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The behavioural ecology of modern families: a longitudinal study of parental investment and child development
Abstract
In recent decades, behavioural ecologists have contributed to our understanding of the family
through extensive studies of animal and traditional human populations. This research
emphasises the importance of sibling competition for parental resources and adaptive
patterns of biased parental care. In contrast, modern human families are rarely considered by
behavioural ecologists, with increases in wealth generally considered to decrease the
importance of resource dilution within families and modern cultural rules discouraging of
unequal treatment of children. In this thesis, I question the validity of these assumptions and
use rich longitudinal data to consider family structure effects on parental investment and child
development in contemporary Britain. I consider time-based and financial investment in
offspring and measures of physical, cognitive and behavioural development over a 10 year
period. The following specific hypotheses are tested. First, parents will face a trade-off
between fertility, investment per child and ultimately child well-being. This hypothesis is
supported for all measures, except for behavioural well-being. Second, parents will bias
investment towards early-born offspring. This hypothesis is largely supported. Later-born
children receive lower investment and have reduced physical and cognitive well-being.
However, mental health is improved in the presence of older siblings. Third, parents will bias
investment towards male offspring. Support for this hypothesis is mixed. Measures of
investment indicate a male-bias driven by fathers, while number of brothers relative to sisters
is associated with reduced cognitive, but not physical or behavioural well-being. Fourth,
children with unrelated father figures will receive less investment. This hypothesis is
supported. Unrelated father figures are associated with lower investment from both parents
and reduced physical and behavioural well-being. Finally, I test the hypothesis that higher
socio-economic status will alleviate family size trade-offs. This hypothesis is rejected, with
some evidence that resource competition is of increased importance in relatively wealthy and
well-educated families.
... Previous work on ALSPAC by Mace (2009b, 2010), focusing on sibling competition, simultaneously explored the effects of household structure on multiple child outcomes. They found that stepfather presence did not have a significant effect on children's educational achievement or IQ (Lawson & Mace, 2009b). In contrast, stepfather presence was associated with detrimental effects on children's behavioural difficulty, whereby children in stepfather households scored higher in behavioural difficulties compared to children in single-mother or father-present households. ...
... Furthermore, stepfather investment itself has a positive effect, and the negative stepfather effect on educational achievement may be overcome if stepfathers are encouraged to interact more with their stepchildren. Our results differ from those of Lawson and Mace (2009b), who found that stepfather presence had no significant effect on educational achievement at age 4/5 years and 6/7 years. However, we believe this difference to be driven by sample size. ...
... However, we believe this difference to be driven by sample size. Lawson and Mace (2009b) took a cross-sectional approach as we have done, but did not impute missing values. This left their analyses with a comparatively smaller sample size (N = 3762 for 4/5 years, N = 4638 for 6/7 years), potentially leading to lack of power through small numbers of stepfathers and less accurate estimates. ...
In contemporary developed populations, stepfather presence has been associated with detrimental effects on child development. However, the proximate mechanisms behind such effects are yet to be fully explored. From a behavioural ecological perspective, the negative effects associated with stepfathers may be due to the reduced quantity and quality of investments children receive within stepfather households. Here, we build on previous studies by investigating whether the effects of stepfather presence on child outcomes are driven by differences in maternal and partner (i.e., father or stepfather) direct investments. We use data from the Avon Longitudinal Study of Parents and Children to explore stepfather effects on children’s educational achievement and behavioural difficulties at age 7. Our results indicate that, for educational achievement, stepfather effects are due to the lower levels of direct investments children receive. For behavioural difficulty, stepfather effects are due to multiple factors whereby stepfather presence is associated with greater difficulties independent of investment levels, and direct investments from stepfathers are ineffective. Our results suggest that the negative effects of stepfathers on child outcomes can be explained, in part, by the reduced quantity and the ineffectiveness of direct investments children receive from stepfathers. Furthermore, the effects of stepfather direct investments seem to vary between child outcomes.
... As a likely consequence of these deficits, children with many siblings perform worse on IQ tests and on formal educational assessments throughout life, a pattern recognised as one of the most stable relationships in the study of education (Blake, 1989;Downey, 1995Downey, , 2001Lawson, 2009;Steelman, Powell, Werum, & Carter, 2002;Zajonc, 1976). Number of siblings also has an important negative effect on achieved socio-economic status in adulthood, particularly on wealth ownership (Kaplan, Lancaster, Bock, & Johnson, 1995;Keister, 2003Keister, , 2004). ...
... In most cases, later-born children are at the biggest disadvantage in terms of both the division of parental investment (Lawson & Mace, 2009;Price, 2008) and relatively poor educational and physical health outcomes (Kristensen & Bjerkedal, 2007;Lawson, 2009;Lawson & Mace, 2008;see also: Modin, 2002). This pattern may be explained by the simple fact that older siblings, being alive both before and after a child's birth, have an increased potential to dilute parental resources (Downey, 2001). ...
... Unlike most past studies we consider both the effects of sibship size and birth order simultaneously. Furthermore, replicating the methodology of our past research into related family structure effects on parental investment and child development in the ALSPAC sample (Lawson & Mace, 2008, 2009, in press), we used detailed longitudinal data to estimate relationships net of an unusually large range of important covariates. ...
The social and health sciences have often emphasised the negative impacts of large sibship size and late birth order on childhood. For example, it is now well established that, other things being equal, children in large families and/or with many older siblings, receive lower allocations of care time from both parents, are more likely to grow up in conditions of economic hardship, and, as a likely consequence, exhibit relatively poor educational and physical health outcomes. Few researchers have, however, quantitatively assessed how siblings may influence indicators of mental health, where it is conceivable that social interactions with siblings may have a positive influence. Here, using data from a large British cohort survey (the Avon Longitudinal Study of Parents and Children), we explored the effects of sibling configuration on the Strengths and Difficulties Questionnaire, as a multidimensional index for mental health problems. We demonstrate a significant socio-economic gradient in mental health between the ages of three and nine years, but little evidence for negative effects of large sibship size. Rerunning this analysis to examine birth order, a much clearer pattern emerged; the presence of older siblings was associated with relatively good mental health, while the presence of younger siblings was associated with relatively poor mental health. This suggests that being born into a large family, providing the child is not joined by subsequent siblings, may carry important benefits unconsidered by past research. We discuss possible interpretations of this pattern and the wider implications for understanding the family context of child development.
... Parents therefore face a tradeoff between number of children and the success of those children. Studies find, for example, that a child's sibling number is negatively associated with cognitive outcomes including years of schooling (Gibbs et al., 2016), performance in schools (Lawson, 2009;, as well as physical outcomes, such as height (Lawson & Mace, 2008). Reductions to offspring outcomes, of course, have a negative effect on an individual's long-term reproductive success. ...
... Parents therefore face a tradeoff between number of children and the success of those children. Studies find, for example, that a child's sibling number is negatively associated with cognitive outcomes including years of schooling (Gibbs et al., 2016), performance in schools (Lawson, 2009;, as well as physical outcomes, such as height (Lawson & Mace, 2008). Reductions to offspring outcomes, of course, have a negative effect on an individual's long-term reproductive success. ...
... Parents therefore face a tradeoff between number of children and the success of those children. Studies find, for example, that a child's sibling number is negatively associated with cognitive outcomes including years of schooling (Gibbs et al., 2016), performance in schools (Lawson, 2009;, as well as physical outcomes, such as height (Lawson & Mace, 2008). Reductions to offspring outcomes, of course, have a negative effect on an individual's long-term reproductive success. ...
... Similarly, childhood play, both supervised by and involving adults, has been argued to be a necessary component of childhood for optimal child development (Ginsburg, 2007). This paternal caregiving measure has been found to vary by household characteristics (Lawson & Mace, 2009) and associated with various child outcomes (Lawson, 2009). This suggests that the current measure has predictive validity meeting our theoretical assumptions, functioning as an appropriate proxy of paternal direct investments for our current study. ...
Studies show that fathers across Western populations tend to provide more care to sons than daughters. Following a human behavioral ecological framework, we hypothesize that son-biases in fathering may (at least in part) be due to differences in fitness returns to paternal direct investments by child’s sex. In this study, we investigate sex-differences in the associations between paternal caregiving and children’s outcomes in stable, two-parent families. Using data from the Avon Longitudinal Study of Parents and Children, we test whether paternal caregiving in early childhood is associated with different effects on children’s school test scores and behavioral difficulties by children’s sex. Overall, we find that paternal caregiving is associated with higher school test scores and lower behavioral difficulty scores, but the association between paternal caregiving and school test scores was stronger for boys. Our findings highlight possible sex-differences in returns to paternal caregiving for certain domains of child outcomes in England.
... In post-industrial environments, increased family size is associated with decreased child development outcomes [72][73][74]93]. The ALSPAC data analysed here have previously been used to demonstrate a trade-off between number of offspring and offspring developmental outcomes [72,94], and to show that parental investment in a child is inversely related to a child's number of siblings [73]. If the trade-off between offspring outcomes and offspring quantity is owing to a dilution of parental resources, then parents who can draw resources from larger and stronger support networks can be expected to be partially buffered from these trade-offs. ...
Many aspects of religious rituals suggest they provide adaptive benefits.
Studies across societies consistently find that investments in ritual behaviour
return high levels of cooperation. Another line of research finds that alloparental
support to mothers increases maternal fertility and improves child
outcomes. Although plausible, whether religious cooperation extends to
alloparenting and/or affects child development remains unclear. Using
10 years of data collected from the Avon Longitudinal Study of Parents
and Children (ALSPAC), we test the predictions that church attendance
is positively associated with social support and fertility (n = 8207 to n =
8209), and that social support is positively associated with fertility and
child development (n = 1766 to n = 6561). Results show that: (i) relative to
not attending, church attendance is positively related to a woman’s social
network support and aid from co-religionists, (ii) aid from co-religionists
is associated with increased family size, while (iii) fertility declines with
extra-religious social network support. Moreover, while extra-religious
social network support decreased over time, co-religionist aid remained constant.
These findings suggest that religious and secular networks differ
in their longevity and have divergent influences on a woman’s fertility.
We find some suggestive evidence that support to mothers and aid from
co-religionists is positively associated with a child’s cognitive ability at
later stages of development. Findings provide mixed support for the premise
that ritual, such as church attendance, is part of a strategy that returns high
levels of support, fertility and improved child outcomes. Identifying the
diversity and scope of cooperative breeding strategies across global religions
presents an intriguing new horizon in the evolutionary study of religious
systems.
... Family size also has a strong negative influence on allocations of care-time to individual children from both mothers and fathers; with family size having a larger influence on parental time investment over the first decade of life than any other covariate considered, including socio-economic indicators and parental age (Lawson & Mace, 2009). Studies throughout the developed world show that children in larger families perform significantly worse on IQ tests and on formal educational assessments throughout life, a pattern recognised as one of the most stable relationships in the study of education (Blake, 1989;Downey, 1995;Downey, 2001;Lawson, 2009;Steelman et al., 2002). There is also evidence that the presence of siblings is associated with deficits childhood growth, which may stem from reduced parental attention to healthcare or nutrition in early life (Lawson & Mace, 2008). ...
The purpose of this article is to develop a model of life history theory that incorporates environmental influences, contextual
influences, and heritable variation. I argue that physically or psychologically stressful environments delay maturation and
the onset of reproductive competence. The social context is also important, and here I concentrate on the opportunity for
upward social mobility as a contextual influence that results in delaying reproduction and lowering fertility in the interest
of increasing investment in children. I also review evidence that variation in life history strategies is influenced by genetic
variation as well. Finally, I show that cultural shifts in the social control of sexual behavior have had differential effects
on individuals predisposed to high- versus low-investment reproductive strategies.
We use the German Socio-Economic Panel to investigate to what extent the number of siblings has an effect on the frequency of cognitively stimulating activities mothers undertake with their children aged 2-3 and 5-6. The resource dilution model assumes that this relationship should be negative. We introduce the resource augmentation hypothesis and argue that increasing sibship size can also have a positive effect when efficiency gains, the public good character of maternal activities, reallocation of maternal time, and child care by older siblings are considered. Our results from intra-individual and between family analyses suggest that resource dilution is unavoidable in large families but that resource-augmenting processes are also at work. The relationship between the number of siblings and activity frequency is not linear and can be reversed from negative to positive in smaller families depending on the presence of younger siblings.
Human reproductive behaviour is marked by exceptional variation at the population and individual level. Human behavioural ecologists propose adaptive hypotheses to explain this variation as shifting phenotypic optima in relation to local socioecological niches. Here we review evidence that variation in fertility (offspring number), in both traditional and modern industrialized populations, represents optimization of the life-history trade-off between reproductive rate and parental investment. While a reliance on correlational methods suggests the true costs of sibling resource competition are often poorly estimated, a range of anthropological and demographic studies confirm that parents balance family size against offspring success. Evidence of optimization is less forthcoming. Declines in fertility associated with modernization are particularly difficult to reconcile with adaptive models, because fertility limitation fails to enhance offspring reproductive success. Yet, considering alternative measures, we show that modern low fertility confers many advantages on offspring, which are probably transmitted to future generations. Evidence from populations that have undergone or initiated demographic transition indicate that these rewards to fertility limitation fall selectively on relatively wealthy individuals. The adaptive significance of modern reproductive behaviour remains difficult to evaluate, but may be best understood in response to rising investment costs of rearing socially and economically competitive offspring.
The equity heuristic is a decision rule specifying that parents should attempt to subdivide resources more or less equally among their children. This investment rule coincides with the prescription from optimality models in economics and biology in cases in which expected future return for each offspring is equal. In this article, the authors present a counterintuitive implication of the equity heuristic: Whereas an equity motive produces a fair distribution at any given point in time, it yields a cumulative distribution of investments that is unequal. The authors test this analytical observation against evidence reported in studies exploring parental investment and show how the equity heuristic can provide an explanation of why the literature reports a diversity of birth order effects with respect to parental resource allocation.
