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

20 PINNING DOWN POTENTIAL

complexity.” And in 2015, he admits that “scientists have not identifi ed a
single gene that would meet any reasonable standard as a ‘gene for’ schizo-
phre nia, intelligence, depression, or extraversion.”^22
Th e dilemma is now widely discussed as the “missing heritability”
prob lem. Twin studies have estimated IQ and other aspects of potential
to be at least 50  percent heritable. To the behavioral ge ne ticist, this indi-
cates that there must be many variable genes under lying individual dif-
ferences in it. So where are they? As we shall see in chapter 2, given the
fl aws in twin studies and the true nature of intelligence, the heritability
estimates are prob ably inaccurate in the fi rst place.
In spite of these disappointments, the gene hunters continue to lace
reports with such terms as “exciting,” “breathtaking,” and “momentous
shift s,” telling us that the fruits “ will soon be available” and so on. Th ese
are the sort of subjective terms we would not normally expect to fi nd in
scientifi c papers. And, increasingly, we see truth being merely asserted
through wishful thinking rather than empirical demonstration. “We now
know that many genes of very small eff ect are responsible for the herita-
bility of intelligence,” say Nicholas Shakeshaft and colleagues in a paper
in the journal Intelligence (February 2015, emphasis added)— even though
no such responsibility has been shown. Th ere are many biological reasons
why not.
For a start, consider the scale of it all. Humans possess, in the original
fertilized egg and in nearly every cell of their bodies, about twenty thou-
sand of the gene “words.” Each word is made up from diff er ent sequences
of four nucleotide “letters”— the chemicals adenine (A), thymine (T),
guanine (G) and cytosine (C). Each of these letters occupies a locus in
a sequence in the DNA strand. And each locus can, in diff er ent people,
have a diff er ent nucleotide (hence the term “single nucleotide polymor-
phism,” or SNP).
For example, a DNA sequence... AAGGCTAA... , as part of a
sequence in one person, may occur as... ATGGCTAA... in another. Th e
second nucleotide has under gone a substitution. Th e trou ble is that there
are more than six billion of these nucleotides in the twenty- three pairs of
chromosomes in each human cell. On average, an SNP occurs only once
in every three hundred nucleotides, which means there are at least ten
million SNPs in the human genome.

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