Monogamy does not normally occur in species where female habitat ranges are large. Social monogamy is relatively common among monogamous species, but genetic monogamy is the exception. Among birds, faithful sex partnership had been thought for many years to be widespread. Only recently, using advanced genetic fingerprinting techniques to determine parentage, have scientists determined that some thirty percent of the time, male birds help to raise chicks that are not their own. The same phenomenon exists in humans.
Many theories propose possible causes of sexual infidelity.
Experts say that female birds and mammals may be biologically driven to stray in order to pick up the best genes possible for her offspring. Males, on the other hand, may be driven by an impulse to father as many offspring as possible. They are biologically driven to stray by the desire to pass their genes through as many offspring as possible. Among species such as lions, mountain gorillas, and grizzly bears, this drive to influence the future genetically leads dominant males to kill and even eat the young of competing males. As a consequence of this complexity, groundbreaking research in molecular biology seeks to explain not only why people are unfaithful, but also why they are faithful at all.