PINNING DOWN POTENTIAL 15entirely from twin studies that (even if valid) impute sources of statisti-
cal variance, not gene identities.
Over the past two de cades, however, brilliant advances in molecular
biology have made it pos si ble to describe more or less directly the specifi c
genes, or diff er ent versions of them, that diff er ent individuals actually
have. Th is is what the Human Genome Proj ect has been about. Immedi-
ately it occurred to behavioral ge ne ticists that the limitations of twin
studies could at last be transcended. Now we could potentially describe
direct associations between variation in genes and variation in ability on
a one- to- one basis—or so it has been thought.
Th ese days, of course, most people know that genes derive their indi-
vidual identity from the diff er ent ways that four molecules (called nucle-
otides) are combined in diff er ent sequences to form the genes on the DNA
strands that make up the chromosomes. Th e new methods enable the
identifi cation not only of the whole genes (the diff er ent “words” on the DNA
strand) but also of the nucleotides (the “letters” that make up the words)
at each location in each gene. Variations in these, from person to person,
are called single nucleotide polymorphisms, or SNPs. Such sequencing was
the pro cess that made the Human Genome Proj ect pos si ble, culminating
in fi rst publication of a representative sequence in 2000.
Th e sequencing pro cess has rapidly developed into an industrial- scale
enterprise done by machines and computers. It has been accelerated by
the commercial production of DNA “microarrays” or “chips.” Th ese are
templates of known nucleotide components against which unknown se-
quences can be compared and matched for identifi cation. Th ey have per-
mitted the sequencing of large samples of individual genomes at rapid
speed and rapidly reducing cost.
Matching diff erences in these SNPs to diff erences in IQ seems a simple,
and obviously hugely appealing, step. Th e DNA can be extracted from
a few drops of blood or a few cells scraped from the lining of the cheek.
Little in- depth knowledge or technical expertise is required, because the
sequencing is done by machine, with any associations printed out by a
computer program. Surely this will identify “genes for intelligence,” or
other aspects of potential, it was immediately reasoned.
So large- scale, international consortia of gene sequencers and psychol-
ogists have formed, backed by huge funds from traditional funding
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