A socio-economic hypothesis for lower birth sex ratios at racial, national and global levels.

The sex ratio at birth may be depressed by a variety of events or conditions due to an excess of male foetal losses. Evidence is accumulating that observed differences in this ratio between races, nations and even at regional global level, may be due to socio-economic stress. This review summarises findings pertaining to Blacks in the United States, and to a global United Nations dataset, as well as to other smaller studies. Extant evidence reinforces the theory that chronic socio-economic stress may be the reason for a lower birth sex ratio at racial, national and global levels. While innate periconceptual and gestational (e.g. hormonal) racial differences have been cited as possible causes for these M/T differences, this may be a mechanism/pathway and not the actual cause for lower M/T. Stress may, however, act by using hormonal pathways to effect the observed M/T disparities.