Historical outlook on the study of Secondary Sex Ratio

The sex ratio at birth, also called the secondary sex ratio, and here denoted SSR, is usually defined as the number of males per 100 females. Among newborns there is always a slight excess of boys. Consequently, the SSR is greater than 100, mainly around 106. In this study, we consider SSR as the rate of males as a percentage of among all births. This transformed SSR variable is mainly about 51.5. This choice was made because the rate of males is statistically more easily studied. The SSR shows regional and temporal variations. In a long series of papers, attempts have been made to identify factors influencing the SSR, but statistical analyses have shown that comparisons demand large data sets. Variations in the SSR that have been reliably identified in family data have in general been slight and rare, consequently lacking a notable influence on national birth registers. Attempts to identify consistent associations between SSRs and stillbirth rates have been made, but no reliable results have emerged.