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American Journal of Epidemiology Vol. 150, No. 9: 957-962
Copyright © 1999 by The Johns Hopkins University School of Hygiene and Public Health

other

Sex Ratios, Family Size, and Birth Order

Robert J. Biggar1, Jan Wohlfahrt2, Tine Westergaard2 and Mads Melbye2

1Viral Epidemiology Branch, National Cancer Institute Bethesda, MD
2Department of Epidemiology Research, Danish Epidemiology Science Centre, Statens Serum Institut Copenhagen, Denmark

In many countries, the male: female ratio at birth has varied significantly over the past century, but the reasons for these changes have been unclear. The authors observed a close parallel between decreasing family size and declining male: female sex ratio in Denmark from 1960 to 1994. To explain this finding, they examined the sex ratio and birth order of 1,403,021 children born to 700,030 couples. Overall, 51.2% of the first births were male. However, families with boys were significantly more likely than expected to have another boy (biologic heterogeneity). By the fourth birth to families with three prior boys, 52.4% were male. The increase varied directly with the number of prior boys (p for trend = 0.0007). Furthermore, couples with boys were more likely to continue to have children. In summary, the authors found that the declining male: female ratio in Denmark and probably other European populations is mainly attributable to three effects: declining family size, biologic heterogeneity, and child sex preference. Why families with boys are more likely to have additional boys is unknown. Am J Epidemiol 1999;150:957-62.

birth order; family characteristics; infant; reproduction; sex distribution; sex ratio


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