Mother's occupation and sex ratio at birth

Abstract

Background: Many women are working outside of the home, occupying a multitude of jobs with varying degrees of responsibilities and levels of psychological stress. We investigated whether different job types in women are associated with child sex at birth, with the hypothesis that women in job types, which are categorized as "high psychological stress" jobs, would be more likely to give birth to a daughter than a son, as females are less vulnerable to unfavourable conditions during conception, pregnancy and after parturition, and are less costly to carry to term.

Methods: We investigated the effects of mother's age, maternal and paternal job type (and associated psychological stress levels) and paternal income on sex ratio at birth. Our analyses were based on 16,384 incidences of birth from a six-year (2000 to 2005 inclusive) childbirth dataset from Addenbrooke's Hospital in Cambridge, UK. We obtained a restricted data set from Addenbrooke's hospital with: maternal age, maternal and paternal occupations, and whether or not the child was first-born.

Results: Women in job types that were categorized as "high stress" were more likely to give birth to daughters, whereas women in job types that were categorized as "low stress" had equal sex ratios or a slight male bias in offspring. We also investigated whether maternal age, and her partner's income could be associated with reversed offspring sex ratio. We found no association between mother's age, her partner's job stress category or partner income on child sex. However, there was an important interaction between job stress category and partner income in some of the analyses. Partner income appears to attenuate the association between maternal job stress and sex ratios at moderate-income levels, and reverse it at high-income levels.

Conclusions: To our knowledge this is the first report on the association between women's job type stress categories and offspring sex ratio in humans, and the potential mitigating effect of their partners' income.

Figure 1

Association between women's stress levels, partners' income and sex ratio at birth. A response plane illustrating the effect of the interaction between mother's perceived job stress (x-axis) and partner income (z-axis) on the probability of producing male offspring (y-axis) for parents from Cambridge, UK from 2000 to 2005. Partner income is in £. The blue line illustrates the point (29,991 £) where the effect of partner income cancels the effect of mother stress, so that the probability of male offspring is equal across all levels of mother stress (probability = 0.509). The area between the red and blue lines is where higher levels of maternal stress decrease the probability of having a male. The area between the blue and green lines is where increasing partner income increases the probability of having a male. The individual black dots represent individual data points.