Table 3.
Within-family offspring sex associations. Left, the within-nuclear-family sex associations for families of different sizes (final number of children 2 to 10). Right, prediction (OR) of the sex of the nth child based on the sex of each older sibling, regardless of final family size. Odds ratios above 1 indicate a higher likelihood individuals' offspring having the same sex, odds ratios below 1 a higher likelihood of opposite sex. NA, not applicable. Analyses performed with generalized estimating equations with logit link and standard errors clustered on parental sibling clusters.
| final family size (no. of children) |
final family size analysis |
birth order analysis |
|||||
|---|---|---|---|---|---|---|---|
| no. of pairsa | odds ratio (95% confidence intervals) | p-value | nth born child | no. of pairsa | odds ratio (95% confidence intervals)b | p-value | |
| 1 | NA | NA | NA | 1 | NA | NA | NA |
| 2 | 1 085 061 | 0.854 (0.847, 0.860) | <0.001 | 2 | 1 748 196 | 0.997 (0.991, 1.003) | 0.363 |
| 3 | 1 461 533 | 1.049 (1.042, 1.056) | <0.001 | 3 | 1 349 407 | 1.005 (0.998, 1.012) | 0.148 |
| 4 | 803 803 | 1.077 (1.068, 1.087) | <0.001 | 4 | 559 402 | 0.999 (0.989, 1.010) | 0.916 |
| 5 | 354 620 | 1.073 (1.058, 1.088) | <0.001 | 5 | 206 348 | 1.003 (0.985, 1.021) | 0.772 |
| 6 | 156 337 | 1.073 (1.051, 1.095) | <0.001 | 6 | 79 373 | 0.993 (0.965, 1.023) | 0.660 |
| 7 | 69 366 | 1.071 (1.037, 1.106) | <0.001 | 7 | 32 318 | 1.008 (0.963, 1.055) | 0.747 |
| 8 | 34 331 | 1.043 (0.997, 1.090) | 0.065 | 8 | 14 593 | 1.057 (0.988, 1.131) | 0.106 |
| 9 | 16 540 | 1.064 (0.996, 1.137) | 0.064 | 9 | 6978 | 0.982 (0.890, 1.084) | 0.724 |
| 10 | 10 779 | 1.091 (1.003, 1.186) | 0.042 | 10 | 3706 | 0.969 (0.847, 1.109) | 0.651 |
aBecause nuclear families were considered as a unit, each pair could only contribute to one family. However, individuals can be part of more than one pair as each first-born offspring may be part of cousin pairs both on her/his mother's and father's side and because each first-born offspring can be part of multiple pairs on mother's and/or father's side. Therefore, the numbers of pairs here do not match numbers of offspring presented in table 1.
bOR of the sex of the nth child based on the sex of each older sibling; e.g. for the fourth-born child, the dependent variable is the sex of child 4, and independent variable is from siblings born first, second and third.