Sex Differences in the Distribution of Mental Ability

[Roger L. Bagula]

Sex Differences in the Distribution of Mental Ability

Kevin Langdon and David Seaborg

Published in Noesis #144, November 1999
Copyright © 1999 by Kevin Langdon and David Seaborg

An earlier version of this article (by Kevin Langdon) was
published in Vidya #155/156 (August 1996). This version
appeared in Gift of Fire #100, October 1998 (a few further
slight modifications were made for publication in Noesis and
on the Web).

Dr. Arthur Jensen s Bias in Mental Testing explores the question of
male/female differences in ability in a very interesting section of
Chapter 13. Jensen cited studies indicating a difference of
approximately one IQ point in the standard deviation of males and
females in general intelligence, with males having the greater variability.

Differences in general population means are less clear-cut. The
following table is derived from Table 13.1 in Bias in Mental Testing:

Number of Studies Showing Significant (p<.05) Difference in Favor of:

Type of Test Neither Males Females
General Intelligence 40 3 15
Verbal Ability 81 13 37
Quantitative Ability 15 16 4
Visual-Spatial Ability (nonanalytic) 24 9 2
Visual-Spatial Ability (analytic) 35 25 3

All the studies made use of tests published since 1966. Jensen defined
analytic visual-spatial ability tests as those which require
analysis, that is, mentally breaking up a gestalt into smaller units in
ways that facilitate spatial problem solving.

According to Jensen:

[T]he sample sizes are generally adequate for even quite small
differences, equivalent to a tenth of a standard deviation or less,
to show up as statistically significant.

Jensen added a note of caution:

The most widely used standardized tests of general intelligence have
explicitly tried to minimize sex differences in total score by
discarding those items that show the largest sex differences in the
normative sample and by counterbalancing the number of remaining
items that favor either sex. This is true, for example, of the
Stanford-Binet and Wechsler scales of intelligence. Such tests,
therefore, obviously cannot be used to answer the question of
whether there is in fact a true difference between males and females
in general intelligence.

The table below is adapted from Table 2 of an article in the July 7,
1995 issue of Science, Sex Differences in Mental Test Scores,
Variability, and Numbers of High-Scoring Individuals, by Larry V.
Hedges and Amy Nowell, both in the Department of Education at the
University of Chicago.

Subject Area Difference of Means Ratio of Variance Ratio of 90-Plus
Scores Ratio of 95-Plus Scores

Reading Comprehension

-.15 1.16 0.90 1.00
-.05 1.03 0.94
-.18 1.16 0.83
.00 1.10 1.03 1.06
-.09 1.16 0.80 0.83

Vocabulary

.25 1.05 1.57 1.50
-.06 1.00 0.89 0.87
-.03 1.08 0.87
.07 1.05 1.06 1.06

Mathematics

.12 1.20 1.33 1.50
.24 1.05 1.76
.26 1.25 1.90 2.20
.08 1.19 1.70 1.90
.22 1.16 1.67 2.06
.03 1.06 1.34 1.64

Nonverbal Reasoning

.04 1.04 1.09 1.00
-.22 1.15 0.74 0.67

Spatial Ability

.13 1.27 1.86 2.33
.25 1.27 1.90 2.39

Differences are male minus female means, in general population standard
deviation units. All ratios are male/female; 90 and 95 refer to the 90th
and 95th percentiles.

One striking pattern that appears here is the strong advantage of males
over females in mathematical and (especially) spatial reasoning.

Among . . . mathematically gifted 13-year-olds, [sex] differences
favour males in mathematical reasoning ability, but not in verbal
reasoning, where there are no [sex] differences. Our gifted males
score approximately one half of a standard deviation higher than
females on the SAT-M, our measure of mathematical reasoning. Males
SAT-M scores are also more dispersed, yielding an upwardly shifted
distribution of male scores (Benbow, Behavioral & Brain Sci. 11,
1988). The resulting proportion of males and females at age 13 at
various cut-off scores on SAT-M is approximately as follows: 500
(average score of college-bound 12th-grade [18-year-old] males):
2:1; 600: 4:1; 700 (top 1 in 10,000 for 7th graders [13-year-olds]):
13:1. These ratios have remained relatively stable over the past 20
years, and have now been observed among mathematically gifted
students in the 3rd grade (8-year-olds), and cross-culturally
(though they are smaller in Asian populations).
Camilla P. Benbow and D. Lubinski, 1993, in Ciba Foundation
Symposium 178,
The Origins and Development of High Ability

Although controversy exists about the magnitude of the sex
difference in spatial ability under various testing conditions,
reviews by Pool (Eve s Rib, 1994) and Voyer et al. (Psychological
Bulletin 117, 1995) show that on the purest spatial measures, such
as rotating an imaginary object, or shooting at a moving rather than
a stationary target, the sex difference approaches one standard
deviation.
J.P. Rushton and C.D. Ankney, Psychonomic Bulletin & Review, 1996

. . . it took five years of extensive nation-wide search to find 36
extremely mathematically talented girls.
Camilla P. Benbow, Neuropsychologia 24, 1986

The mean variance ratio in the table is 1.13, indicating a difference of
approximately two points of IQ in standard deviation between men and
women, twice Jensen s figure.

Norming data for the Langdon tests show an even greater difference in
favor of males. In populations with average IQ s of approximately 140
(sigma=16), the following differences were observed:

Test/Norming LAIT (Norming #2) LAIT (Omni Sample) PIAS
N 553 20,000 1464
Male Mean 145.7 137.5 142.0
Female Mean 139.3 132.0 131.5
Difference 6.4 5.5 11.5

Calculations based on the above data give figures ranging from 2.2 to
4.6 for the difference in standard deviation between men and women. The
PIAS data may be less reliable than that for the LAIT, which accords
more closely with the lower figures mentioned above.

Data from Ronald K. Hoeflin s Mega Test provides additional evidence for
a high level of male/female difference in standard deviation, based on a
far-right-tail population similar to those attempting the Langdon tests.
According to data contained in a letter from Dr. Hoeflin to Kevin
Langdon dated December 4, 1985, ten times as high a percentage of those
scoring 5 or below (out of 48) on the Mega Test were women than of those
scoring 30 or above.

One possible factor in the observed male/female differences on the
Langdon and Hoeflin tests is that both include spatial elements, but
neither is highly loaded on the spatial factor, which has to do
specifically with visualizing the rotation of objects in two- and
three-dimensional space.

At this point, we may ask why the observed differences in variance exist.

We are diploid organisms, like all higher animals. Unlike haploid
organisms, which have only one set of chromosomes, we have two parallel
sets, with 23 chromosomes in each set. Each chromosome has one or
another allele (alternate form) for every gene it carries; our 23
chromosome pairs are the vehicle for the approximately 100,000 genes in
the human genome. (Many plant species alternate between haploid and
diploid generations; in social insects, males are haploid.)

Diploid organisms shuffle the alleles passed on to the next generation
through sexual reproduction, in which genetic material is systematically
varied. A further refinement is what geneticists call dominance. Certain
alleles mask the effect of others, which are called recessive. The
alleles of other genes are additive, resulting in an intermediate
phenotypic expression in heterozygotes. (E.g., breeding a Manx cat with
an ordinary, long-tailed cat produces stubby-tailed offspring.)

The following quotation is from a Web page entitled Diploidy and
Dominance, by Deborah Stacey, Associate Professor of Computing and
Information Science, University of Guelph.
<http://hebb.cis.uoguelph.ca/~deb/27662/Lectures/diploidy.html
<http://hebb.cis.uoguelph.ca/%7Edeb/27662/Lectures/diploidy.html>>:

A famous biological example is that of the peppered moth in Great
Britain. The original, dominant form of the moth had white wings
with small black specks which functioned as camouflage against its
habitat of lichen-covered trees. During the Industrial Revolution,
black forms of the moth started to dominate the population.
Industrial pollution had killed off the lichen covering the trees in
the moth s habitat. The new black form thus had a survival advantage
over the speckled version. The colour was controlled by a single
gene. And not only had the population percentage changed dominance
among alleles had changed also the black allele was now dominant.
When the population balance had shifted towards the dark form, it
became dominant and the speckled form was held in abeyance. The
black version of the moth was not new it had been invented earlier
and was only expressed when the environment had changed. This
demonstrates that diploidy and dominance can permit alternative
solutions to be shielded against overselection. It also demonstrates
that dominance is not an absolute state of affairs dominance can
evolve as well.

When an organism carries identical alleles of a given gene, it is said
to be homozygous for that gene; otherwise it is said to be heterozygous.
The pattern of alleles for a given organism is its genotype. Its pattern
of externally-expressed genetic factors is its phenotype. Recessive
alleles are not expressed phenotypically unless they are homozygous.
This permits organisms to retain alleles that are not favorable if
expressed in existing conditions; this genetic material can be drawn on
in adapting to changing conditions.

As most of the genes on the X chromosome do not have a corresponding
locus on the Y chromosome, males are effectively haploid for the sex
chromosome pair; recessive alleles of these genes, including nature s
more advanced experiments and (far more numerous) lethal and
debilitating mutations, are much more likely to be expressed in males,
giving rise to the observed greater variability in males on various
measurable traits, including g. This is adaptive for the species, as
males are expendable; the size of the next generation depends on the
number of females that survive long enough to reproduce.

In every society, human and animal, males do the fighting. This is
not only because of their greater physical strength, but it is due
also to the most basic requirement of a species: survival of the
next generation. In a population of ten males and ten females, if
nine females died in battle, only two or three children could
possibly be born to the group the next year. . . . So males are more
expendable. Robert E. Ornstein, Psychology: The Study of Human
Experience. San Diego: Harcourt Brace Jovanovich, 1985.

The vast majority of animal species are polygynous. In a polygynous
species, it is greatly to the male s advantage to be more variable in
general, because only a few of the males the most fit, the best
competitors get all (or substantially all) of the females, while the
rest of the males are left out in the cold. Gambling on a long-shot
mutation is a strategy with a high expected return under these
circumstances, as a male of only average fitness will not mate and pass
on his genes anyway. A successful mutant could become an alpha male and
mate with many females. We recognize that there is another, sneaky
mating strategy used by the males of some species. Although unable to
compete as alpha males, lower-status males may still find opportunities
to mate when the alphas aren t looking. This may reduce the advantage of
variability, but cannot eliminate it, in polygynous species.

Birds, for some reason, have evolved in the opposite direction; it is
the male who has a matched pair of sex chromosomes and the female in
whom one chromosome is incomplete.

Dinosaurs, from which birds are believed to have evolved, were
ground-nesters, as are certain birds today. Many ground-nesting species
are polyandrous; the cause of this may be that only the cooperative
efforts of a group can protect their nests effectively. Male birds are
locked into this evolutionary strategy, even though, as most are
polygynous or monogamous, this isn t the best adaptive stategy for them.

Butterflies and moths are another exception to the general rule, as are
snakes and monitor lizards (from which snakes are believed to have
evolved). The hetero-gametic sex in some fly families of the order
Diptera is the male and in other families the female. This is also the
case for some other orders of insects and also for frogs and toads.
Also, within some families of insects and lizards there are differences
between genera. The inconsistent pattern here does not support the more
general conclusions suggested above, but there may still be some merit
in our argument with respect to mammals and birds; other considerations
may predominate in other cases.

Intelligence: Essays and Reviews
<http://www.polymath-systems.com/intel/essayrev/index.html>

Consciousness and Mind <http://www.polymath-systems.com/conscmnd.html>

Polymath Systems Home Page <http://www.polymath-systems.com/index.html>

http://www.polymath-systems.com/intel/essayrev/sexdiff.html

--
Respectfully, Roger L. Bagula
tf...@earthlink.net, 11759Waterhill Road, Lakeside,Ca 92040-2905,tel: 619-5610814 :
URL : http://home.earthlink.net/~tftn
URL : http://victorian.fortunecity.com/carmelita/435/