PRETEND GENES 65Th e heritabilities listed in table 2.1, for example, were estimated, not by
fallible twin studies using rough and ready mea sures. Th ey come from
well- controlled breeding studies in the house martin carried out by Philippe
Christe and colleagues.^34
As you can see, these estimates are miniscule compared with those
(0.5–0.8) typically reported for human potential derived from twin
studies. Little won der that Peter Schönemann sarcastically warned that
heritability estimates for IQ “surpass anything found in the animal
kingdom.”^35 Any objective observer would surely reach the conclusion that
human potential is unlikely to be so unusual. Rather it is the method by
which it is estimated that is anomalous.
As I emphasize further in chapter 4, such low heritabilities do not
necessarily mean little ge ne tic variation; they simply mean that, for a
variety of reasons, there is little association (correlation) between ge ne-
tic variation and phenotypic variation. Why make intelligence such an
exception? We could, aft er all, apply the same logic in many subjective
ways. Perhaps we could establish from twin studies that behavioral
ge ne ticists have a surfeit of genes for “belief in heritability.” And per-
haps critics have a surfeit of genes for “skepticism.” In such a gene-
determined world, of course, objective science would no longer be pos-
si ble, because every one’s appraisal of data is hopelessly biased by their
genes!
Th e bottom line is that if we really want to know the causes of varia-
tion in human potential, then twin studies and the pursuit of heritability
TABLE 2.1 Heritabilities of vari ous traits
of the house martinTRAIT HERITABILITYWing length 0.156
Ta rsu s leng t h 0.079
Body mass 0.000
Immunoglobulin 0.051
T- c e l l r e s p o n s e 0.0 0 7
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