Monogamy

Monogamy is a relationship of two individuals in which they form a mutual and exclusive intimate partnership. Having only one partner at any one time, whether for life or serial monogamy, contrasts with various forms of non-monogamy.
The term monogamy, derived from Greek for "one marriage," has multiple context-dependent meanings—genetic, sexual, social, and marital—each varying in interpretation across cultures and disciplines, making its definition complex and often debated. The term is typically used to describe the behavioral ecology and sexual selection of animal mating systems, referring to the state of having only one mate at any one given time. In a human cultural context, monogamy typically refers to the custom of two individuals, regardless of orientation, committing to a sexually exclusive relationship.
Monogamy in humans varies widely across cultures and definitions. While only a minority of societies are strictly monogamous, many practice serial monogamy or tolerate extramarital sex. Genetic monogamy is relatively unstudied and often contradicted by evidence of extrapair paternity. Monogamy in humans likely evolved through a combination of biological factors such as the need for paternal care and ecological pressures, alongside cultural developments like agriculture, property inheritance, and religious or societal norms promoting social stability.
Biologists distinguish between social, sexual, and genetic monogamy to reflect how animal pairings may involve cohabitation, sexual exclusivity, and reproductive fidelity in varying combinations, while serial monogamy describes successive exclusive relationships over time.

Terminology

The word monogamy derives from the Greek μονός, monos, and γάμος, gamos, referring to the functional social behaviour of pair-bonding. The term can then be subsequently subclassified by context-dependent relational types. Generally, there are four intersecting definitions.

genetic monogamy refers to sexually monogamous relationships with genetic evidence of paternity.
sexual monogamy refers to two partners remaining sexually exclusive with each other and having no outside sex partners.
social monogamy refers to two individuals co-habitating, maintaining a sexual relationship, and sharing basic resources such as shelter, food, and parenting responsibilities.
marital monogamy refers to marriages of only two people, within the context of the institution of marriage.

For instance, biologists, biological anthropologists, and behavioral ecologists often use monogamy in the sense of sexual, if not genetic, exclusivity. When cultural or social anthropologists and other social scientists use the term monogamy, the meaning is social or marital monogamy.
Marital monogamy may be further distinguished between:

classical monogamy, "a single relationship between people who marry as virgins, remain sexually exclusive their entire lives, and become celibate upon the death of the partner"
serial monogamy, marriage with only one other person at a time, in contrast to bigamy or polygamy

Defining monogamy across cultures can be difficult because of different cultural assumptions. Some societies believe that monogamy requires limiting sexual activity to a single partner for life. Others accept or endorse pre-marital sex prior to marriage. Some societies consider sex outside of marriage or "spouse swapping" to be socially acceptable. Some consider a relationship monogamous even if partners separate and move to a new monogamous relationship through death, divorce, or simple dissolution of the relationship, regardless of the length of the relationship. The need to accurately define monogamy was highlighted in a 2012 work, which defined practices as either formal or informal polyandry. The researchers found 53 communities studied between 1912 and 2010 that practiced polyandry. This broader definition indicated that polyandry was more common worldwide than previously believed.
Terminology may also affect how data on polygamy is interpreted. While the genetic record indicates that genetic monogamy increased within the last 5,000-10,000 years, the form of prehistoric non-monogamy is less clear. A lack of genetic monogamy could be interpreted as polygamy despite other [|plausible explanations]. Anthropological observations indicate that even when polygyny is accepted in the community, the majority of relationships in the society are monogamous in practice – while couples remain in the relationship, which may not be lifelong. Thus, in prehistoric communities and communities categorized as polygamous, short- or long-term serial monogamy may be the most common practice rather than a lifelong monogamous bond.

Frequency in humans

Distribution of social monogamy

According to the Ethnographic Atlas by George P. Murdock, of 1,231 societies from around the world noted, 186 were monogamous; 453 had occasional polygyny; 588 had more frequent polygyny; and 4 had polyandry.
Divorce and remarriage can thus result in "serial monogamy", i.e. multiple marriages but only one legal spouse at a time. This can be interpreted as a form of plural mating, as are those societies dominated by female-headed families in the Caribbean, Mauritius and Brazil where there is frequent rotation of unmarried partners. In all, these account for 16 to 24% of the "monogamous" category.

Prevalence of sexual monogamy

The prevalence of sexual monogamy can be roughly estimated as the percentage of married people who do not engage in extramarital sex. The Standard Cross-Cultural Sample describes the amount of extramarital sex by men and women in over 50 pre-industrial cultures. The amount of extramarital sex by men is described as "universal" in 6 cultures, "moderate" in 29 cultures, "occasional" in 6 cultures, and "uncommon" in 10 cultures. The amount of extramarital sex by women is described as "universal" in 6 cultures, "moderate" in 23 cultures, "occasional" in 9 cultures, and "uncommon" in 15 cultures.
Surveys conducted in non-Western nations also found cultural and gender differences in extramarital sex. A study of sexual behavior in Thailand, Tanzania and Côte d'Ivoire suggests about 16–34% of men engage in extramarital sex while a much smaller percentage of women engage in extramarital sex. Studies in Nigeria have found around 47–53% of men and to 18–36% of women engage in extramarital sex. A 1999 survey of married and cohabiting couples in Zimbabwe reports that 38% of men and 13% of women engaged in extra-couple sexual relationships within the last 12 months.
Many surveys asking about extramarital sex in the United States have relied on convenience samples: surveys given to whoever happens to be easily available. Convenience samples may not accurately reflect the population of the United States as a whole, which can cause serious biases in survey results. Sampling bias may, therefore, be why early surveys of extramarital sex in the United States have produced widely differing results: such early studies using convenience samples reported the wide ranges of 12–26% of married women and 15–43% of married men engaged in extramarital sex. Three studies have used nationally representative samples. These studies in 1994 and 1997 found that about 10–15% of women and 20–25% of men engage in extramarital sex.
Research by Colleen Hoffon of 566 homosexual male couples from the San Francisco Bay Area found that 45% had monogamous relationships. However, the Human Rights Campaign has stated, based on a Rockway Institute report, that "LGBT" young people... want to spend their adult life in a long-term relationship raising children." Specifically, over 80% of the homosexuals surveyed expected to be in a monogamous relationship after age 30.

Prevalence of genetic monogamy

The incidence of genetic monogamy may be estimated from rates of extrapair paternity. Extrapair paternity is when offspring raised by a monogamous pair come from the female mating with another male. Rates of extrapair paternity have not been extensively studied in people. Many reports of extrapair paternity are little more than quotes based on hearsay, anecdotes, and unpublished findings. Simmons, Firman, Rhodes, and Peters reviewed 11 published studies of extra-pair paternity from various locations in the United States, France, Switzerland, the United Kingdom, Mexico, and among the native Yanomami Indians of Amazon forest in South America. The rates of extrapair paternity ranged from 0.03% to 11.8% although most of the locations had low percentages of extrapair paternity. The median rate of extrapair paternity was 1.8%. A separate review of 17 studies by Bellis, Hughes, Hughes, and Ashton found slightly higher rates of extrapair paternity. The rates varied from 0.8% to 30% in these studies, with a median rate of 3.7% extrapair paternity. A range of 1.8% to 3.7% extrapair paternity implies a range of 96% to 98% genetic monogamy. Although the incidence of genetic monogamy may vary from 70% to 99% in different cultures or social environments, a large percentage of couples remain genetically monogamous during their relationships. A review paper, surveying 67 other studies, reported rates of extrapair paternity, in different societies, ranging from 0.4% to over 50%.
Covert illegitimacy is a situation which arises when someone who is presumed to be a child's father is in fact not the biological father. Frequencies as high as 30% are sometimes assumed in the media, but research by sociologist Michael Gilding traced these overestimates back to an informal remark at a 1972 conference.
The detection of unsuspected illegitimacy can occur in the context of medical genetic screening, in genetic family name research, and in immigration testing. Such studies show that covert illegitimacy is in fact less than 10% among the sampled African populations, less than 5% among the sampled Native American and Polynesian populations, less than 2% of the sampled Middle Eastern population, and generally 1–2% among European samples.
Pedigree errors are a well-known source of error in medical studies. When attempts are made to try to study medical afflictions and their genetic components, it becomes very important to understand non-paternity rates and pedigree errors. There are numerous software packages and procedures that exist for correcting research data for pedigree errors.