To the Editor,
The sex ratio at birth is expected to be approximately 0.515, calculated as male births divided by total births (M/T), thus a slight excess of males. Acute stress in the general population may cause M/T depressions, as evidenced by the M/T dip observed after the Great Recession of 2007 in the United States (#*#ref1#*#). Significant racial differences have been noted and attributed to innate and minor physiological differences, but chronic stress has been proposed as a possible cause (#*#ref2#*#). A recent paper showed that M/T was depressed in Mexico when compared to a global reference dataset (#*#ref3#*#,#*#ref4#*#). This study was carried out in order to ascertain whether Mexican M/T was similar to M/T in non-Mexican births in the same study cohort (#*#ref4#*#).
Ethical approval was not required as data was comprised of free and anonymous datasets from the Instituto Nacional de Estadística y Geografía, Mexico’s National Institute of Statistics and Geography. For the same reason, informed consent was not obtained. Live births by sex, year and nationality [Mexican or non-Mexican (“Extranjero”)] were available for the period January 2010-December 2020.
The equations of Fleiss (binomial) were used to calculate 95% confidence intervals (CI) for proportions. A bespoke Excel sheet was used to perform chi square tests. A p-value <0.05 was taken to represent a statistically significant result. Mexican and non-Mexican births by sex, and M/T with 95% CIs are shown in Table 1. There was no significant M/T difference between the two groups.
While racial disparities in M/T could theoretically be caused by innate physiological differences, it is also possible that the differences seen may be due to chronic stress (#*#ref2#*#). Support for this comes from a comparison of racial M/T in the United States which showed that M/T was higher in Whites than in American Indian/Alaska Native, and Black/African American births (#*#ref5#*#). This was probably due to the fact that in the United States, race remains the primary determinant of socioeconomic status and stress. This accords with the Trivers-Willard hypothesis of male foetal loss in this type of stress (#*#ref2#*#).
This study indirectly supports the hypothesis that innate physiological differences do not appear to affect M/T, as there were no significant differences between the low Mexican M/T and non-Mexican M/T. However, the study was limited by small numbers and a lack of breakdown of what races or ethnicities constituted the non-Mexican births.
