  Families form the basis of society, and anthropologists have characterised
various family systems. This study developed a multi-level evolutionary model
of pre-industrial agricultural societies to simulate the evolution of family
systems and determine how each of them adapts to environmental conditions and
forms a characteristic socio-economic structure. In the model, competing
societies evolve, which themselves comprise multiple evolving families that
grow through family labour. Each family has two strategy parameters: the time
children leave the parental home and the distribution of inheritance among
siblings. The evolution of these parameters demonstrates that four basic family
systems emerge; families can become either nuclear or extended, and have either
an equal or unequal inheritance distribution. Nuclear families emerge where
land resources are sufficient, whereas extended families emerge where land
resources are limited. Equal inheritance emerges where the amount of wealth
required for a family to survive is large, whereas unequal inheritance emerges
where the required wealth is small. Analyses on the wealth distribution of
families demonstrate a higher level of poverty in extended families, and that
the accumulation of wealth is accelerated for unequal inheritance. By comparing
wealth distributions in the model with historical data, family systems are
associated with characteristic economic structures and modern social
ideologies. Empirical data analyses using the cross-cultural ethnographic
database verify the theoretical relationship between the environmental
conditions, family systems, and socio-economic structures. Theoretical studies
by this simple constructive model, as presented here, will integrate the
understandings of family systems in evolutionary anthropology, demography, and
socioeconomic histories.
