Why are Male IQ Scores More Variable?
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- University of Chicago, Molecular Genetics and Cell Biology
- Massachusetts Institute of Technology, MIT Computer Science and Artificial Intelligence Laboratory
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- Document Type:
- Manuscript
- Date:
- Received 12 May 2009 01:32 UTC; Posted 12 May 2009
- Subjects:
- Genetics & Genomics
- 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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Our answer to the comment is as follows:
This comment would be correct if ALL the cells in the female chose one or the other option. But X inactivation works differently so that a female is actually a mixture of cells, approximately 50% of the cells having one X allele active and 50% having the other. If the gene product is diffusible then the organism as a whole will have an average value. If a particular organ is a mosaic-with some cells displaying one allele and other cells the alternate it is hard to predict but once again the most probable value is the average. X inactivation does provide a “wild card” in the calculations because there is evidence that different females may inactivate different genes and because some portions of the X actually escape inactivation . Once again, the simplest assumption is that the overall effect is approximated by the average. In this case the overall male/female variance are different.
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- This document is licensed to the public under the Creative Commons Attribution 3.0 License
- How to cite this document:
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Strauss, Bernard and Strauss, Jacob. Why are Male IQ Scores More Variable?. Available from Nature Precedings <http://hdl.handle.net/10101/npre.2009.3238.1> (2009)
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Bob O'Hara on 14 May 2009 08:59 UTC
p2, 2nd paragraph:
_Assuming that X inactivation occurs
randomly, the mean would still be 50 but the variance would be half that of
males: 50^2/2._
This is wrong. If inactivation is random, females will only have one copy having an effect. That’s chosen at random, so it’s just picking one copy from the mother, and seeing the genotype. But that’s exactly like male situation: there is one active copy. The variance is thus also the same variance as for the male.