Twins Support the Absence of Parity-Dependent Fertility Control in Pretransition Populations

Abstract

A conclusion of the European Fertility Project in 1986 was that pretransition populations mostly displayed natural fertility, where parity-dependent birth control was absent. This conclusion has recently been challenged for England by new empirical results and has also been widely rejected by theorists of long-run economic growth, where pre-industrial fertility control is integral to most models. In this study, we use the accident of twin births to show that for three Western European-derived pre-industrial populations-namely, England (1730-1879), France (1670-1788), and Québec (1621-1835)-we find no evidence for parity-dependent control of marital fertility. If a twin was born in any of these populations, family size increased by 1 compared with families with a singleton birth at the same parity and mother age, with no reduction of subsequent fertility. Numbers of children surviving to age 14 also increased. Twin births also show no differential effect on fertility when they occurred at high parities; this finding is in contrast to populations where fertility is known to have been controlled by at least some families, such as in England, 1900-1949, where a twin birth increased average births per family by significantly less than 1.

Mothers with higher twinning propensity had lower fertility in pre-industrial Europe

“…A drop in parity progression after a twinning event is expected since such an event may increase the risk of maternal death and reduce reproductive health 3 , 5 , 40 . Furthermore, mothers may show a reduced inclination to continue reproducing after the birth of twins, as suggested for some modern populations 41 , but see 42 . Fitting a GLMM to our parity progression data (see “Methods”) confirmed that mothers were indeed less likely to keep reproducing following the delivery of twins versus a singleton (Fig.…”

Re-introducing the Cambridge Group Family Reconstitutions

“…Some selection criteria have very little impact on results, but others can bias results in important ways. Unfortunately, some recent publications do not follow important rules for analyzing family reconstitutions, and they include results that are biased and incorrect (see critiques found in Alter, 2019;Clark, Cummins, & Curtis, 2019).…”