MANKIND QUARTERLY 2016 57:1 5-8
Sex Differences in Intelligence
This special issue presents a number of papers challenging the widespread
consensus that there is no sex difference in general intelligence defined as the
IQ obtained from tests of a suite of reasoning, verbal, perceptual, memory and
spatial abilities like the Wechsler, Stanford-Binet and numerous others. This
position was asserted a century ago by Terman (1916) in reporting his
standardization of the Stanford-Binet in the United States in which he wrote “the
superiority of girls over boys is so slight … that for practical purposes it would
seem negligible.” This conclusion was frequently reasserted in the second half of
the twentieth century. Typical conclusions by leading authorities were those of
Cattell (1971, p. 131): “it is now demonstrated by countless and large samples
that on the two main general cognitive abilities – fluid and crystallized intelligence
– men and women, boys and girls, show no significant differences”; Brody (1992,
p. 323): “gender differences in general intelligence are small and virtually non-
existent”; and Jensen (1980, p. 360): “the sex difference in psychometric gis
either totally non-existent or is of uncertain direction and of inconsequential
magnitude.” Others who stated the same conclusion included Hutt (1972, p. 88),
Maccoby and Jacklin (1974, p. 65), Eysenck (1981, p. 40), and Herrnstein and
Murray (1994, p. 275).
This consensus was disputed by Lynn (1994), who advanced a
developmental theory of sex differences in intelligence stating that while there is
virtually no sex difference up to the age of 16 years, from this age onwards males
develop an advantage that increases with age reaching approximately 4 IQ points
among adults (Lynn, 1994). Further data documenting this male advantage was
given in Lynn (1999) and in a meta-analysis of sex differences on the Progressive
Matrices by Lynn and Irwing (2004) concluding that among adults males obtain a
5 points higher IQ than females.
The most frequent reaction to Lynn's theory has been to ignore it and
continue to assert that there is no sex difference in intelligence. For instance:
“Women’s brains are 10% smaller than men’s, but their IQ is on average the
same” (Butterworth, 1999, p. 293); “It is an important finding of intelligence testing
that there is no difference between the sexes in average intellectual ability; this is
true whether general ability is defined as an IQ score calculated from an omnibus
test of intellectual abilities such as the various Wechsler tests, or whether it is
MANKIND QUARTERLY 2016 57:1
defined as a score on a single test of general intelligence, such as Raven’s
Matrices... the evidence that there is no sex difference in general ability is
overwhelming” (Anderson, 2004, p. 829); “Men and women have equal cognitive
capacity” (Speke, 2007, p. 65); “There appears to be no sex difference in general
intelligence; claims that men are more intelligent than women are not supported
by experimental data” (Hines, 2007, p. 103); “General intelligence does not differ
between men and women” (Haier, 2007); “There is no difference in intelligence
between males and females…overall, the sexes are equally smart” (Halpern,
2007, p. 123); “The two sexes do not differ consistently in average IQ”
(Mackintosh, 2011, p. 380); “Women tend to do better than men on verbal
measures, and men tend to outperform women on tests of spatial ability; these
small differences balance out so that the average general score is the same”
(Ritchie, 2015, p. 105); “In adulthood, there is scant evidence for sex differences
in g, although women tend to perform better than men in verbal tasks, whilst men
outperform women slightly in spatial tasks” (Cooper, 2015, p. 207).
These quotations illustrate that there are two questions that need to be
examined. One is the possibility that there is a sex difference in the general ability
factor g, which is technically defined as the unrotated first factor or principal
component of a factor analysis or principal components analysis and is very close
to the IQ calculated from a battery of diverse tests. The other is the possibility that
there are sex differences on specialized abilities rather than on g, such that
women excel in some abilities and men in others. The likely presence of both
kinds of difference adds complexity to the question, and explains the often
inconsistent results that studies of sex differences at the latent factor level have
produced in the past (e.g., Keith et al., 2008; Lemos et al., 2013).
The ongoing debate about sex differences is based almost exclusively on
results from modern Western societies. It does not take account of the possibility
that there could be systematic differences between countries with different school
systems, cultural traditions, and gender roles. Thus there is an urgent need to
expand the evidence base on which theories about sex differences are built by
including results from a greater variety of countries.
Filling this gap is the aim of this special issue of Mankind Quarterly, which
presents new results about sex differences in Brazil, Chile, the United States,
Sweden, Russia, Korea, Taiwan, Thailand, Egypt, Libya, Sudan and Yemen.
Most results show sex differences in “general” ability to be small, and most show
marginal to solid male advantages. Those who expect large and systematic
differences between countries in the size, direction or pattern of sex differences
may be disappointed. The results indicate that such differences between
countries and world regions, if present, are not of large magnitude and may be
difficult to demonstrate in more systematic surveys of the literature.
However, this “null” result is itself interesting because it suggests that cultural
and in particular educational and ideological forces have limited ability in shaping
sex differences in the levels and patterns of ability and achievement. For
example, if it is considered desirable to masculinize female ability and
achievement patterns by raising women’s mechanical comprehension and
reducing their psychomotor speed and social skills to male levels, education and
indoctrination may not be sufficient. Something else may be required. One option
would be prenatal testosterone treatment for female fetuses. Clinical studies
suggest that this may actually work (Hampson, Rovet & Altmann, 1998; Hier &
Anderson, M. (2004). Sex differences in general intelligence. In: R.L. Gregory (ed.), The
Oxford Companion to the Mind. Oxford: Oxford University Press.
Brody, N. (1992). Intelligence. San Diego, CA: Academic Press.
Butterworth, B. (1999). The Mathematical Brain. London: Macmillan.
Cattell, R.B. (1971). Abilities: Their Structure, Growth and Action. Boston: Houghton
Cooper, C. (2015). Intelligence and Human Abilities. London: Routledge.
Eysenck, H.J. (1981). In H.J. Eysenck and L. Kamin: Intelligence: The Battle for the Mind:
H.J. Eysenck versus Leon Kamin, pp. 11-89. London: Pan.
Haier, R. (2007). Brains, bias, and biology: Follow the data. In: S.J. Ceci & W.M. Williams
(eds.), Why Aren’t There More Women in Science? Washington, D.C.: American
Halpern, D. (2007). Science, sex and good sense: Why women are underrepresented in
some areas of science and math. In S.J.Ceci & W.M.Williams (eds.), Why Aren’t There
More Women in Science? Washington, D.C.: American Psychological Association.
Hampson, E., Rovet, J.F. & Altmann, D. (1998). Spatial reasoning in children with
congenital adrenal hyperplasia due to 21-hydroxylase deficiency. Developmental
Neuropsychology 14: 299-320.
Herrnstein, R. & Murray, C. (1994). The Bell Curve. New York: Random House.
MANKIND QUARTERLY 2016 57:1
Hier, D.B. & Crowley, W.F. (1982). Spatial ability in androgen-deficient men. New England
Journal of Medicine 306: 1202-1205.
Hines, M. (2007). Do sex differences in cognition cause the shortage of women in
science? In: S.J. Ceci & W.M. Williams (eds.), Why Aren’t There More Women in
Science? Washington, D.C.: American Psychological Association.
Hutt, C. (1972). Males and Females. Harmondsworth, UK: Penguin Books.
Jensen, A.R. (1980). Bias in Mental Testing. London: Methuen.
Keith, T.Z., Reynolds, M.R., Patel, P.G. & Ridley, K.P. (2008). Sex differences in latent
cognitive abilities ages 6 to 59: Evidence from the Woodcock-Johnson III tests of cognitive
abilities. Intelligence 36: 502-525.
Lemos, G.C., Abad, F.J., Almeida, L.S. & Colom, R. (2013). Sex differences on gand
non-gintellectual performance reveal potential sources of STEM discrepancies.
Intelligence 41: 11-18.
Lynn, R. (1994). Sex differences in brain size and intelligence: A paradox resolved.
Personality and Individual Differences 17: 257-271.
Lynn, R. (1999). Sex differences in intelligence and brain size: A developmental theory.
Intelligence 27: 1-12.
Lynn, R. & Irwing, P. (2004). Sex differences on the Progressive Matrices: A meta-
analysis. Intelligence 32: 481-498.
Maccoby, E.E. & Jacklin, C.N. (1974). The Psychology of Sex Differences. Stanford, CA:
Stanford University Press.
Mackintosh, N.J. (2011). IQ and Human Intelligence, 2nd edition. Oxford: University Press.
Ritchie, S. (2015). Intelligence. London: John Murray Learning.
Speke, E. (2007). Sex, math and science. In: S.J. Ceci & W.M. Williams (eds): Why Aren’t
There More Women in Science? Washington, D.C.: American Psychological Association.
Terman, L.M. (1916). The Measurement of Intelligence. Boston, MA: Houghton Mifflin.