Short answers, pulled from the story.
Monogamy, derived from the Greek for "one marriage", has four distinct definitions: genetic, sexual, social, and marital monogamy. Genetic monogamy requires DNA confirmation of exclusive reproduction; sexual monogamy means no outside sex partners; social monogamy describes shared living and resources; marital monogamy applies within the institution of marriage and includes both classical and serial forms.
In George P. Murdock's Ethnographic Atlas, which recorded 1,231 societies, only 186 were monogamous; 453 had occasional polygyny; 588 had more frequent polygyny; and 4 had polyandry. However, this does not account for relative population size, and in societies that tolerate polygamy, the majority of individuals often live in de facto monogamous relationships.
Genetic studies suggest that monogamy increased significantly within the last 10,000 to 20,000 years, corresponding to the Neolithic agricultural revolution. Paleoanthropological evidence based on sexual dimorphism in Australopithecus afarensis, from approximately 3.9 to 3.0 million years ago, is inconclusive and contested among researchers.
A review of 11 published studies by Simmons, Firman, Rhodes, and Peters found rates of extrapair paternity ranging from 0.03% to 11.8%, with a median of 1.8%. A separate review of 17 studies by Bellis, Hughes, Hughes, and Ashton found rates from 0.8% to 30%, with a median of 3.7%. Together, these imply that 96% to 98% of couples are genetically monogamous.
Anthropologist Jack Goody's study using the Ethnographic Atlas found a statistical correlation between intensive plough agriculture and social monogamy, running from Japan to Ireland. In plough farming societies, land was primarily men's work and tied to private property, making monogamous marriage a mechanism for keeping property within the nuclear family. A molecular genetic study also found that polygyny was typical until the shift to sedentary farming around 10,000 to 5,000 years ago in Europe and Asia.
Studies using the prairie vole, Microtus ochrogaster, show that oxytocin in females and vasopressin in males play a central role in developing pair bonds during mating. Uniquely in prairie voles, receptors for these hormones are located along the mesolimbic dopamine reward pathway, which appears to condition the animal to its mate's scent and consolidate social memory of the mating episode, a distribution not found in non-monogamous montane voles.