Many factors influence human reproductivity, which in the last analysis determines world populations both qualitatively and quantitatively. The results of long years of study of the biologic and statistical factors involved, interspersed with personal lines of thought which have been provoked thereby, furnishes the subject of a recent book by Pearl.1 Both the survival urge and the reproductive urge have a fundamental biologic role in influencing reproductive patterns, and there are recognizable differences in the strength of these factors. Just as there are demonstrable differences in the libido, there is likewise no reasonable doubt of variations between individual human beings in respect to innate reproductive capacity, though these are extremely difficult to measure precisely. One definitely determining factor is the limited span of reproductive life in the human female. Less important—because it varies less frequently and to a lesser degree—is the litter size or frequency of multiple births. Closely
HUMAN REPRODUCTIVE PATTERNS. JAMA. 1939;113(18):1645–1646. doi:10.1001/jama.1939.02800430037013
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