Sex ratios, family size, and birth order

Abstract

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.

PIP: The relationship between the sex ratio and birth order of 1,403,021 children born to 700,030 couples was examined by analyzing the birth records from 1960 to 1994 in Denmark. The analysis revealed that about 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 4th birth to families with 3 prior boys, 52.4% were males. 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. The researchers found that the declining sex ratio between male and female in Denmark, and perhaps in other European populations, is mainly attributable to 3 factors: 1) declining family size; 2) biologic heterogeneity; and 3) child sex preference. Lastly, it is not known why families with boys are more likely to have additional boys.