40 PRETEND GENES
too. So the distinction was made between qualitative traits, for which
individuals can be discretely categorized (Mendelian traits), and quanti-
tative traits, for which individuals have to be mea sured and/or ranked
(non- Mendel ia n t ra its). Th e latter are also referred to as biometric or
polygenic traits. It soon became commonplace among psychologists
to assume that human potential or intelligence is a continuous trait and
is distributed like one.
IDENTIFYING CAUSES OF VARIATIONBy the 1920s, great interest in ge ne tics had developed in agricultural con-
texts. It was realized that knowing how much variation in par tic u lar traits
could be attributed to ge ne tic variation could help guide crop and animal
breeding to maximize yields. If variation in milk yield in cows, say, is
largely due to ge ne tic variation ( there is a high heritability), then select-
ing for breeding those individuals who already exhibit high milk yield
may boost the average yield in off spring. If the variation is estimated to be
mainly due to environmental variation ( there is low heritability) selective
breeding will not make much diff erence.
However, estimating heritability is much more diffi cult for continuous
traits than for Mendelian traits. If we look at a group of individuals vary-
ing in a Mendelian trait, we can observe who has what gene. We cannot
do this with polygenic traits because (a) there are many genes involved,
(b) the variation is continuous, and (c) part of it will be due to diff er ent
genes and part of it to diff er ent environments. We cannot distinguish one
kind of eff ect from the other.
Th en along came the statistician Ronald Fisher, who envisaged what
he thought would be a pos si ble solution. Fisher developed an interest in
heredity in part because of his interest in eugenics in humans. His solu-
tion to modeling polygenic, continuous traits was published in 1918. In it
he introduced new methods of statistical analy sis of variation. Using
these methods, he proposed to work out the relative contributions of
nature and nurture to mea sured variation.
Th is was his suggestion. Th ere are many genes, he acknowledged. But
let’s assume that their eff ects on individuals are in de pen dent of one an-
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