Table 2.
Mean number of children per family and index of opportunity for fertility selection If, in various human populations
| Population | Mean number of children | Ij |
| Rural Quebec, Canada | 9.9 | 0.20 |
| Gold Coast, Africa | 6.5 | 0.23 |
| New South Wales, Australia (1898–1902) | 6.2 | 0.42 |
| United States, women born in 1839 | 5.5 | 0.23 |
| United States, women born in 1871–1875 | 3.5 | 0.71 |
| United States, women born in 1928 | 2.8 | 0.45 |
| United States, women born in 1909 | 2.1 | 0.88 |
| United States, Navajo Indians | 2.1 | 1.57 |
If is calculated as the variance divided by the square of the mean number of children. The opportunity for selection usually increases as the mean number of children decreases. Reprinted from ref. 8.