[evol-psych] Raymond: Book review of Jensen on Intelligence-g-Factor
- psycoloquy.99.10.057.intelligence-g-factor.7.raymond Sat Dec 4 1999
ISSN 1055-0143 (10 paragraphs, 9 references, 235 lines)
PSYCOLOQUY is sponsored by the American Psychological Association (APA)
Copyright 1999 B. Raymond
BIOLOGICAL DETERMINISM UNWARRANTED
Book review of Jensen on Intelligence-g-Factor
Institute of Virology and Environmental Microbiology
University of Oxford
Oxford OX1 3SR
ABSTRACT: Jensen (1998) does not present any evidence for the
external validity of g other than through biological correlations
with IQ. Hence it is impossible to assess the validity of g in
comparison with competing theories from this book. The author's
interpretation of heritability estimates of IQ and of racial
differences in IQ adoption studies ignores any potential maternal
effects on IQ. His insistence that the primary cause of variation
in IQ within and between races is genetic does not seem warranted
by the evidence presented.
1. Jensen (1998, 1999) makes a wide ranging series of claims about the
nature of intelligence, many of them controversial and potentially
socially divisive. I would expect any treatment of sensitive issues
such as the differences in IQ scores between races to be especially
rigorous and objective. In this, the book under review is
disappointing. I find examples of poor understanding of evolution,
over-eager assumptions of causality, selective presentation and
interpretation of data, and conclusions that are hard to warrant in the
light of the evidence presented.
2. It is difficult to extract any concise definition of g itself from
the book. The best was "g is created from the correlations among second
order factors, whose residual variance consists of whatever variance
they do not have in common with each other". My understanding of g and
factor analysis is that it is an attempt to reduce the
multidimensionality of a battery of mental test results to one
dimension which best represents the common variance shared between
tests. As such, it is a mathematical abstraction, and it is not
possible on theoretical grounds to dismiss other theories of
intelligence (e.g., Thurstone's simple structure) which represent an
equal amount of information from the data on more than one dimension.
Spearman's original method of tetrad differences for testing for g was
abandoned precisely because it often demonstrated that it was difficult
to represent mental tests on one dimension (see p. 75). A hierarchical
analysis which correlates these first order factors (e.g. spatial,
verbal, and memory first order factors), avoids this problem only
because these factors also tend to be correlated.
3. Some inconsistencies also arise when g is used as a fundamental
theory of intelligence; for example, the increase in IQ (a heavily g
loaded parameter) over the last 50 years has been matched by a decrease
in scholastic ability (another highly g loaded indicator; p. 322). Has
g both increased and decreased simultaneously? Jensen blames this
inconsistency on the use of different 'vehicles', yet there is no
direct means of measuring g other than through 'vehicles' of one form
or another. Jensen also indicates that g 'typically' accounts 'for a
larger proportion of the total variance than any other factor and often
accounts for more of the variance than all of the other factors
combined' (p. 79). Exactly what 'typically' or 'often' represents is
not discussed further, despite being fundamental to the justification
of the method. Furthermore, in one cited African study on elementary
cognitive tasks (p. 392) the largest common factor among tests is
described as being possibly something other than g. How is it possible
to make this distinction, unless the author is assuming an existence of
g external to the test results?
4. An assertion that g has validity beyond a mathematical abstraction
requires that it be connected to biological reality, and moreover
associated with biology in such a way that it has increased explanatory
power relative to competing theories. This connection to basic biology
is based on correlations with "stature, head size, brain size,
frequency of alpha brain waves, latency of evoked potentials, rate of
brain glucose metabolism, and general health" (p. 137). First,
correlations themselves prove no causal link between g and biology, and
the separation (p. 130-143) of correlations into intrinsic (causal) and
extrinsic (spurious) is simply not valid. Correlation with body size is
dismissed by Jensen himself as not causal (p. 146). EEG is correlated
with IQ; but IQ, even if it is strongly correlated with g, is not g,
and we cannot assess whether alternative theories of intelligence
describe this relationship better (e.g., whether higher g-loaded
measures of mental ability are better correlated with EEG results).
Cerebral glucose metabolism, head size, and nerve conduction are also
only correlated with IQ. Thus, Jensen presents no evidence of causal
links, nor any evidence that g is better correlated with basic biology
than multi-dimensional theories of intelligence.
5. The study of brain size in relation to race has historically been
controversial and riddled with error and bias (Gould, 1981). Figure
12.4 does nothing to relieve my admittedly pre-conceived scepticism
concerning this issue. The high correlation coefficient is cited but
not the significance value (p = 0.037, which is just significant).
Moreover, the plotted values of cranial capacity are not those
corrected for body size; the correlation is not significant (p > 0.05)
when the corrected values are used. The method of deriving IQ/race
medians from means is obscure, and no reason for preferring means over
medians is given. The black IQ of 80 is surprisingly low given the
American mean of 85. There may well be evidence for racial difference
in brain size, but the best study of adults cited is not described as
controlling for obvious but potentially confounding environmental
effects such as alcohol use (Ho et al., 1980). There are well
established differences in brain size between males and females,
without significant differences in IQ, and this is accounted for by
differences in neuronal packing density (p. 438). Since this factor has
not been studied in relation to racial differences, sweeping causal
conclusions seem premature.
6. Jensen's chapter on the heritability of g fails to address a recent
meta-analysis of 212 IQ on this topic. That paper estimates that 20% of
the covariance in twins IQ can be explained by maternal effects and
that broad heritability is 48% (Devlin et al., 1997). Thus, the shared
environment effects on twins reared apart are underestimated by Jensen
to give inflated values of heritability. Furthermore, the meta-analysis
supported a maternal effects model better than models which assume
heritability increases with age. The types of maternal effects
associated with IQ changes described in the literature include:
dietary supplements (Harell et al., 1955) and PCBs (Jacobson &
Jacobson, 1996). Moderate alcohol consumption of mothers can affect
psychomotor development (Larroque et al., 1995); and polycyclic
aromatic hydrocarbons are associated with reduced head circumference of
newborns (Perera et al., 1999). It is not unreasonable to hypothesize
that a more socially disadvantaged group will be subject to poorer
diet, increased levels of environmental pollutants, as well as
increased drug and alcohol use.
7. This section also includes some fundamental errors in evolutionary
understanding. The assertion that "traits that show genetic dominance
provide evidence that they have been subjected to natural selection"
(p. 170) is not correct. Recessive traits, and purely additive traits
can also be subject to natural selection, and dominant alleles may be
selectively neutral. The fact that a trait is heritable and variable
means that it is potentially subject to natural selection, but even
this does not mean we can infer selection; other evolutionary forces
such as mutation and genetic drift can be responsible for changes in
gene frequency. In this light, the discussion relating dominance to
hybrid vigour and inbreeding depression is misguided (p. 189-197).
8. Jensen's "default hypothesis" is that differences in white and black
IQ scores are made up of genetic and environmental effects and that
environmental effects are often small relative to genetic effects (e.g.
pp. 177-175, 475, 476, 489). These conclusions seem best supported by
estimates of heritability, discussed above, and from the Minnesota
transracial adoption study. This adoption study has several flaws.
Maternal effects were not considered as a serious possibility for the
lower IQ of blacks relative to whites. The IQ of parents of adopted
children was unknown. The mean age of adopted infants also differed
between treatment groups. And most fundamentally, the parents of
children were not selected randomly from the population. Infants of
parents of varying socioeconomic status and race might give children up
for adoption, or have them taken into care, for very different reasons.
In contrast, Jensen dismisses data from a racial admixture study in
Germany which found no significant or consistent differences in IQ with
race, because parental IQ was unknown, and because white and black
fathers were not randomly sampled, two faults shared with the Minnesota
adoption study. The absence of differences in the German study is also
consistent with a maternal effects hypothesis, since the mothers were
9. Within the "g factor" there is abundant evidence for various
environmental correlates of IQ, and for effects which may be specific
to the black population. On p. 385 a study that controlled for
socio-economic status reduced the g factor difference between American
blacks and whites by 12 IQ points. On p. 513, "stereotype threat", a
form of test anxiety, could account for a 5 IQ point difference between
black and white college students. 12 plus 5 is 17, and the mean
difference between blacks and whites in the US is only 15 IQ points.
There are also two strong patterns which are highly suggestive of
environmental effects, namely, the steady increase in IQ scores of 3 IQ
points per decade in the last half century (p. 307), and the strong
geographical gradient in IQ among blacks from south to north. For
example, a group of black schoolchildren in rural Georgia has a mean IQ
of 71, and Jensen's comments are "we would be hard put to find a more
socially disadvantaged black community... anywhere in the United
States". Whereas the black mean in Minnesota is given as 90. The most
poorly founded conclusion in the entire book also concerns
environmental effects, namely that 'mother's education alone account[s]
for 13% of the childrens' IQ variance, but this is most likely a
genetic effect' (p. 502). Given these potentially numerous and powerful
effects, Jensen's insistence on the primacy of genes is surprising.
10. Overall, the book leaves an impression of biological determinism;
the correlation between IQ and socioeconomic status is consistently
presented as resulting from the causal effects of IQ. Jensen paints a
picture of America as a country with perfect social mobility, where an
absence of racial prejudice is effectively demonstrated by the
overrepresentation of blacks in jobs of high socioeconomic status
relative to their distribution of IQ (p. 568), despite the much lower
correlation between IQ and socioeconomic status for blacks relative to
whites (p. 358). This is not an impression that rings true.
Devlin, D., Daniels, M., & Roeder K., 1997. The heritability of IQ.
Nature 388: 468-471.
Gould, S.J., 1981. The Mismeasure of Man. Penguin, London.
Harrell, R.F., Woodyard, E., & Gates, A.I., 1955. The effects of
mothers diet on the intelligence of offspring. Teacher's College, New
Ho, K-c., Roessman, U., Stramfjord, J.V., & Monroe, G., 1980. Analysis
of brain weight: II. Adult brain weight in relation to body height,
weight , and surface area. Archives of Pathology and Laboratory
Jacobson, J.L., & Jacobson, S.W., 1996. Intellectual impairment in
children exposed to polychlorinated biphenyls in utero. New England
Journal of Medicine 335:783-789.
Jensen, A. (1998) The g Factor: The Science of Mental Ability. Praeger
Jensen, A. (1999) Precis of: "The g Factor: The Science of Mental
Ability" PSYCOLOQUY 10 (23).
Larroque, B., Kaminski, M., Dehaene, P., Subtil, P., Delfosse, M.J., &
Querleu, D., 1995. Moderate prenatal alcohol exposure and psychomotor
development at preschool age. American Journal of Public Health 85:
Perera, F.P., Jedrychowski, W., Rauh, V., & Whyatt, R.M., 1999.
Molecular epidemiological research on the effects of environmental
pollutants on the fetus. Environmental Health Perspectives 107: