Genes, Brains, and Human Potential The Science and Ideology of Intelligence

44 PRETEND GENES

mea sures are from samples of a population, rather than from every indi-
vidual, many things have to be assumed before the statistical formulas
can be applied. In the age of opinion polls, most people are at least
vaguely aware of such prob lems. Basically, the logic involves asking what
the results (the twin correlations) should look like if the variation were
ge ne tic in origin. Th e assumptions create a model, and the procedure is
called “model fi tting.” Here is the basic model.

Basic Model
Th e model assumes that intelligence is an ordinary quantitative trait. Th at
means that it develops and varies like milk yield in cows, back fat in pigs,
or height in humans. I correct that grossly false notion in chapter 3 and
in later chapters. In addition, the model assumes that all genes associated
with a trait (including intelligence) are like positive or negative charges,
G+ or G– : like batteries in a fl ashlight, but in their hundreds or thou-
sands. For the statistical model, it has to be assumed that these charges
are randomly distributed among individuals to constitute, in each, the
individual’s total “ge ne tic charge” (or genotype; fi gure 2.1). Accordingly,
the so- called power of an individual’s intelligence, say, lies in the par tic-
u lar permutation of strong or weak alleles. So behavioral ge ne ticists
speak of “intelligence- enhancing alleles” and “intelligence- depleting
alleles.”

FIGURE 2.1

Ge ne tic eff ect for three individuals in the behavioral ge ne tic model.

low charge medium charge high charge

G 2

G 2

G 2

G 2

G 1

G 2 G^2

G 2 G^2

G 2

G 1 G 2

G 1 G^1

G 1

G 1

G 1 G 1

G 1 G 1

G 1

G 1 G 1

G 1

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