Sex ratios, family size, and birth order

Citation
Rj. Biggar et al., Sex ratios, family size, and birth order, AM J EPIDEM, 150(9), 1999, pp. 957-962
Citations number
13
Categorie Soggetti
Envirnomentale Medicine & Public Health","Medical Research General Topics
Journal title
AMERICAN JOURNAL OF EPIDEMIOLOGY
ISSN journal
00029262 → ACNP
Volume
150
Issue
9
Year of publication
1999
Pages
957 - 962
Database
ISI
SICI code
0002-9262(19991101)150:9<957:SRFSAB>2.0.ZU;2-B
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 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.