The male female difference in the variance of quantitative traits including IQ can be accounted for by the observation of the concentration of genes involved in brain development on the X chromosome and by the mechanism of sex determination in mammals. Females have two X chromosomes with one randomly inactivated. Males have a single X chromosome, which results in increased variance. It is shown that these factors are sufficient to account for the observed differences

Bernard Strauss

Jacob Strauss

English

Nature Precedings

 1
Why are Male IQ Scores More Variable? 
Bernard S. Strauss and Jacob A. Strauss 
Department of Molecular Genetics and Cell Biology, The University of Chicago 
and MIT Computer Science and Artificial Intelligence Laboratory 
 
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Abstract: The male female difference in the variance of quantitative traits 
including IQ can be accounted for by the observation of the concentration of 
genes involved in brain development on the X chromosome and by the 
mechanism of sex determination in mammals. Females have two X 
chromosomes with one randomly inactivated. Males have a single X 
chromosome, which results in increased variance.  It is shown that these factors 
are sufficient to account for the observed differences. 
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The average (mean) scores for males and females for a variety of 
quantitative traits, including IQ, are almost identical. What is different is that the 
variance for males is higher 1,2. This means that there are more males at both 
ends of the distribution.  There are about 40 percent more mentally defective 
males than females 3 and it has been argued that the excess of males in the very 
high IQ regions could account for the differential representation of males in 
tenured positions on the faculties of research universities particularly in the 
physical sciences 4.  Although there are sociological and economic reasons that 
might account for this differential, there is a simple biological reason, related to 
the mechanism of sex determination in mammals, for the greater trait variance of 
males.  
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Females have two X chromosomes and in humans the X includes about 
1100 genes.  The Y chromosome on the other hand includes about 50 genes, 
most related to sex determination and fertility.  The X chromosome also seems to 
have an unusually high representation of genes involved in brain development 5.  
Nature compensates for the excess representation of X-linked genes.  In humans 
this occurs by a process of random inactivation of most of the genes on one or 
the other of the two X chromosomes.  As far as gene function is concerned, 
females are mosaics 6,7. The net result of these biological facts is that males 
must be more variable for any trait involving the X chromosome 6,8. 
If IQ were simply related to one gene on the X chromosome with 
no representative on the Y, and this gene had two  alleles (A) and (a) with (A) 
providing 100 IQ units and (a) providing (0) units, and if all genes acted equally 
(no dominance) then males would be either X(A)Y or X(a)Y and would have 
values of 100 or 0.  Given equal gene frequencies the 
mean would be 50 and the variance 502.  Females would be X(A)X(A) or 
X(A)X(a) or X(a)X(a) in a ratio 1:2:1.  Assuming that X inactivation occurs 
randomly, the mean would still be 50 but the variance would be half that of 
males: 502/2.  Of course IQ cannot be related to only one gene but is more likely 
affected by many genes on the X chromosome and on the autosomes.  
Assuming that each one of these genes can be considered as contributing to IQ 
in an additive fashion, the contribution of genes on the X chromosome can be 
generalized as shown in Model 1 (Fig. 1).  For any number of loci the male 
variance is greater than the female variance by the fraction of all loci that are on 
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the X chromosome .  If IQ were determined solely by genetic factors the variance 
ratio between males and females could take any value between 1 and 2.  Of 
course this does not describe the real world and a model taking into account 
additional sources of variance is given in the supporting online material.  
 How well do observed variance ratios fit these models?  All are consistent 
with the predicted range between 1 and 2.  The observed variance ratio falls 
between 1.07 to 1.17 1 depending on the component tested.  In a study on the 
number of words used by males and females, the mean was statistically 
indistinguishable but the male variance was 1.40 times greater 9.   A simple 
prediction based on Model 1 is that between 7% and 17% or 40% of all the 
genes involved in these traits are located on the X chromosome. 
  If a major reason for the difference in variance is as straightforward as 
this analysis indicates then the selection of women at the highest IQ values 
merely requires increasing the size of the cohort. 
 
References 
1. Strand, S., Deary, I.J. & Smith, P. Sex differences in cognitive abilities test 
scores: a UK national picture. Br J Educ Psychol 76, 463-80 (2006). 
2. Hedges, L.V. & Nowell, A. Sex differences in mental test scores, 
variability, and numbers of high-scoring individuals. Science 269, 41-5 
(1995). 
3. Leonard, H. & Wen, X. The epidemiology of mental retardation: challenges 
and opportunities in the new millennium. Ment Retard Dev Disabil Res 
Rev 8, 117-34 (2002). 
4. Summers, L.H. Remarks at NBER Conference on Diversifying the Science 
and Engineering Workforce by Lawrence H. Summers, Cambridge Mass. 
Jan. 14, 2005. in 
http://www.president.harvard.edu/speeches/summers_2005/nber.php 
(2005). 
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5. Graves, J.A., Koina, E. & Sankovic, N. How the gene content of human 
sex chromosomes evolved. Curr Opin Genet Dev 16, 219-24 (2006). 
6. Lange, K. & Sobel, E. Variance component models for X-linked QTLs. 
Genet Epidemiol 30, 380-3 (2006). 
7. Payer, B. & Lee, J.T. X chromosome dosage compensation: how 
mammals keep the balance. Annu Rev Genet 42, 733-72 (2008). 
8. Pan, L., Ober, C. & Abney, M. Heritability estimation of sex-specific effects 
on human quantitative traits. Genet Epidemiol 31, 338-47 (2007). 
9. Mehl, M.R., Vazire, S., Ramirez-Esparza, N., Slatcher, R.B. & 
Pennebaker, J.W. Are women really more talkative than men? Science 
317, 82 (2007). 
 
 
 
 
 
Fig.1.  The predicted male/female variance ratio for a quantitative trait as a 
function of the fraction α of related genes on the X chromosome to total related 
genes.  The model assumes that all genes contribute additively in the same 
proportion, and are uncorrelated.  Since 0≤α≤1, the resulting variance ratio falls 
between 1 and 2, with increased autosomal contribution diluting the observed 
variance ratio toward 1. 
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SUPPLEMENTAL MATERIAL 
 
Fig S1.  An extension to Model 1 that groups together additional sources of 
observed variance such as measurement errors or non-additive sources.  The 
resulting model is no longer dependent only on the fraction of X-linked genes, but 
instead relates several parameters through a construct β.  Since β≥0, the 
resulting range of variance ratios is again between 1 and 2.   
 
 
 
 
 
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Are women really more talkative than men?

Heritability estimation of sex-specific effects on human quantitative traits.

in http://www.president.harvard.edu/speeches/summers_2005/nber.php (2005). N a t u r e P r e c e d i n g s : h d l : / n p r e .

Sex differences in cognitive abilities test scores: a UK national picture.

Sex differences in mental test scores, variability, and numbers of high-scoring individuals.

The epidemiology of mental retardation: challenges and opportunities in the new millennium. Ment Retard Dev Disabil Res Rev 8,

X chromosome dosage compensation: how mammals keep the balance.

Why are Male IQ Scores More Variable?

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Why are Male IQ Scores More Variable?

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