Group marriage is a marriage-like arrangement where three or more adults live together, all considering themselves partners, sharing finances, children, and household responsibilities. Group marriage is considered a form of polyamory. The term does not refer to bigamy as no claim to being married in formal legal terms is made.
The concept reentered popular consciousness in 1974 with the publication of Group Marriage: a study of contemporary multilateral marriage by Larry Constantine and Joan Constantine.
Depending on the sexual orientations of the individuals involved, all adults in the group marriage may be sexual partners of all others with whom they are compatible. For instance, if all members are heterosexual, all the women may have sexual relationships with all the men. If members are bisexual or pansexual, they may have evolved sexual relationships with either sex.
Group marriage implies a strong commitment to be "faithful" by having sex only within the group and intending to remain together for an extended period. The group may be 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.
The most common form of group marriage appears to be a triad of two women and one man, or less often two men and one woman.[better source needed] There are also polyfidelitous families formed by two heterosexual couples who become a foursome and live together as a family.
In most countries, it is not explicitly illegal for three or more people to form and share a sexual relationship (subject sometimes to laws against homosexuality), though such relational forms risk running afoul of state or local ordinances banning unmarried cohabitation. No Western country permits statutory marriage between 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.
The following instances are cited in Thomas 1906.
Group marriage occasionally occurred in communal societies founded in the 19th and 20th centuries.
A long-lived example was the Oneida Community founded by John Humphrey Noyes in 1848. Noyes taught that he and his followers, having reached 200 in number, had thus 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. The group began to falter about 1879-1881, eventually disbanding after Noyes fled arrest. Several dozen pairs of Oneidans quickly married in traditional fashion.
It is difficult to estimate the number of people who actually practice group marriage in modern societies, as such a 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 to be a group marriage.
Group marriage has been a literary theme, particularly in science fiction, and especially in 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. In at least The Moon Is a Harsh Mistress and Friday it is called "line marriage".
In several of her Hainish Cycle stories, Ursula Le Guin describes a type of four-person marriage known as a sedoretu, practiced on the planet O. In this arrangement, two men and two women are married to each other, but each member of the marriage has a sexual relationship only with one male and one female spouse.