Glimpses at the History of Sex Ratio Studies

The sex ratio at birth (SR) is defined as the number of males per 100 females and is almost always around 106. John Graunt (1620–1674) was the first to compile data showing an excess of male births to female births and to note spatial and temporal variation in the SR. John Arbuthnot (1667–1735) demonstrated that the excess of males was statistically significant and asserted that the SR is uniform over time and space. Arbuthnot suggested that the regularity in the SR and the dominance of males over females must be an indication of divine providence. Nicholas Bernoulli’s (1695–1726) counter-argument was that chance could give uniform dominance of males over females. Later, Daniel Bernoulli (1700–1782), Pierre Simon de Laplace
(1749–1827) and Siméon-Denis Poisson (1781–1840) also contributed to this discussion. Attempts have been made to identify factors influencing the SR, but comparisons demand large data sets. Attempts to identify associations between SRs and stillbirth rates have failed to yield consistent results. A common pattern observed in different countries is that during the first half of the twentieth century the SR showed increasing trends, but during the second half the trend decreased. A common opinion is that secular increases are caused by improved socio-economic conditions. The recent downward trends have been attributed to new reproductive hazards. Factors that affect the SR within families remain poorly understood. Although they have an effect on family data, they have not been identified in large national birth registers.