A heritable positive and causal relationship will have been selected between pairbond strength and stability and reproductive success if a male-female pair bond has been the major unit of reproduction during the evolutionary history of a population. All the descriptive studies reported on pair-bonding birds and on humans are consistent with this evolutionary deduction. Pair-bonded individuals with poor reproductive success are predicted to alter their behavior so that their reproductive success is increased or so that their inclusive fitness is increased through kin selection. Specific examples of these alterations are elaborated for a monogamous and territorial population. Five hypotheses are formulated so that the assumptions on which the deduction is based may be tested experimentally in specific subject populations of animals.