Hammer and his colleagues analyzed X chromosome sequence variation and compared to autosomal sequence variation to understand the male-female asymmetrical demographic patterns observed. They believe that the large variance in male reproductive success due to wide spread practice of polygyny is the major contributing factor for sex-biased demographic pattern.
Hammer et al. used the same 90 individuals from six populations (Biaka, San, Mandenka, Han Chinese, Oceanians, and Basque) that Cox et al. analyzed, and they analyzed about 210 kb sequence from 40 independently evolving non-gene coding regions on autosomes and X chromosome.
Then, they examined the ratio of effective population size between X chromosome and autosomes (NX/NA). Note that females carry two X chromosome and males carry one X chromosome, but both sexes carry a pair of autosomal chromosome. So, the expected ratio (NX/NA) is 0.75, if male and female effective population sizes are equal. The observed ratios, on the other hand, ranges from 0.85 to 1.08 indicating that female effective population size is larger than male effective population size.
They explored the possible explanations (sequencing error, background selection, changes in population size, sex-biased migration, and high variance in male reproductive success) for the observed pattern. They believe that sequencing error is minor and background selection, changes in population size, and sex-biased migration do not alter the ratio significantly.
They hypothesize that high variance in male reproductive success is the major factor affecting the ratio, though all other factors may have contributed. They explain:
The human mating system is considered to be moderately polygynous, based on both surveys of world populations and on characteristics of human reproductive physiology. The practice of polygyny, in both the traditional sense and via ‘effective polygyny’ (whereby males tend to father children with more females than females do with males-a common practice in many contemporary western culture), would tend to increase the variance in reproductive success among males.
It is important to note that Hammer and colleagues have argued that wide spread practice of polygyny causes sex-biased demographic history, but in this article they are saying polygyny is not only cultural practice that causes some males to be reproductively more successful than the others, but they extend into other polygynous reproductive behaviors (not specific about what these cultural practices are) or physiological processes (e.g., sperm competition). In addition, age structure, lower male survival rate to the adulthood or higher mortality rate among young male, and delayed maturity of males also influence the ratio.
Another important thing to note is that Hammer and his colleagues in Cox et al. (2008) argued that female gene flow was important factor understanding human demographic history. Yet, they did not ask how female gene flow, sex-biased migration might have affected the ratio in detail, and they conducted computer simulations, but I think the two-deme island model they used is not realistic.