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

166 INTELLIGENT DEVELOPMENT

system, so long as a basic threshold is reached, the exact levels do not
matter. Indeed, as described earlier, it is part of the function of physiol-
ogy to create adaptable variation in changing environments. And such
functioning seems, in the vast majority of individuals, to apply quite ad-
equately. It is also worth mentioning that few of the distributions assume
a normal curve, a basic assumption of IQ test construction and of statis-
tical attempts to mea sure heritability in human intelligence.
Th at appears to be the case for other aspects of physiology. Take im-
munocompetence, which is defi ned as the ability of the body to produce
a normal immune response following exposure to foreign proteins,
pathogens, or other toxins. It evolved early in vertebrates and in humans;
it rapidly develops in the fi rst month or so of embryonic life. Vari ous
specifi c mea sures of level of functioning can signify disease states. But
functioning is aff ected by many experiential factors, including physical
and mental stress, nutrition, and age.
In other words, the immune system interacts intensively with many
other aspects of physiology, resulting in wide variations in indices at par-
tic u lar times. Again, there are categorical disease conditions, some of
which can be associated with rare genes. But, so far as I know, no one
dreams of ranking individual diff erences on a general scale of immuno-
competence. Most of the time in most people it functions well enough
and only draws attention in the relatively few cases when something is
seriously wrong.
In sum, no physiologist would dream of suggesting the following:

(a) that within the normal ranges of physiological diff erences, a higher

level is better than any other (as is supposed in the construction of

IQ tests);

(b) that there is a general index or “quotient” (à la IQ) that could mean-

ingfully describe levels of physiological suffi ciency or ability and

individual diff erences in it;

(c) that such “normal” variation is associated with ge ne tic variation

(except in rare deleterious conditions); and

(d) that ge ne tic causation of such variation can be meaningfully sepa-

rated from the environmental causes of variation.

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