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The behavioural ecology of modern families: a longitudinal study of parental investment and child development

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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.
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... 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. ...
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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.
... Some studies, conducted in the West, suggest a positive relationship between religiosity and parental investment in children (Bartkowski and Xu 2000), while here we find a negative relationship between religiosity and maternal investment. Reductions in maternal investment found here may be because of the higher fertility of religious mothers and the dilution of resources that occurs with larger family sizes (Becker and Lewis 1973;Lawson 2009;Lawson and Mace 2011). Our results may also reflect the costs of religious involvement (Sosis and Alcorta 2003), for example through religious attendance or charitable provisioning, and/or the higher levels of allomothering engaged in by religious mothers. ...
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Objectives Human childrearing is cooperative, with women often able to achieve relatively high fertility through help from many individuals. Previous work has documented tremendous socioecological variation in who supports women in childrearing, but less is known about the intracultural correlates of variation in allomaternal support. In the highly religious, high‐fertility setting of The Gambia, we studied whether religious mothers have more children and receive more support with their children. Methods We randomly sampled 395 mothers and 745 focal children enrolled in the Kiang West (The Gambia) Longitudinal Population Study cohort. Structured interviews asked mothers who and how often people invest in their children, and about their religious practices. Data were collected at participants' homes on electronic tablet‐based long‐form surveys and analyzed using the Bayesian hierarchical models. Results Religiosity was weakly associated with women's higher age‐adjusted fertility. Maternal religiosity was negatively related to maternal investment in focal children, but positively associated with total allomaternal support. Specifically, a woman's religiosity was positively associated with allomaternal support from matrilineal kin, other offspring, and affinal kin, but unrelated to paternal, patrilineal, and non‐kin investment. Conclusions These results suggest that higher fertility among religious mothers may be supported by high levels of investment from biological and affinal kin. Matrilineal kin, other siblings, and affinal kin seem to be the most responsive to a woman's religiosity. Our findings cast doubt on interpretations of women's religious behaviors as signals of fidelity, and instead suggest they may be part of strategies to enable collective allomaternal resources and higher relative fertility.
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... 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. ...
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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. ...
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... 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). ...
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