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.
One of the central arguments surrounding monogamy is that of "naturalness," or whether the practice is inherent in human biology. Pepper Schwartz, a professor of sociology at the University of Washington in Seattle, does not believe humans are a "monogamous animal." She compared the species to geese: "A really monogamous animal is a goose – which never mates again even if its mate is killed." Obviously, many humans seek out alternative or additional partners in these situations and others.
The argument on whether or not monogamy comes naturally gets a bit moot at this point - after all, most of human behavior could potentially be described as unnatural. As Belinda Luscombe said, "There’s nothing natural about reading, using toilet paper, or skydiving."
As is the case with all scientific theories, no conclusive answer points to why humans practice monogamy, but it's not for a lack of hypotheses. Rather, so many believable theories on the origins of monogamy exist that scientists simply cannot come to a common consensus.
From the onslaught of disease transmitted by humans to a genetic instinct in fathers to protect their children, a host of different theories suppose why humans became (mostly) monogamous. Historians will note the importance of marriage in protecting wealth and status, whereas biologists would deem the practice "unnatural" solely based on the fact that monogamy rarely occurs in mammals.
Doctor Dieter Lukas with the University of Cambridge calls monogamy "a problem," and said he and his fellow biologists consider it something of an "evolutionary puzzle."
As Lukas points out, monogamy possesses no inherent positives. From a strictly biological standpoint, "male mammals could theoretically have more offspring by giving up on monogamy and mating with lots of females."
This brings Lukas to conclude monogamy comes about when females "spread out," making it more difficult for males to compete with other males for mating opportunities. In these situations, it makes more sense for a male to remain committed to a female to ensure mating potential. Though failing to explain why humans, in particular, are monogamous, this explanation does provide some answers as to why humans developed committed relationships in the first place.
Some Canadian scientists theorize monogamy became prevalent after the rise of large societies and, therefore, disease. Chris Bauch and his peers at the University of Waterloo came out with a study in which they mathematically mapped the evolution of human mating norms. They discovered the larger societies grew, the more widespread sexually transmitted disease became.
Bauch concluded that disease put pressure on humans to embrace monogamy as a way to prevent the spread of STDs. Furthermore, one of the side effects of certain STDs, such as syphilis and Chlamydia, is infertility, which could be a potential explanation for the push toward monogamy.