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

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