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Father's occupation and sex ratio of offspring

Unit of Statistics and Epidemiology, Örebro University Hospital, Örebro, Sweden, anders.magnuson{at}orebroll.se

Lennart Bodin

Unit of Statistics and Epidemiology, Örebro University Hospital, Örebro, Sweden, Department of Statistics, Örebro University, Örebro, Sweden

Scott M. Montgomery

Unit of Statistics and Epidemiology, Örebro University Hospital, Örebro, Sweden, Clinical Epidemiology Unit, Department of Medicine, Karolinska Hospital, Karolinska Institutet, Stockholm, Sweden, Department of Primary Care and Social Medicine, Charing Cross Hospital, Imperial College, London, UK

Aims: Ecological studies have demonstrated that when national economies contract the proportion of male live births is reduced. It has been suggested that the relative disadvantage this causes influences sex ratio among births. Here the authors use individual data to investigate whether there is a persistent association of father's occupation with the sex of offspring and if it varies by industry sector. Associations with season of birth are also investigated. Methods: All fathers with a first singleton live birth in Sweden between 1940 and 1949 were identified using population registers (n=523,671). Fathers' occupations from the 1960 Census was categorized into: manual workers; agricultural sector (divided into workers and owners/ managers); and office sector (divided into workers and managers). Results: Compared with manual workers, the routine workers in other sectors were not statistically significantly more likely to have a male first offspring. Agricultural owners/ managers and office managers were both statistically significantly more likely to have male offspring with adjusted odds ratios (and 95% confidence intervals) of 1.045 (1.024—1.066, p<0.001) and 1.021 (1.003—1.039, p=0.022), respectively. Compared with autumn births, spring births were less likely to be male: 0.983 (0.967—0.998, p=0.029). Conclusions: Fathers' occupation level, even 10—20 years after childbirth, but not labour market sector is associated with the sex ratio of offspring, indicating that material or social conditions are responsible. Spring births are less likely to be male, probably due to infections differentially reducing male foetal survival.

Key Words: Occupation • season of birth • sex ratio

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