The ratio of the tMRCA between chromosome X and the autosomes in West Africans, 0.763 ± 0.026, is consistent with the expected 3/4, but it is lower than 3/4 in non-African populations: 0.635 ± 0.024 in North Europeans and 0.613 ± 0.026 in East Asians (Table 2, Supplementary Note and Supplementary Table 2 online).
I am fairly allergic to explanations invoking genetic drift, and I think that both selection and demography might play a role in the observed discrepancy.
Nature Genetics doi:10.1038/ng.303
Accelerated genetic drift on chromosome X during the human dispersal out of Africa
Alon Keinan et al.
Comparisons of chromosome X and the autosomes can illuminate differences in the histories of males and females as well as shed light on the forces of natural selection. We compared the patterns of variation in these parts of the genome using two datasets that we assembled for this study that are both genomic in scale. Three independent analyses show that around the time of the dispersal of modern humans out of Africa, chromosome X experienced much more genetic drift than is expected from the pattern on the autosomes. This is not predicted by known episodes of demographic history, and we found no similar patterns associated with the dispersals into East Asia and Europe. We conclude that a sex-biased process that reduced the female effective population size, or an episode of natural selection unusually affecting chromosome X, was associated with the founding of non-African populations.