Consensually nonmonogamous relationships are those in which all partners explicitly agree that each partner may have romantic or sexual relationships with others (Conley, Ziegler, Moors, Matsick, & Valentine, 2013 ). In this article, research examining the associations between consensual nonmonogamy, psychological well-being, and relationship quality is reviewed. Specifically, three types of consensual nonmonogamy are examined: swinging, open relationships (including sexually open marriage and gay open relationships), and polyamory. Swinging refers to when a couple practices extradyadic sex with members of another couple; open relationships are relationships in which partners agree that they can have extradyadic sex; and polyamory is the practice of, belief in, or willingness to engage in consensual nonmonogamy, typically in long-term and/or loving relationships. General trends in the research reviewed suggest that consensual nonmonogamists have similar psychological well-being and relationship quality as monogamists. Methodological challenges in research on consensual nonmonogamy and directions for future research are discussed.

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Consensual nonmonogamy (CNM) is a relationship in which individuals agree that romantic or sexual relationships with others is permissible or desirable (e.g., polyamory or open relationships). Although anti-CNM prejudice is prevalent, it is not well understood. We propose that one of the bases of anti-CNM prejudice is zero-sum thinking about love-the perception that one person's love gained is another's love lost. We outline our theory and then present three studies that test our predictions. In these studies, participants read a vignette that depicted characters who were in a CNM or monogamous relationship, and then judged aspects of the characters and their relationship. In Study 1, participants who read the CNM vignette judged the protagonist's love for their initial romantic partner before and after they became involved with a second partner. Zero-sum thinking was operationally defined as the within-subject change in love ratings. In Studies 2 and 3, participants rated their agreement with items from a new preliminary measure of zero-sum romantic beliefs. We measured CNM devaluation by asking for ratings of the relationships and of individuals in the relationships. Supporting our predictions, in all three studies we found that zero-sum thinking about love was associated with increased CNM devaluation. We end by briefly discussing the implications of our findings.

Despite a growing interest in polyamory, it is unknown how many polyamorists there are in the general population. In acknowledging that the meaning of “polyamory” is contested (e.g. Klesse, 2014), we estimated the prevalence of polyamory when it was defined as: (1) an identity, (2) relationship beliefs/preferences, (3) relationship status, and (4) relationship agreements. We recruited 972 individuals from Mechanical Turk and used a sample weighting procedure to approximate a representative sample of the population of the USA. Point prevalence estimates ranged from about 0.6% to 5%, and lifetime estimates ranged from about 2% to 23%. Thus, we estimate that there are at least 1.44 million adults in the USA who count as polyamorous.