54 PRETEND GENESOf course, identical twins who have been reared apart are rather rare.
Bouchard and colleagues have accumulated increasing numbers of pairs
as the study has progressed; up to the eighty- one pairs in a fi nal total in
- Th ey have consistently reported high average correlations between
them in IQ and other tests. Th is proves, they claim, a heritability for IQ
of 0.75 (i.e., 75 percent of diff erences in IQ between individuals are attrib-
utable to ge ne tic diff erences). Papers have been readily accepted in other
prestigious journals and books, and popu lar renderings have grabbed the
media’s attention on a worldwide scale.
It is diffi cult to overstate the infl uence of these results on the nature-
nurture debate surrounding human potential. Sandra Scarr, president of
the American Psychological Association in 1993, said that “the remark-
able studies of MZ twins reared in diff er ent families challenges many
cherished beliefs in developmental psy chol ogy.” In 1993, Robert Plomin
likewise wrote about “the power ful design of comparing twins reared
apart” suggesting “a heritability for g of 80%.”^16
However, regardless of whether the theoretical assumptions are
correct, there are many dubious aspects of these data. Th ey have been
subjected to forensic dissection by investigators like Leon Kamin and Jay
Joseph. I do not review their work in detail here, but quickly note the
main prob lems they have noted.^17
Th ey Are Not Separated Twins
At the very least, the method demands that twins have developed in truly
separate environments. Th e fi rst prob lem confounding this requirement,
of course, is that the twins share an environment in the womb before
birth. We now know that experiences in the womb can have lasting eff ects
on development throughout life (see chapter 10). Th at could explain much
of the reported resemblances in cognitive and other potentials and dis-
qualify the method.
But even aft er birth, the usual meaning of the word “separated” seems
to be stretched beyond credulity. Placement of pairs is, aft er all, usually
down to family con ve nience and circumstance rather than careful re-
search design. It turns out that many, if not most, of the earlier studies of
twins were simply moved into diff er ent branches of the same family and
brought up by grandparents, aunts, or cousins. Th ey very oft en lived in theThis content downloaded from 139.184.14.159 on Tue, 17 Oct 2017 13:51:26 UTChttp://www.ebook3000.com