164 INTELLIGENT DEVELOPMENTconvinced that natu ral ability varies as if it were a physiological trait. For
that reason, he and his followers based their fi rst intelligence tests on
sensory motor tests, like visual discrimination and reaction times.
Somewhat similarly, Ian Deary and colleagues, pursuing the notion of
a general intelligence factor, g, have suggested the existence of a general
biological fi tness infl uencing the growth and maintenance of all bodily
systems. Th at is what IQ tests mea sure, they say. And the same notion is
refl ected in attempts to relate IQ scores to physiological mea sures, like
speed of pro cessing or using fMRI scans to relate IQ with ce re bral activ-
ity levels (on which, more in chapter 6).
Th e main aim, of course, has been to imply something genuinely
fundamental in IQ testing— and the concept of g— when there have always
been doubts about their validity. Th e logic is that individual diff erences
in IQ are expressions of the same so- called biological forces that are found
in physiology, with some of the variation traceable to genes and some to
environment. So it should be in ter est ing to look at individual diff erences
in physiology to see whether there really is a parallel.
No doubt individuals vary on a wide range of physiological mea sures.
Take, for example, a simple mea sure of physiological state, such as basal
metabolic rate. Th is is the minimal rate of energy expenditure per unit
time by individuals at rest. In humans there are big individual diff erences
in basal metabolic rate. One study of one hundred and fi ft y adults in
Scotland reported basal metabolic rates between 1,027 and 2,499 kilocal-
ories per day. Th e researchers calculated that 62.3 percent of this varia-
tion was explained by diff erences in fat- free body mass (i.e., bigger bodies
use more calories). Variation in fat mass accounted for 6.7 percent and
age for 1.7 percent. Th e rest of the variation (26.7 percent) was unexplained,
but it was not due to sex diff erences or the size of diff er ent energy-
demanding organs, like the brain.^24
Estimates of how much of the variation in such physiological functions
is associated with variable genes (i.e., the heritability) have, in humans,
relied on the twin method. As described in chapter 2, such estimates are
unreliable, because they include unknown amounts of environmental
and interactive sources of variance (mislabeled as “ge ne tic”). Estimates
of the heritability of basal metabolic rate in animals tend to be small,
although subject to mea sure ment and other diffi culties.^25 However, this
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