8 PINNING DOWN POTENTIALmessage was that social inequalities are really just smudged expressions
of ge ne tic inequalities, and the test proves it.
A vigorous testing movement grew, clearly nourished by those beliefs.
Lewis Terman, the founder of the fi rst English- speaking test in the United
States, declared how the test would help “preserve our state for a class of
people worthy to possess it.”^6 Certainly, turning social classes into nu-
merical ranks, with the upper class at the top, made the enterprise look
scientifi c. It furnished the fi rst scientifi c- seeming tool of social policy.
Th e po liti cal spin- off was huge, as were the social consequences. Th e
IQ testing movement became highly infl uential in the United States in the
adoption of eugenic and anti- immigration policies in the 1920s.^7 Like-
wise, the test became used to help justify education se lection policies on
both sides of the Atlantic. Th e issue went quiet during and aft er the Sec-
ond World War, following the horrors of Nazi Germany. But it soon
took off again. Th e racist undertones of Arthur Jensen’s counterblast to
compensatory education in the 1970s, and of Richard Herrnstein and
Charles Murray’s Th e Bell Curve in 1994, became notorious.
Nearly all of what is said scientifi cally about human potential today
is still derived from IQ testing. But to this day, its supporters are not
sure what IQ tests mea sure. In the absence of a clear theory, they can
only resort to meta phors from common experience— a kind of general-
ized mental “strength” or “energy” that they call “g”; a “power” or “capac-
ity.” As Mark Fox and Ainsley Mitchum explained in a paper in 2014,
“Virtually all psychometric models... take for granted that a score on
a test can be accurately interpreted as placement along one and only
one dimension.”^8
Indeed, such a proposition put by Charles Spearman in 1915 has been
described as the single biggest discovery in psy chol ogy. He introduced the
concept of “general intelligence,” or “g,” although he was later to admit
that the idea is poorly defi ned. In their standard textbook Behavioral
Genetics— basic fodder for generations of psy chol ogy students— Robert
Plomin and colleagues candidly admit, albeit with some understatement,
that “it is rather less certain what g is.” And, in a popu lar introductory
book on the subject, Ian Deary also admitted that “ there is no such thing
as a theory of human intelligence diff erences— not in the way that grown-
up sciences like physics or chemistry have theories.”^9
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