Maternal Risk of Breeding Failure Remained Low throughout the Demographic Transitions in Fertility and Age at First Reproduction in Finland

Abstract

Radical declines in fertility and postponement of first reproduction during the recent human demographic transitions have posed a challenge to interpreting human behaviour in evolutionary terms. This challenge has stemmed from insufficient evolutionary insight into individual reproductive decision-making and the rarity of datasets recording individual long-term reproductive success throughout the transitions. We use such data from about 2,000 Finnish mothers (first births: 1880s to 1970s) to show that changes in the maternal risk of breeding failure (no offspring raised to adulthood) underlay shifts in both fertility and first reproduction. With steady improvements in offspring survival, the expected fertility required to satisfy a low risk of breeding failure became lower and observed maternal fertility subsequently declined through an earlier age at last reproduction. Postponement of the age at first reproduction began when this risk approximated zero–even for mothers starting reproduction late. Interestingly, despite vastly differing fertility rates at different stages of the transitions, the number of offspring successfully raised to breeding per mother remained relatively constant over the period. Our results stress the importance of assessing the long-term success of reproductive strategies by including measures of offspring quality and suggest that avoidance of breeding failure may explain several key features of recent life-history shifts in industrialized societies.

Future Fertility in Low Fertility Countries

“…In Finland, for example, two surviving children per mother has been the dominant pattern for well over a century (Liu et al 2012). Women born in the 1960s had their first child two to three years later than those born in the 1950s and mean age at first birth is now around 29 years.…”

Lifetime reproduction and the second demographic transition: Stochasticity and individual variation

“…The demographic transition is characterized both by a decline in the number of children and a postponement in age of first childbearing, with declining mortality rates being a main driver and component of the transition (Liu, Rotkirch, and Lummaa 2012). Lower fertility levels are the main feature of the first demographic transition, which occurred in most European countries, starting in the latter half of the 19th century.…”

Changes in Length of Grandparenthood in Finland 1790-1959

“…This large and near-continuous increase in the number of years grandparents (both grandmothers and grandfathers) were available for their grandchildren, and the number of years a grandparent had grandchildren, coincided with the onset and progress of the demographic transition (Scranton et al 2016) and industrialisation in Finland, which brought about decreases in child mortality and fertility rates (Liu, Rotkirch and Lummaa 2012). It may be that an increasing presence of grandparents in early childhood because of increasing grandparental lifespan had as much a role in this expansion of shared time as decreasing childhood mortality.…”