We present a simple model for the evolution of social behavior in family-structured, finite sized
populations. Interactions are represented as evolutionary games describing frequency-dependent
selection. Individuals interact more frequently with siblings than with members of the general
population, as quantified by an assortment parameter r, which can be interpreted as “relatedness”.
Other models, mostly of spatially structured populations, have shown that assortment can promote the
evolution of cooperation by facilitating interaction between cooperators, but this effect depends on the
details of the evolutionary process. For our model, we find that sibling assortment promotes cooperation
in stringent social dilemmas such as the Prisoner's Dilemma, but not necessarily in other situations.
These results are obtained through straightforward calculations of changes in gene frequency. We also
analyze our model using inclusive fitness. We find that the quantity of inclusive fitness does not exist for
general games. For special games, where inclusive fitness exists, it provides less information than the
straightforward analysis.