An evolutionary approach to change of status–fertility relationship in human fertility transition

Abstract

The change in benefits of high socioeconomic status to fertility in humans during the demographic transition from high to low fertility has interested both demographers and evolutionary biologists. Evolutionary analyses add to demographic analyses by considering also males and status-related differential in male mating success, but they have been limited to time cross-sections and have not linked this differential to differentials in other determinants of male fertility. We use life-history records of males (n = 3791) entering marriage market before (1810s-1880s) and during (1890s-1960s) the Finnish fertility transition to investigate associations between socioeconomic status and chance and timing of marriage, choice for spouse, and lifetime fertility. Low status invariantly brought a lower marriage chance throughout these 160 years, which partly explained why ever-married high-status men lost advantage at early marriage when the system of achieving a high status shifted from inheritance to self-effort. The loss, coupled with assortative mating by age, promoted disappearance of differential in wife's age at marriage and thus, disappearance of differential in fertility between ever-married high-and low-status men in the Finnish fertility transition. Consequently, among all men (married and unmarried), statusrelated differential in lifetime fertility-not selection coefficient-declined over the transition. This study is among the first to show the interrelated dynamics of status-related differentials in male mating and reproductive traits; by doing so, it contributes to an evolutionary understanding of change of status-fertility relationship in human fertility transition and confirms continuing phenotypic selection on male status/wealth in modern societies.

Effects of the demographic transition on the genetic variances and covariances of human life-history traits

“…Thus, for all analyses, we required complete information for a number of parameters that were entered as fixed and random effects. As fixed effects, we included socioeconomic status as a two‐level factor (“landowner” vs. “landless”) because it is related to key life‐history trait differences in these populations (Pettay et al ; Gillespie et al ; Liu and Lummaa ), twinning status at birth as a two‐level factor (singleton or multiple) because it affects fitness (Lummaa et al ), birth order as a two‐level factor (firstborn son vs. others) because the firstborn son inherited the majority of the wealth (Faurie et al ), parish of origin as a factor with seven levels and sex of the individual as a two‐level factor (Bolund et al ). The reported variances and covariances are estimated after removing the variation due to these fixed effects.…”

“…Here, we use genealogical data over 15 generations from seven parishes in Finland to study changes in the genetic variance and covariance of four key life‐history traits; age at first and last reproduction, number of offspring, and longevity. Accurate church records of all births, marriages, and deaths across the country since the 18th century provide complete life‐history data for a representative sample of individuals living both before and after the demographic transition to lower fertility and mortality rates (Liu et al ; Liu and Lummaa ). During the study period, the society transformed from an agriculture‐ and fishing‐based economy to a modern industrialized nation (Singleton ).…”

The effects of resource availability and the demographic transition on the genetic correlation between number of children and grandchildren in humans

“…Before the demographic transition, landless individuals suffered more from frequent famines and epidemics of infectious diseases in this population, while landowners were protected from these negative extremes of fluctuations in resource availability (Hayward et al, 2012). In general, SES has been shown to influence long-term fitness (Pettay et al, 2007) and life history trade-offs (Gillespie et al, 2008;Liu and Lummaa, 2014) in this, as well as other preindustrial populations (Voland, 1990;Skjaervø et al, 2011). After industrialisation, these differences between socioeconomic classes with respect to resource availability diminished, which is reflected in smaller differences in reproductive success between poor and rich individuals (Liu and Lummaa, 2014, Results section).…”

“…Women who did not have a profession were assigned the SES of the (first) husband with which they reproduced. Although this dichotomous assignment of SES is based on occupation data only (in contrast to the inclusion of, for example, education, which however, was not available in the studied population during most of the study period), these two categories have consistently been associated with variation in many aspects of fitness in previous studies on this population (Pettay et al, 2007;Gillespie et al, 2008;Hayward et al, 2012;Liu and Lummaa, 2014). Furthermore, broad categories are necessary to achieve sufficient sample sizes within each SES group for quantitative genetic analyses.…”

Reduced costs of reproduction in females mediate a shift from a male-biased to a female-biased lifespan in humans

“…Importantly, over the last 200 years, many human populations have gone through the demographic transition to low birth and death rates 42 . This has led to changes in selection pressures on life history traits 43 44 45 46 and likely radically altered life-history trade-offs in modern societies. A key example is the rapid decline in fertility rates that likely result in reduced physiological costs of reproduction, particularly in females, and thus an average resource allocation to reproduction that may be too low to constrain life-history allocations 15 36 .…”