There are two types of monogamy: social and genetic.
In the first case, partners work together to raise their offspring. With genetic monogamy, parents are faithful sex partners.
While social monogamy is relatively common, the genetic issue is the exception rather than the rule.
In fact, there are only two species of monkeys, the marmoset and the tamarin, that are truly monogamous. All other primates, humans included, often mate outside their partnerships.
Among birds, it has been thought for years that faithful sex partnership is widespread.
In such a concern, the eastern bluebird was considered a prime example, with male and female partners working together to build nests, incubate eggs, then feed and raise their young. However, studies using genetic testing techniques showed that 15% to 20% of chicks cared for by a pair of bluebirds were not fathered by the male (*).
Among several hypotheses to explain the above facts, what follows stands up:
1. Genetic male infidelity
Females stray to gather the best possible genes for their offspring, while males are driven to father as many and as often as possible. The first pattern would support the fact that females seek males of high status, to produce offspring of higher quality that will be able to survive better. The second pattern is in accordance with the biological role of males, in order to scatter their genes into as many members of the next generation as possible (**).
2. Social monogamy
Monogamy could have been originated among species whose young survived best when raised by a bonded pair. This may have led to the rise of monogamy among people, since human children take so long to mature.
References
(*) P. A. Gowarty. Behavioural ecology. University of Georgia.
(**) T. Emlen. Evolutionary behaviour. Cornell University.