Men may have outnumbered women during the human migration out of Africa, according to a new paper appearing in yesterday’s advance online version of Nature Genetics.

Researchers from Harvard Medical School, the Broad Institute, and the National Human Genome Research Institute assessed more than 130,000 SNPs and a billion bases of sequence data representing individuals of West African, European, and East Asian descent. Together, the X chromosome and autosomal chromosome data suggest that the effective female population size was unexpectedly low when humans migrated from Africa to other parts of the world some 60,000 years ago.