I describe an agent-based model to study the evolution of paternal care. The reported
Iterated Prisoner’s Dilemma shows that the relative differences in the reproductive effort between sexes can explain the evolution of paternal care. When female reproductive costs are higher than male reproductive costs, males cooperate with females even when females do not reciprocate. Paternal care is thus an evolutionary achievement to compensate for the energy demands that reproduction involves for mothers. Paternal care, in turn, produces a sustained population growth, since females can reproduce at higher rates.