Jensen redux

Mike Cole (mcole who-is-at weber.ucsd.edu)
Sun, 28 Nov 1999 10:31:00 -0800 (PST)

>From: PSYCOLOQUY <journal who-is-at princeton.edu>
Subject: psyc.99.10.049.intelligence-g-factor.6.tan (166 lines)
To: PSYC who-is-at PUCC.PRINCETON.EDU
Status: RO

psycoloquy.99.10.049.intelligence-g-factor.6.tan Sun Nov 28 1999
ISSN 1055-0143 (5 paragraphs, 13 references, 160 lines)
PSYCOLOQUY is sponsored by the American Psychological Association (APA)
Copyright 1999 Uner Tan

THE BIOLOGICAL CORRELATES OF THE G FACTOR
Book review of Jensen on Intelligence-g-Factor

Uner Tan
Department of Physiology
Medical Faculty, Blacksea Technical University
Trabzon, Turkey
unertan who-is-at mailcity.com
http://www.ktu.edu.tr/jensenbook.txt

ABSTRACT: The biological correlates of g (brain size, evoked
potentials, and sex hormones) analyzed by Jensen (1998, 1999) are
critically considered in the light of the current literature.

1. Jensen (1998) frequently accentuates the biological origins of the g
Factor in the book under review and concludes in the abstract of the
Precis (Jensen, 1999) that "Psychometric g has more direct biological
correlates than any other independent source of test variance, for
example, brain size, brain evoked potentials, nerve conduction
velocity, and the brain's glucose metabolic rate during cognitive
activity... The current frontier of g research is the investigation of
the anatomical and physiological features of the brain that cause g". I
will review Jensen's book in this biological context.

2. HEAD SIZE AND BRAIN SIZE (Chapter 6, pp. 146-149). Jensen devotes a
relatively detailed section to the relation of IQ to head and brain
size. The quintessence of this section is that brain size correlates
significantly with IQ. Jensen emphasizes that there are at present
eight MRI studies indicating a significant correlation between brain
volume and IQ, which is close to +.40 after removing variance due to
body size. Interestingly, we have also found an overall correlation of
+.40 between MRI-measured total cerebral area (midsagittal section) and
IQ, thereby further supporting the IQ-brain size hypothesis (Tan et
al., 1999). On the other hand, we found in the same study that there
was no significant correlation between IQ and MRI-measured cerebral
area in right-handed men; although there was no sex difference in IQ,
women had smaller cerebral areas than men, as frequently reported by
others for IQ-brain size relations. This issue is nicely discussed in
Jensen`s Book (p. 149). Nevertheless, Jensen does not mention a
modification in IQ-brain size relation. Unfortunately, he largely
neglects the studies inconsistent with the IQ-brain size hypothesis
(see for instance Cain & Vanderwolf, 1990; Peters et al., 1998).

3. AVERAGE EVOKED POTENTIAL (AEP) CORRELATES OF G. Jensen's arguments
about the importance of cortical evoked potentials for IQ have already
been critically considered by Verleger (1999) in this journal, with the
conclusion that "it appears that these results do not support the idea
that ERPs provide a simple biological basis for the measurement of
intelligence". Jensen (1998, p. 153) concludes that higher IQ subjects
exert shorter latencies in AEPs (no reference), without providing
evidence. This conclusion is not consistent with the current
literature. For instance, Tan (1992) reported that N49 latency
(somatosensory AEP) has a negative linear relation to IQ in females,
but no correlation was found for males, indicating the importance of
sex difference in neural speed hypothesis. Moreover, Tan et al. (1993)
have reported that the N1 and P1 latencies of visual evoked potentials
correlated inversely with IQ, but only for the right side of the
brain. The H-reflex latency was inversely correlated with IQ, but this
correlation was not always observed: it occurred in left-handers only
for the left side and in right-handers only for the right side (Tan,
1991). These findings suggest that the simple hypothesis that speed is
one of the biological correlates of g does not seem to be tenable. Yet
Jensen believes that " higher IQ subjects show shorter latencies
(faster neural reaction)".

4. SEX DIFFERENCES IN G (Chapter 13). The visuo-spatial abilities
favoring males have the largest and most consistent sex differences in
any psychometric abilities (Jensen`s book, p. 533). Jensen states that
"Generally, females with markedly above-average testosterone levels
(for females) and males with below-average levels of testosterone (for
males) tend to have higher levels of spatial ability, relative to the
average spatial ability for their own sex". Indeed, it seems to be well
established that males outperform females in spatial abilities. If so,
the male hormone testosterone (T) should be beneficial for this kind of
cognitive function (cf. Mazur & Booth 1998). It has frequently been
reported that T may be advantageous for spatial abilities in animals
and humans (see Tan & Tan, 1998). There are also studies -- not
mentioned by Jensen -- indicating that serum T level is correlated with
spatial IQ in men (Tan, 1990; Tan & Akgun, 1992; Tan, 1998). Hence it
is not justified to conclude that low T levels are associated with high
spatial abilities in men.

5. CONCLUSIONS. My review has attempted to discuss the biological
correlates of g summarized in Arthur Jensen`s book. Jensen emphasizes
in his abstract that "the current frontier of g research is the
investigation of the anatomical and physiological features of the brain
that cause g", and "psychometric g also has more direct biological
correlates than any other independent source of test variance, for
example brain size, brain evoked potentials, nerve conduction
velocity". Despite this main aim, only one chapter (Chapter 5, pp.
137-169) is devoted to the biological correlates of g. Here it seemed
to me more as if the author were expressing his own subjective ideas
rather than objective scientific observations and conclusions.

REFERENCES

Cain, D.P., & Vanderwolf, C.H. (1990) A critique of Rushton on race,
brain size and intelligence. Personality and Individual Differences,
11: 777-784. Jensen,

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).
ftp://ftp.princeton.edu/pub/harnad/Psycoloquy/1999.volume.10/
psyc.99.10.023.intelligence-g-factor.1.jensen
http://www.cogsci.soton.ac.uk/cgi/psyc/newpsy?10.23

Mazur, A., & Booth, A. (1998). Testosterone and dominance in men.
Behavioral and Brain Sciences, 21: 353-398.
http://www.cogsci.soton.ac.uk/bbs/Archive/bbs.mazur.html

Peters, M., Jancke, L., Staiger, J.F., Schlaug, G., Huang, Y., &
Steinmetz, H. (1998) Unsolved problems in comparing brain sizes in Homo
sapiens. Brain and Cognition, 37: 254-285.

Tan, U. (1990) Testosterone and nonverbal intelligence in right-handed
men and women. International Journal of Neuroscience, 54: 277-282.

Tan, U. (1991) The inverse relationship between nonverbal intelligence
and the latency of the Hoffmann reflex from the right and left thenar
muscles in right- and left-handed subjects. International Journal of
Neuroscience, 57: 219-238.

Tan, U. (1992) Relation of nonverbal intelligence assessed by Cattell`s
Culture Fair Intelligence Test to latencies of the somatosensory evoked
potentials elicited by stimulation of the posterior tibial nerves in
right-handed male and female subjects. International Journal of
Neuroscience, 64: 107-112.

Tan, U., & Akgun, A. (1992) There is a direct relationship between
nonverbal intelligence and serum testosterone level in young men.
International Journal of Neuroscience, 64: 213-216.

Tan, U., Akgun, A., Komsuoglu, S., Telatar, M. (1993) Inverse
relationship between nonverbal intelligence and the parameters of
pattern reversal visual evoked potentials in left-handed male subjects:
importance of right brain and testosterone. International Journal of
Neuroscience, 71: 189-200.

Tan, U., & Tan, M. (1998). Curvilinear correlations between total
testosterone levels and fluid intelligence in men and women.
International Journal of Neuroscience, 95: 77-83.

Tan, U., Tan, M., Polat, P., Ceylan, Y., Suma, S., and Okur, A. (1999)
Magnetic Resonance Imaging Brain Size/IQ Relations in Turkish
University Students. To appear in Intelligence, 27: 83-92.

Verleger, R. (1999) The g factor and event-related EEG potentials. Book
Review of Jensen on Intelligence-g-Factor. PSYCOLOQUY 10(39).
ftp://ftp.princeton.edu/pub/harnad/Psycoloquy/1999.volume.10/
psyc.99.10.039.intelligence-g-factor.2.verleger
http://www.cogsci.soton.ac.uk/cgi/psyc/newpsy?10.39