and Eric Turkheimer set out in 2012 that the heritability of test scores varies with social class, being much lower in poor families, and that essentially no genetic variants reliably track normal-range cognitive ability across groups. Adoption and intervention studies show substantial environmental movement.

Where this lands is that the gap is measured and its causes are not established. The claim of a genetic cause is not supported by evidence meeting the standard the claim requires.

What it does not say

It does not license the figures as they appear in the popular history this reaches the vault through. Two errors in Peter Watson's The Modern Mind are confirmed and recorded so they do not travel. At page 534 he prints the gap as approximately fifteen per cent, where it is fifteen points, as he himself writes correctly on the following page. At page 698 he describes the National Longitudinal Survey of Youth as a database of about four million Americans, where the cohort is about twelve thousand seven hundred respondents, which misstates the evidential base of the chapter's biggest controversy by roughly three orders of magnitude.

It does not include a sentence attributed to Jensen at page 534 about the education of black children. That sentence appears inside quotation marks in the book, reads as a paraphrase rather than a quotation, and could not be located verbatim in the source article. It is not reproduced here in any form.

It does not tell anyone anything about an individual. A test score is a position in a distribution on a particular instrument, group averages carry no information about any person, and nothing on this page is guidance about anybody's education, employment or care.

It does not inherit authority from the book it arrived in. Watson is a journalist writing an intellectual history, his book is a map rather than a source, it is cited here only for the fact that the idea existed and mattered, and it predates the modern literature on the Flynn effect, on heritability and on gene-by-environment interaction entirely.

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