Elsevier

Evolution and Human Behavior

Volume 30, Issue 5, September 2009, Pages 329-341
Evolution and Human Behavior

Original Article
Social and biological determinants of reproductive success in Swedish males and females born 1915–1929

https://doi.org/10.1016/j.evolhumbehav.2009.03.007Get rights and content

Abstract

Studying biological and social determinants of mortality and fertility provides insight into selective pressures in a population and the possibility of trade-offs between short- and long-term reproductive success. Limited data is available from post-demographic transition populations. We studied determinants of reproductive success using multi-generational data from a large, population-based cohort of 13,666 individuals born in Sweden between 1915 and 1929. We studied the effects of birthweight for gestational age, preterm birth, birth multiplicity, birth order, mother's age, mother's marital status and family socioeconomic position (SEP) upon reproductive success, measured as total number of children and grandchildren. We further tested the hypothesis that number of grandchildren would peak at intermediate family size, as predicted by some life history explanations for fertility limitation. Reproductive success was associated with both social and biological characteristics at birth. In both sexes, a higher birthweight for gestational age, a term birth and a younger mother were independently associated with a greater number of descendants. A married mother and higher family SEP were also associated with a greater number of descendants in males (but not in females), while higher birth order was associated with a greater number of descendents in females (but not males). These effects were mediated by sex-specific effects upon the probability of marriage. Marriage was also affected by other early life characteristics including birthweight, indicating how ‘biological’ characteristics may operate via social pathways. Number of grandchildren increased with increasing number of children in both sexes, providing no evidence for a trade-off between quantity of offspring and their subsequent reproductive ‘quality’.

Introduction

The central driving force of evolution through natural selection is differential mortality and fertility among individuals. Together these determine an individual's reproductive success, usually defined as number of children or grandchildren. Reproductive success, in turn, influences an individual's long-term genetic contribution to future generations, this being at the core of the concept of ‘fitness’ in evolutionary biology (Endler, 1986). Investigating the biological and social characteristics associated with individual reproductive success in humans provides insight into the nature and magnitude of selective pressures operating in a particular society at a particular time. In combination with observations from different populations, hypothesis testing and evolutionary theory, this can clarify the selective pressures which shaped human evolution in the past, and the contexts in which these pressures become most evident (Clarke & Low, 2001). Understanding the social and biological determinants of mortality, fertility and reproductive success is also of much interest from a public health perspective as it can illuminate how systematic differences in health between social groups reappear in each subsequent generation and why social inequalities in health tend to persist across generations.

Yet studying reproductive success is challenging in humans because of the long generation time. Studies of contemporary populations usually use short-term and partial outcomes such as death during childhood (e.g., Sear, Steele, McGregor & Mace, 2002) or the probability of giving birth within an observation period (e.g., Ekholm, Carstensen, Finnstrom & Sydsjo, 2005) rather than direct and detailed measurements of survival and reproduction over the whole life-course. Using historical data can preserve the ability to trace long-term outcomes (Clarke & Low, 2001), but tends to be compromised by poor data quality as there is often substantial uncertainty regarding the completeness and accuracy with which mortality and/or fertility was recorded. Furthermore, such data is generally only available for pre-demographic transition populations; that is, populations predating the radical declines in family sizes observed across Western Europe in the 18th and 19th centuries and coinciding with increased urbanisation, industrialisation and material prosperity (Coale & Treadway, 1986). Historical populations therefore cannot be used to address the challenges which, as discussed in more detail below, are posed for evolutionary theory by the demographic transition. Finally, it is rare in both contemporary and historical studies for information to be available for different stages of an individual's life. This makes it difficult to apply a life-course approach in exploring how childhood characteristics are mediated by adult experiences in affecting survival and reproduction.

We had a unique possibility to explore social and biological determinants of reproductive success in Sweden during the 20th century, using multi-generational data from a large, population-based cohort born 1915–1929. This cohort was born at a time when the Swedish demographic transition was largely concluded: completed family size had fallen from over four for cohorts born 1736–1856 to under three in the 1886 cohort and to under two in the 1901 cohort (Eckstein, Mira & Wolpin, 1999). Most unusually for a cohort this old, high-quality data was collected at birth on a number of characteristics including birthweight and gestational age; birth multiplicity and birth order; mother's age and marital status at the time of birth; and family socioeconomic position (SEP). Sociodemographic and socioeconomic information was collected about cohort members during their adult lives, as was their number of descendants.

Two questions which relate to subcomponents of reproductive success have previously been examined in this cohort, specifically the role of birthweight for gestational age upon the probability of marriage (Vagero & Modin, 2002) and the role of birth order upon mortality (Modin, 2002). Reproductive success has not, however, previously been examined explicitly, in detail and over more than one generation. The aims of the current study were to investigate whether early life characteristics predict subsequent reproductive success; to ascertain the pathways mediating any observed effects; to examine the relationship between number of children and number of grandchildren; to investigate intergenerational effects upon number of grandchildren independent of number of children; and to examine whether any of the above effects were gender specific.

In the medical literature, recent years have seen some attention to how low birthweight, premature birth and other indicators of adverse health in early life predict long-term health outcomes, including a reduced probability of reproduction (Ekholm et al., 2005, Swamy et al., 2008). The effects of such biological markers of early development have received less attention from evolutionary anthropologists. While effects have been demonstrated in animal studies (Lindstrom, 1999), little research has addressed this question in human populations, particularly with regard to reproduction (Lummaa & Clutton-Brock, 2002).

More attention has been paid to factors such as sibling composition and the attendant consequences for parental investment and sibling resource competition. A number of studies of historical and traditional societies support the predictions of a life history theory in finding that having more siblings, particularly older siblings, has negative consequences for individual development, survival and long-term fertility (Gillespie et al., 2008, Hagen et al., 2006, Low, 1991, Mace, 1996). In post-transitional populations, the implications for reproductive success are less clear cut. There is some evidence that adverse effects on health and development persist (Downey, 2001, Lawson & Mace, 2008, Modin, 2002), but some studies have also found that larger natal family size and/or higher birth order is associated with higher fertility (Murphy, 1999, Murphy & Knudsen, 2002, Murphy & Wang, 2001, Pouta et al., 2005). Not all study populations born in the early 20th century show this relationship (Lopreato & Yu, 1988, Mueller, 2001) and several report it to be stronger in women than in men (Murphy, 1999, Murphy & Wang, 2001). The mechanisms of the association are also unclear, although they may result from a large natal family increasing an individual's own fertility preferences. It is plausible that this increase in fertility could compensate for any deleterious effects of high birth order on health and lead to higher reproductive success overall. To our knowledge no study has explicitly tested this.

Evolutionary anthropologists have also paid considerable attention to how reproductive success is influenced by social status or SEP in early and adult life. A strong positive correlation is consistently observed in traditional and hunter-gatherer populations (Hill & Kaplan, 1999, Low, 2000), but in modern post-demographic transition populations this association is often greatly attenuated or even reversed (Clarke & Low, 2001). In part this is because the fertility reductions associated with demographic transition usually occurred first and fastest in richer areas and among social elites (Livi-Bacci, 1986).

Post-demographic transition populations therefore show substantial society-wide fertility limitation at a time of unsurpassed affluence and often also feature a within-society levelling of reproductive success. Both features pose important theoretical challenges to the neo-Darwinian assumption that humans evolved to use their available resources to maximise reproductive success (Vining, 1986). Various theories have been proposed which try to account for these observations within an evolutionary framework (reviewed in Borgerhoff Mulder, 1998). Some theories suggest that fertility limitation is maladaptive in the current environment (e.g., Boyd & Richerson, 1985, Kaplan, 1996) or reflects behaviour designed to maximise some other aspect of fitness (e.g., Boone & Kessler, 1999). A prominent alternative argument, however, is that fertility limitation may optimise reproductive success in post-transition populations through a life history trade-off between offspring ‘quality’ and ‘quantity’ (Hagen et al., 2006, Mueller, 2001). The central hypothesis is that because total parental resources are limited, offspring quality will decrease with larger total family sizes. This in turn leads to each additional child making a smaller marginal contribution to the parent's long-term fitness (often operationalised as number of grandchildren). If at some point this marginal return becomes negative, then reproductive success will be maximised at intermediate family sizes in which each child receives more investment.

A key prediction of this hypothesis is that the average number of grandchildren will peak at an intermediate family size (Kaplan et al., 1995, Mueller, 2001). The limited empirical work conducted in pretransition populations provides support for such a relationship in some cases (Borgerhoff Mulder, 2000, Gillespie et al., 2008) but not others (Stevenson, Everson & Grimes, 2004). There is no support for this prediction in any of the studies we know of which test this hypothesis in a post-transition population. Rather in range of 20th century populations from the US (Kaplan et al., 1995, Mueller, 2001) and Germany (Mueller, 2001), the number of grandchildren increased across the range of number of children. Moreover, there was not even any suggestion of a reduction in the marginal fitness return of additional children in larger family sizes. Instead, number of grandchildren either increased in an approximately linear fashion with increasing number of children or, if anything, increased somewhat more rapidly at higher numbers of children.

Section snippets

Study population

The Uppsala Birth Cohort (UBCoS) includes all live births at the Uppsala University Hospital from 1915 to 1929 (N=14,193). Of these, 13,811 (97.3%) subjects were successfully traced through parish archives until death, emigration or until their unique personal registration number was assigned, usually in 1947 (further details in Leon et al., 1998).

Information about descendants of the cohort was obtained through linkage to the Swedish Multigeneration Registry for the 12,168 (85.7%) individuals

Determinants of total number of children and grandchildren

In total, 22,376 children and 41,153 grandchildren are registered for the 13,666 cohort members. This includes biological descendants given away for adoption (1.6% of children, 0.6% of grandchildren) and excludes nonbiological descendants adopted into the families of cohort members (1.6% of children and grandchildren). 99.4% of the children had fathers aged 15–49 at the time of their birth, and 99.9% had mothers aged 15–49, providing justification for our choice of this age range as defining

Discussion

To our knowledge, our study of 13,666 infants born in Uppsala between 1915 and 1929 is unique in providing a detailed investigation of the determinants of reproductive success over three generations. It demonstrates that an individual's social and biological characteristics at birth do predict long-term reproductive success even in a post-demographic transition population. In both sexes, a higher birthweight for gestational age, a term birth and a younger mother were independently associated

Conclusion

The UBCoS is representative of the Uppsala region and Sweden in 1915–1929 in terms of infant mortality (Rajaleid et al., 2008, SCB, 1918-1935, SCB, 1919-1932). The fertility of the cohort is also similar to that reported from large nationally representative Swedish cohorts from the same period (Bernhardt, 1971). This suggests that it may be possible to generalise our observations from this cohort to the Swedish native-born cohort of 1915–1929. One implication is that our findings can

Acknowledgments

We would like to thank Gunnar Andersson, Johan Fritzell, James MacCabe and Bitte Modin for advice given in the course of this study, and Dave Leon and David Lawson for advice and comments on draft versions of this manuscript.

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    The UBCoS and UBCoS Multigen study has received grants from the UK Medical Research Council, the Swedish Research Council and the Swedish Council for Working Life and Social Research. AG conducted this research while funded by the UK Economic and Social Research Council, and IK is currently funded by the Swedish Council for Working Life and Social Research.

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