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.