Group marriage (a form of polyamory) is a marriage-like arrangement between more than two people. Usually consisting of three to six adults, all partners live together, share finances, children, and household responsibilities. Depending on the sexual orientation of the members, all adults in the family are sexual partners. For instance, if all members are heterosexual, all the women may have sexual relationships with all the men. If the members are bisexual, they may have sexual relationships with the women as well as the men. And so on. However, this is a closed system (closed relationships are good for avoiding Sexually transmitted diseases if everyone is tested and is healthy) and sex is only allowed within the group - no outside sexual relationships are allowed. Some families are open to taking on new partners, but only if all members of the family agree to accept the new person as a partner. The new person then moves into the household and becomes an equal member of the family. Currently, the most common form of group marriage is a triad of two women and one man, or two men and one woman. However, there have recently been a number of polyfidelitous families formed by two heterosexual couples who become a four-some and live together as a family.
Line marriage is a form of group marriage found in fiction in which the family unit continues to add new spouses of both sexes over time so that the marriage does not end.
Legal aspects 
For group marriage as a type of marriage, that includes weddings and describing/considering each other as married, spouses, wives/husbands, etc. see Bigamy, Polygamy in North America or Polygamy in Christianity.
"Group marriage" can more informally describe a polyamorous relationship, "… our little group marriage", when no claim to being married is made.
In most countries, it is legal for three or more people to form and share a sexual relationship (subject sometimes to laws against homosexuality). However, no Western countries permit marriage among more than two people. Nor do they give strong and equal legal protection (e.g., of rights relating to children) to non-married partners – the legal regime is not comparable to that applied to married couples. Individuals involved in polyamorous relationships are considered by the law to be no different from people who live together, or "date", under other circumstances.
Non-European cultures 
- Among the Sandwich Islanders, the relationship of punalua involved "the fact that two or more brothers with their wives, or two or more sisters with their husbands, were inclined to possess each other in common".
- In some parts of Melanesia, there are "sexual relations between a group of men formed by the husband's brothers and a group of women formed by the wife's sisters".
- Friedrich Ratzel in The History of Mankind reported in 1896 that In Hawaii a kind of incipient polyandry arose by the addition to the marriage establishment of a cicisbeo, known as Punalua.
- Toda people, who live on the isolated Nilgiri plateau of Southern India practiced adelphic polyandry for centuries, but no longer do so. Adelphic polyandry occurs when brothers share the same wife or wives. Such arrangements has happened in Himalayian tribes too.
The following instances are cited in Thomas 1906.
- In North America there is "group marriage as existing among the Omahas … adelphic polygyny."
- Among the Dieri of Australia exist forms of spouse-sharing known as pirrauru, in two categories "according as the man has or has not a tippa-malku wife. In the first case it is, taken in combination with the tippa-malku marriage, a case of bilateral dissimilar adelphic (M. and F.) polygamy. In the latter case it is dissimilar adelphic (tribal) polyandry". The pirrauru "relation arises through the exchange by brothers of their wives".
- Among the Kurnandaburi of Australia, "a group of men who are own or tribal brothers are united … in group marriage".
- Among the Wakelbura of Australia, there is "adelphic polyandry."
- Among the Kurnai of Australia, "unmarried men have access to their brothers' wives."
In modern U.S. cultures 
Group marriage occasionally occurred in communal societies founded in the 19th and 20th centuries.
An exceptionally long-lived example was the Oneida Community founded by John Humphrey Noyes in 1848. Noyes taught that he and his followers, which reached 200 in number, had undergone sanctification; that is, it was impossible for them to sin, and that for the sanctified, marriage (along with private property) was abolished as an expression of jealousy and exclusiveness. The Oneida commune lived together as a single large group and shared parental responsibilities. Any given male-female combination in the group was free to have sex, usually upon the man's asking the woman, and this was the common practice for many years. In effect it functioned as a large group marriage until about 1879-1881. Nor did the Oneida Community self-destruct as happens with many communes. Noyes heard a New York warrant was out for his arrest, perhaps for adultery, though apparently not for anyone else in the group, and he fled to Canada, not many miles away. He lived there the rest of his life. After some period without his leadership and acting at Noyes' suggestion, the group disbanded. Several dozen pairs of them quickly married in traditional fashion after disbanding. They and others nearby then created the Oneida Silver company that for many decades was a famous US company name in flatware and related items.
The Kerista Commune practiced group marriage in San Francisco from 1971 to 1991.
It is difficult to estimate the number of people who actually practice group marriage in modern societies, as this form of marriage is not officially recognized or permitted in any jurisdiction in the U.S., and de jure illegal in many. It is also not always visible when people sharing a residence consider themselves privately as a group marriage.
In popular culture 
- Interest in, and practice of nonmonogamy is well known in modern science fiction fandom. Group marriage has been a theme in some works of science fiction — especially the later novels of Robert A. Heinlein, such as Stranger in a Strange Land, Friday, Time Enough for Love, and The Moon Is a Harsh Mistress. Stranger in a Strange Land describes a communal group much like the Oneida Society. A domestic partnership consisting of four people who are all married to each other features in Vonda N. McIntyre's Starfarers series.
- The Moon Is a Harsh Mistress describes line families in detail. The characters argue that the line family creates economic continuity and parental stability in an unpredictable, dangerous environment. Manuel's line marriage is said to be over 100 years old. The family is portrayed as economically comfortable because improvements and investments made by previous spouses compounded, rather than being lost between generations. Heinlein also makes it a point that this family is racially diverse. A passing reference to Heinlein's marriage forms is made in David Brin's Infinity's Shore, where a sapient bottlenose dolphin crewmember is noted as belonging to a "line marriage, one of the Heinlein forms."
- Group marriages of three partners (called triples) are described as commonplace in the 1966 novel Babel-17 by Samuel R. Delany. The novel's protagonist, a female starship captain Rydra Wong, once lived in a triple, until one member died and another was put in stasis for an incurable illness. Other crew members, especially those who worked in close three-person teams, are noted for this type of relationship.
- Group marriage is advocated in Robert Rimmer's 1968 novel Proposition 31, the story of two middle-class, suburban California couples who turn to a polyamorous relationship to deal with their multiple infidelities as an alternative to divorce. The novel is written as a case study by a psychologist supporting a fictional ballot initiative "Proposition 31" that would amend the California Constitution to permit polyamory relationships. In the book, the solution to the couples' problems with adultery and the impregnation of one of the couple's wives by the other couple's husband is to commit to a group marriage to raise their five children in a home compound in which the husbands rotate among the wives. The book is a plea to pass this proposed proposition to offer an alternative to divorce.
- Line marriage is also commonly practiced in Joe Haldeman's 1981 novel Worlds. Haldeman describes how individual families joined forces to avoid inheritance taxes.
- Group marriage is a central plot element in Donald Kingsbury's 1982 novel Courtship Rite. A six-partner group marriage (three male, three female) is considered the ideal norm in the alien society described in the novel; the main characters are in a five-partner group marriage, and much of the dramatic tension hinges on there being more than one candidate for the sixth position.
- Group marriage is briefly addressed in the 1989 Star Trek novel Star Trek: The Lost Years, by J.M. Dillard. A minor character, Lt. Nguyen, enters into a group marriage, and this is portrayed as a relatively normal occurrence. Additionally, in Star Trek: Enterprise, the alien Dr. Phlox comes from a world where such relationships are the norm.
- William H. Keith, Jr., under his pseudonym Ian Douglas, describes line marriages as the norm, with large family clans extending through many generations, in his Heritage Trilogy, and its two sequels Legacy Trilogy and Inheritance Trilogy. These three trilogies are about the US Marine Corps battling aliens over a period of a thousand years.
- In the television series Caprica, the character Sister Clarice is a participant in a group marriage.
- Several short stories by Ursula K. Le Guin take place on the planet O, where a four-person marriage, called a sedoretu, is common. The sedoretu consists of both a man and a woman from each of two moieties; since it considered incest to have sex with someone of the same moiety, each participant in the marriage has a sexual relationship with only two out of the three other participants.
- In the Earth's Children series by Jean Auel, the Sharamudoi practice group marriage in the form of "cross-mating" a couple from their land-dwelling clan with a couple from the water-dwelling clan. Children of both couples are considered progeny of all four mates, but the sexual relationships between cross-mates is not clear.
See also 
- Constantine, Larry and Joan (1974). Group Marriage: A Study of Contemporary Multilateral Marriage. Collier Books. ISBN 0-02-075910-X, 978-0020759102 Check
- Dillard, J.M. (1989). The Lost Years. Pocket Books. p. 440. ISBN 0-7434-5422-7, ISBN 978-0-7434-5422-3 Check
- Emens, Elizabeth F. (2004). "Monogamy's Law: Compulsory Monogamy and Polyamorous Existence". New York University Review of Law & Social Change 29 (2): 277.
- Heinlein, Robert (1966). The Moon Is a Harsh Mistress. Tom Doherty Associates, Inc. pp. 260–262. ISBN [[Special:BookSources/0-312-86335-1|0-312-86335-1 [[Category:Articles with invalid ISBNs]]]] Check
- Murdock, George P. (1949). Social Structure. New York: The MacMillan Company. ISBN 0-02-922290-7.
- Murdock, George P. "Ethnographic Atlas Codebook", derived from Ethnographic Atlas
- Westermarck, Edward. The History of Human Marriage. Allerton Book Company, New York, 1922.
- Heinlein, 1966, pp. 260–262.
- Westermarck 1922, Part III, p. 240
- Westermarck 1922, Part III, p. 241
- Ratzel, Friedrich. The History of Mankind, VolI P277. (London: MacMillan, 1896). URL: www.inquirewithin.biz/history/american_pacific/oceania/position-women.htm accessed 11 April 2010.
- Northcote W. Thomas: Kinship Organizations and Group Marriage in Australia. Cambridge, at the University Press, 1906. Chapter XIII.