Answers are grounded in this paper and cite their sources.
Changes in marital fertility have accounted for 83 percent of the decline in total fertility since 1971; changes in marital status accounted for 19 percent, and changes in nonmarital fertility had only a negligible effect. Declines in the level of marital fertility account for a substantial part of the overall decrease; postponement of childbearing did not occur in any significant degree until the 1970s.
No takes yet. Share an insight, caveat, or question.
Campbell Gibson (1976) studied this question.
Synapse has enriched 10 related papers. Consider them for comparative context: