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 d
eclining male:female sex ratio in Denmark from 1960 to 1994. To explain thi
s finding, they examined the sex ratio and birth order of 1,403,021 childre
n born to 700,030 couples. Overall, 51.2% of the first births were male. Ho
wever, families with boys were significantly more likely than expected to h
ave another boy (biologic heterogeneity). By the fourth birth to families w
ith three prior boys, 52.4% were male. The increase varied directly with th
e number of prior boys (p for trend = 0.0007). Furthermore, couples with bo
ys were more likely to continue to have children. In summary, the authors f
ound that the declining male:female ratio in Denmark and probably other Eur
opean 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.