A question commonly asked among historians, biologists, and anthropologists alike: "Are humans monogamous by nature?" Though the evolution of monogamy in humans follows something of a set trajectory, it largely fails to explain why humans are monogamous. If you take a look at any other species, you find that monogamy in animals rarely occurs, making its appearance in humans all the more bizarre. Even the question of when humans became monogamous presents a bit of a conundrum. Though monogamy existed in humans for many years, it was also there right alongside polygamy, making it difficult to trace exactly when monogamy arose.
Overall, no consensus points to exactly when or why humans adopted monogamy. However, a variety of different hypotheses do provide possible factors that likely contributed to its rise.
Even among humans, a seemingly monogamous species, these mating systems can be "flexible." According to a New York Times article: "Only 17 percent of human cultures are strictly monogamous. The vast majority of human societies embrace a mix of marriage types, with some people practicing monogamy and others polygamy. (Most people in these cultures are in monogamous marriages, though)."
Another popular theory supposes that males became monogamous to protect their offspring from other males. A report in the Proceedings of the National Academy of Sciences (PNAS) claims males, after fathering offspring, stayed with females to protect their children from being harmed or eliminated by other males.
The rationale behind this theory goes that if a mother nurses, she is less likely to mate, thereby ruining a male's chances to mate. So if the male disposes of the baby, he then gets to mate with the female. Belinda Luscombe worded the findings of the PNAS study as such:
"Males began balancing the need to spread their gene pool against the need to protect their young from being killed by other nonrelated males. The attacking males needed to kill the young so that they could breed with its mother, who would delay conception of another offspring if they were nursing. So the father hung around to ensure the safety of his genetic line and to help raise the young so that the mother could reproduce again sooner."
E E Smith writes for Psychology Today that "We also carry the biological imprint of polygamy, the opposite of lifelong fidelity to one mate." Smith analyzes the argument present in Professor David Barash's book on polygamy, in which he claims even if monogamy isn't "natural," it never deters humans from practicing it. Barash claims humans purposefully seek out activities that are complicated, such as playing the violin.
In the grand scheme of things, however, it might appear to humans as less complicated than the problems which arise from polygamy, such as disease and lack of a committed partner.
Although the practice developed over millions of years, what humans know today as monogamy emerged during the most recent millennia, according to Kit Opie. Scientists are still working to uncover how it specifically became a human trait, while other primates continue to practice polygamy to this very day.