In cooperatively breeding species, extended living in natal families
after maturity is often associated with limited breeding possibilities
and the ability to gain indirect fitness from helping relatives, with
family dynamics, such as parental presence and relatedness between
family members, playing a key role in determining the timing of own
reproduction. How family dynamics affect marriage and the onset of
reproduction in humans is complex and less well-understood. While
paternal absence can be associated with both earlier puberty and
reproductive behaviour, or with delayed reproduction if marriage
requires parental resources, in step-parent families, half-siblings
could further decrease the benefits from helping and delaying own
reproduction compared to families with only full-siblings. Such costs
and benefits are likely age-dependent, but have not been addressed in
previous studies. Using data from pre-industrial agrarian Finland, we
investigated if parental loss and remarriage affected marriage
probabilities of their differently-aged sons and daughters. We found
that parental composition had divergent effects across adulthood: loss
of a parent resulted in a higher probability to marry in early
adulthood, whereas parental presence increased later adulthood marriage
probability. Whilst the death of either parent was linked to an overall
lowered marriage probability, remarriage of the widowed parent,
especially mother, could mitigate this effect somewhat. Additionally,
the presence of underage full-siblings lowered marriage probability,
suggesting postponement of one's own reproduction in favour of helping
parental reproduction. Overall, our results support the idea that humans
are cooperative breeders, and show the importance of considering both
relatedness and age when investigating family dynamics.