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Open marriage typically refers to a marriage in which the partners agree that each may engage in extramarital sexual relationships, without this being regarded as infidelity. There are many different styles of open marriage (such as swinging and polyamory), each with the partners having varying levels of input on their spouse's activities.
History of the term 
The origins of the term open marriage remain obscure. Researchers in the 1960s used the term open marriage to describe individual freedom in choosing marriage partners. Closed marriage meant individuals had to marry someone based on social prohibitions and social prescriptions. Open marriage meant individuals could choose to marry someone based on personal preferences.
Nena O'Neill and George O'Neill changed the meaning of the term with the 1972 publication of their book Open Marriage, which sold over 1.5 million copies. The O'Neills conceived open marriage as one in which each partner has room for personal growth and can develop outside friendships. Most chapters in the book dealt with non-controversial approaches to revitalizing marriage in areas of trust, role flexibility, communication, identity, and equality. Chapter 16, entitled "Love Without Jealousy", devoted 20 pages to the proposition that an open marriage could include some forms of sexuality with other partners. These concepts entered the cultural consciousness and the term "open marriage" became a synonym for sexually non-monogamous marriage, much to the regret of the O'Neills. In the 1977 publication of The Marriage Premise, Nena O'Neill advocated sexual fidelity in the chapter of that name. By then however, the concept of open marriage as sexually non-monogamous marriage had gained a life of its own.
Relationship maintenance 
The impact of open marriage on relationships varies across couples. Some couples report high levels of marital satisfaction and have long-lasting open marriages. Other couples drop out of the open marriage lifestyle and return to sexual monogamy. These couples may continue to believe open marriage is a valid lifestyle, just not for them. Still, other couples experience serious problems and claim open marriage contributed to their divorces. All couples in open marriages may therefore want to pay attention to their relationship maintenance behaviors.
Ground rules 
Couples involved in open marriages or relationships typically adopt a set of ground rules to guide their activities. Ground rules in relationships allow partners to coordinate their behaviors so they achieve shared goals with fewer conflicts. Some ground rules are universal in the sense that they apply to virtually all relationships in a particular culture. Other ground rules apply to specific kinds of relationships, such as friendships or marriages. Still other ground rules are designed to manage romantic rivalry and jealousy. The ground rules adopted by sexually monogamous couples tend to prohibit behaviors that are viewed as acts of infidelity. The ground rules adopted by sexually open couples tend to prohibit behaviors that provoke jealousy or sexual health concerns. Partners may change the ground rules of their relationships over time. One example of a changing ground rule includes where a married couple decides to separate. Without divorcing, they are still legally married. However, they may choose to continue cohabitation.
Open marriage styles 
Couples in open marriages may prefer different kinds of extramarital relationships. Couples who prefer extramarital relationships emphasizing love and emotional involvement have a polyamorous style of open marriage. Couples who prefer extramarital relationships emphasizing sexual gratification and recreational friendships have a swinging style of open marriage. These distinctions may depend on psychological factors such as sociosexuality and may contribute to the formation of separate Polyamory and Swinging communities. Despite their distinctions, however, all open marriages share common issues: the lack of social acceptance, the need to maintain the health of their relationship and avoid neglect, and the need to manage jealous rivalry.
Many open couples maintain rules forbidding emotional attachment, illegitimate children, extramarital sex in the marital bed, extramarital sex with those known to both partners, and requiring use of barrier contraception.
Some open marriages are one-sided as well. One partner who may need more sexual gratification than the other is free to seek it out where he/she sees fit, all while maintaining a functional emotional relationship with their full-time partner.
Jealousy issues 
Couples in open marriage expose themselves to situations that can potentially provoke jealousy. Most couples in open marriages report experiencing jealousy at some point during their marriage. Ground rules are one way to help manage jealousy in open relationships. However, ground rules may not be sufficient. Couples in open marriages may benefit from a general understanding of jealousy and how to cope with it.
Open marriage acceptance 
Surveys show large majorities of people disapprove of extramarital sexual activity. A few studies show people specifically disapprove of open marriages. Critics have put forward moral, medical, and psychological objections to open marriages. The lack of social acceptance places pressure on couples to hide their open marriages from family, friends, and colleagues. This may limit their social support network.
Legal issues 
The practice of extramarital sex is often illegal in jurisdictions where adultery is illegal, regardless of whether the partner(s) has given their consent. Open marriage is not the same thing as polygamy, where sexual relationships are not necessarily contemplated, but rather one can have more than one simultaneous spouse, which is said to protect individual and marital property rights.
Religious issues 
Some conservative Christians believe that God sees open marriage as adultery even though both people agree they can date others. Still other liberated Christians disagree with this evangelical hermeneutical interpretation and see nothing wrong with open marriages or swinging, believing that what is done in honesty and acceptance out of love is all that matters.
Incidence of open marriage 
The incidence of open marriage is the frequency with which open marriage occurs. Several definitional issues complicate attempts to determine the incidence of open marriage. People sometimes claim to have open marriages when their spouses would not agree. Couples may agree to allow extramarital sex but never actually engage in extramarital sex. Some researchers define open marriages in highly narrow terms. Despite these difficulties, researchers have estimated that between 1.7 percent and 6 percent of married people are involved in open marriages. The incidence of open marriage has remained relatively stable over the last two generations.
Notable people in open marriages 
- Jenny Block, author of Open: Love, Sex and Life in an Open Marriage
- Thomas Braden, journalist
- Bob Crane, actor and Sigrid Valdis, actress
- Robert Crumb and Aline Crumb, cartoonists
- Denholm Elliott, actor
- Shirley MacLaine, actress
- Havelock Ellis, sexologist
- Erica Jong, author
- Larry King, talk show host and Shawn Southwick, singer
- Alfred Kinsey, sexologist and wife Clara Bracken McMillen
- Andrei Kirilenko, professional basketball player[dead link]
- Charles Laughton, actor, and Elsa Lanchester, actress
- Edna St. Vincent Millay, American poet
- Mo'Nique, actress/comedian
- Amanda Palmer, American performer and Neil Gaiman, English author.
- Ayn Rand, novelist, philosopher, playwright, and screenwriter
- Margaret Sanger, founder of Planned Parenthood
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