REVISED AND EXPANDED EDITION 33genetic defect as any other. I welcome the discovery of genetic
causes or influences for such scourges as schizophrenia, bipolar
manic depression, and obsessive-compulsive disorder. No pain can
match that of a parent who "loses" a vibrant and promising child to
the ravages of such illnesses, with their frequently delayed onset
near the end of life's second decade. Let us celebrate the release of
parents from consuming guilt and, more important of course, the
possibility of amelioration, or even cure, supplied by identification
of causes.
But all these genuine discoveries involve definite and specific
pathologies, diseases, or conditions that thwart what we may still
legitimately call "normal" development—that is, the bell curve. (Bell
curves are technically called normal distributions; they arise when
variation is distributed randomly around the mean—equally in both
directions, with greater probability of values near the mean.) Spe-
cific pathologies do not fall on the bell curve, but usually form
clumps or clusters far from the curve's mean value and apart from
the normal distribution. The causes of these exceptions therefore
do not correspond with reasons for variation around the mean of
the bell curve itself.
Just because people with Down's syndrome tend to have quite
short stature as the result of an extra twenty-first chromosome, we
would not infer that short-statured people in the normal distribu-
tion of the bell curve owe their height to possession of an extra
chromosome. Similarly, the discovery of a gene "for" Huntington's
chorea does not imply the existence of a gene for high intelligence,
or low aggressivity, or high propensity for xenophobia, or special
attraction to faces, bodies, or legs of a sexual partner—or for any
other general feature that might be distributed as a bell curve in the
full population. "Category mistakes" are among the most common
errors of human thought: we commit a classic category mistake if
we equate the causes of normal variation with the reasons for pathol-
ogies (just as we make a category error in arguing that because
IQ has moderate heritability within groups, the causes for average
differences between groups must be genetic—see my review of The
Bell Curve in essay 1 at the back). Thus, we should be excited about
advances in identifying the genetic causes of certain diseases, but we
should not move from this style of explanation to the resolution of
behavioral variation in our general population.
Of all the baleful false dichotomies that stymie our understand-