Monogamy is intended to be a life-long comittment, with complete sexual fidelity of the spouses to each other. However, when viewed from an evolutionary psychology perspective, the pressures on men and women to break that committment are not the same. As men age, they retain their capacity for reproduction and are subject to selective forces that may involve increasing their inclusive fittness if they can successfully attract another woman. Women are much more limited in their reproductive opportunities as they age, and have a greater interest in insuring the survival and success of their existing children. The leading factor in the dissolution of marriages in the United States is dissatisfaction of the man.
Additional data come from a survey of primates and their patterns of pair-bonding.
Among the apes, differences in body size between males and females reflect the extent of polygyny. On the other end of the scale, gibbons are completely monogamous, and the males and females are identical in size. Even though polygyny is allowed in most human cultures, it is actually not common. Most marriages, even in polygynous societies, are in fact monogamous. This is because of economic considerations.