Why Might Males Show Greater Variability?

This project is descriptive—it documents patterns, not causes. But the question of why the pattern exists is scientifically important. This page summarizes one biologically plausible partial explanation that has received increasing empirical support. It is not the only possible contributor, and it does not explain most trait variance.

A Possible X-Linked Contribution

One biologically plausible partial explanation for greater male variability is the asymmetric genetics of the X chromosome. Males are hemizygous for most non-pseudoautosomal X-linked loci, so X-linked allelic effects are more directly exposed. Females have two X chromosomes, but one is largely inactivated in each cell, producing mosaic expression rather than uniform diploid expression. Under standard dosage-compensation models, that architecture predicts greater additive genetic variance from X-linked loci in males than in females.1, 2

Brain-Related Traits

This possibility is especially relevant for brain-related traits. The X chromosome is unusually relevant to brain biology, and a 2025 UK Biobank brain XWAS reported that many brain imaging traits fit a full-dosage-compensation model, under which males are expected to show roughly twice the genetic variance of females at X-linked loci. In the same study, the sex-stratified analysis identified 25 male-specific non-pseudoautosomal trait–locus pairs but only 5 female-specific pairs. More broadly, recent UK Biobank and FinnGen analyses also report a pronounced male bias in X-linked heritability, which is consistent with near-complete dosage compensation through X-chromosome inactivation.2, 3

Methodological Underappreciation

A reason this mechanism may be underappreciated is methodological. The X chromosome is still often omitted or inconsistently analyzed in GWAS, and only 25% of genome-wide summary statistics in the GWAS Catalog for 2021 included X-chromosome results. The brain XWAS also notes that sex-specific variance on the X matters statistically: because PLINK2 did not implement the equal-variance X model used in the heritability stage, traits favoring that model had to be analyzed under full-dosage-compensation coding in the association stage. This suggests that models treating the X as an ordinary chromosome by length alone should be read as conservative baselines rather than complete biological accounts.4, 2

Important Caveats

This is a partial explanation, not a complete one

The X chromosome does not explain most heritability for most complex traits. One 2025 study estimated that chrX contributes about 3% of autosomal heritability overall, and the brain XWAS found X-linked heritability enrichment for only 2.9% of brain imaging traits, while 39.3% showed depletion relative to chromosome size. There is also at least one narrower human study that did not find evidence that female X-inactivation “averaging” explained greater male variance in childhood cognitive scores.

The strongest defensible claim is not that the X chromosome fully explains greater male variability, but that male hemizygosity together with female X-chromosome inactivation is a plausible, biologically grounded, and probably undermeasured partial contributor, especially for some brain-related traits.3, 2, 5

Cross-Species Evidence

Cross-species evidence supports the broader sex-chromosome part of the argument. In species with heterogametic females (such as birds, where females are ZW and males are ZZ), variability tends to shift toward females rather than males, which is consistent with a heterogamety or hemizygosity mechanism. But those systems do not use mammalian-style chromosome-wide X inactivation; in birds, dosage compensation is much less effective and more local. So this comparative evidence strengthens the general sex-chromosome hypothesis more than the specifically mammalian claim about female X-inactivation buffering variance.6, 7

References

  1. Sidorenko J, Kassam I, Kemper KE, et al. The effect of X-linked dosage compensation on complex trait variation. Nat Commun. 2019;10(1):3009. PubMed
  2. Jiang Z, Sullivan PF, Li T, et al. The X chromosome’s influences on the human brain. Sci Adv. 2025;11(4):eadq5360. PMC
  3. Fu Y, Kenttämies A, Ruotsalainen S, Pirinen M, Tukiainen T. Role of X chromosome and dosage-compensation mechanisms in complex trait genetics. Am J Hum Genet. 2025;112(6):1330–1343. PubMed
  4. Sun L, Wang Z, Lu T, Manolio TA, Paterson AD. eXclusionarY: 10 years later, where are the sex chromosomes in GWASs? Am J Hum Genet. 2023;110(6):903–912. PubMed
  5. Giummo J, Johnson W. Testing for evidence of an X-linked genetic basis for a greater proportion of males with high cognitive ability. Behav Genet. 2012;42(5):808–819. PubMed
  6. Reinhold K, Engqvist L. The variability is in the sex chromosomes. Evolution. 2013;67(12):3662–3668. PubMed
  7. Itoh Y, Melamed E, Yang X, et al. Dosage compensation is less effective in birds than in mammals. J Biol. 2007;6(1):2. PubMed