Correction
for Biggar et al., Am. J. Epidemiol. 150 (9) 957-962.
American Journal of Epidemiology Advance Access originally published online on October 27, 2007
American Journal of Epidemiology 2008 167(2):249-250; doi:10.1093/aje/kwm299
American Journal of Epidemiology Published by the Johns Hopkins Bloomberg School of Public Health 2007.
RE: "SEX RATIOS, FAMILY SIZE, AND BIRTH ORDER"
In 1999, Biggar et al. (
1) reported findings regarding the effect
that the sex of the newborn had on subsequent birth patterns.
This report had two major components: 1) a decreased likelihood
of choosing to have additional children when parents had mixed-sex
families or girls rather than boys; and 2) an increased likelihood
that the next child would be a boy if the mother had had previous
boys. The finding was statistically significant, but the authors
could not explain the second finding biologically.
They recently sought to verify and further examine the second finding in new data accumulated since that time. In doing so, they discovered a programming error that is responsible for the second finding. They intended to exclude multiple births, as stated in the Materials and Methods of the original article (1, p. 958), because they recognized that including monozygous twins would bias results toward having a sibling of the same sex. However, an error in programming had the effect of transforming twin births into two singleton births that remained in the data set.
The inclusion of twins resulted in erroneous findings regarding the influence that the sex of earlier children had on the sex of subsequent children. When they revised the program to exclude all twins, the new findings showed no significant impact of previous children on the sex of the next child, as shown in the reevaluated table 2 below. The very strong findings of familial choices regarding having more children based on the number and sex of earlier children were unaffected (data not shown here).
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TABLE 2. Sex ratio changes (given as % male*), by birth order, among Danish families with one to five single births, 1960–1994
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The authors and the
Journal apologize for the error and regret
the misleading implications of the earlier report with regard
to the effect of previous siblings on the sex of the next child.
The findings with regard to the effect of offspring sex on birth
choices remain correct.
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References
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- Biggar RJ, Wohlfahrt J, Westergaard T, et al. Sex ratios, family size, and birth order. Am J Epidemiol (1999) 150:957–62.[Abstract/Free Full Text]
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