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Crow’s index of opportunity for natural selection has been computed for the period 1750-1980 using data from the Swedish National Central Bureau of Statistics (SCB). The general trend is a decline in the size of Crow’s index. Over time prenatal mortality became the major contributor to the part of the index related to the prereproductive period. This is due to a decrease in childhood mortality. A large part of If, the index related to reproduction, is due to childlessness. Marital childlessness seems to have been fairly constant over time whereas the percentage of childless women in the population varied. The fertility component for childbearing women, Isf, shows little absolute change with the exception of a peak in the period 1930-39. Its relative contribution to the total index did however increase substantially over time.