Sex Ratios, Family Size, and Birth Order

R. J. Biggar, J. Wohlfahrt, T. Westergaard, M. Melbye
1999 American Journal of Epidemiology  
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
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.
doi:10.1093/oxfordjournals.aje.a010104 pmid:10547141 fatcat:rayzqr5mlzh3nderm44sxgqu2u