Academic mothers have a pronounced seasonal variation in their offspring sex ratio.

OBJECTIVES Environmental and socio-demographic factors can influence the variation of the human sex ratio at birth (SRB = the ratio of males to males plus females). In particular, findings of seasonal, parental education, birth order, and maternal age effects on the SRB are not always in agreement, and a number of works report minimal variation. SETTING Here, we investigated the seasonality of SRB in academic and non-academic mothers employed at the University of Vienna, and who gave birth between 1963 and 2000 (n = 1932 births). METHODS All data were available from an anonymous employee database. RESULTS Both groups, academic and non-academic mothers do not differ between their overall SRB. In academic mothers the SRB is significantly (P = 0.004) increased during the springtime and decreased during the summertime. Although in non-academic mothers the trend is comparable, it is far less pronounced and not significant (P = 0.345). When a multiple logistic model was applied to the data of academic mothers the only significant influencing factor on the SRB is the season, while birth order of children and mothers' age at childbirth has no effect. None of the three independent variables influence the SRB in non-academic mothers. MAIN FINDINGS These findings suggest a more flexible SRB rate in academic mothers than in non-academics within the seasons. CONCLUSION We conclude that the significant seasonal variation of the SRB in academic women cannot be merely interpreted as an effect of socio-economic status but more likely as the interaction between socio-economic and environmental working conditions.