Introduction

Remaining sexually exclusive with a partner (sexual monogamy) is perceived by many as an effective strategy to promote sexual health and safety—including public health officials, researchers, and the general public [1,2,3]. Sexual monogamy is often the cornerstone of sexual health programs, and alternative types of partnering are perceived as responsible for the spread of sexually transmitted infections (with the exception of abstinence) [2, 3]. However, recent research has revealed an interesting twist to these popular conceptions of sexual safety: People engaged in consensually non-monogamous relationships are more likely to practice safer sex strategies than people engaged in ostensibly sexually monogamous relationships [3, 4•]. That is, people who have agreed to engage in multiple concurrent romantic and/or sexual relationships are more likely to correctly and consistently use condoms, implement barrier methods, and sanitize sex toys than people who have agreed to be sexually monogamous with their partner yet are sexually unfaithful [3, 5•].

In light of recent developments in sexual health research, it is critically important to better understand people’s engagement in consensual non-monogamy (CNM). Over the past decade, interest in CNM has grown both among the American public and within the academic literature [6,7,8,9,10]. Yet, relatively little is known about the prevalence of CNM, as well as factors that predict its occurrence and subjective success, including the impact on people’s sexual health and well-being. The bulk of research on CNM relationships has been qualitative or relied on convenience samples of self-identified consensually non-monogamous people. While such research provides important insights about the structure and dynamics of CNM relationships, much of this work is not suited to address questions about the popularity or correlates of CNM relationships.

A critical component to moving research on CNM relationships forward and improving practitioner-patient relationships is elucidating stereotypes regarding who engages in CNM, based on the available empirical evidence. As such, the current paper is intended to synthesize academic articles related to CNM relationships with a focus on the past 5 years of research.Footnote 1 In so doing, we address best research and clinical practices and raise questions to stimulate future scholarship on CNM.

Definitions of Consensually Non-Monogamous Relationships

Consensual non-monogamy is an umbrella term for relationships in which all parties agree that they may engage in sexual activity and/or romantic relationships with other partners [3]. Arrangements of CNM can vary considerably, but the unifying focus remains on the consensual aspect, which is an important distinguishing feature of these relationships as compared to non-consensual extra-dyadic activity such as sexual or emotional infidelity, typically characterized as a form of relationship betrayal and violation of agreed commitment exclusivity. Much of the existing literature on CNM has focused on differentiating its various subtypes [11, 12], and the precise arrangements and agreements that characterize these subtypes [13]. Despite this work, neither scholars nor those involved in CNM relationships themselves have reached clear consensus on definitions for terms that constitute CNM relationships, such as “open relationship” and “polyamory” [e.g., 6, 14].

While there is no indication that the use and connotation of CNM terms will standardize in the foreseeable future, for clarity we will rely on a few terms and commonly drawn distinctions throughout this paper, while acknowledging that these terminological boundaries remain contested. These categories are based on a combination of reported behaviors, self-identifications, and agreements between partners [e.g., 3, 6], which will not always categorize an individual in the same way. We have provided the exact definition used by each researcher when possible. In the context of the current review, we draw on extant scholarship to define those who report engagement in CNM into one of five types (usually according to the term used by the original researchers; [3, 15,16,17,18]. The types used here are as follows:

  • Polyamory refers to multiple romantic, loving, and/or sexual long-term non-exclusive relationships

  • Swinging denotes non-exclusive sexual (not typically romantic) relationships, especially between couples

  • Open relationship typically means non-exclusive long-term romantic relationships that are accompanied by additional relationships that are primarily sexual but may also include romance

  • Monogamish refers to relationships in which partners engage in sexual activity with others outside the relationship but primarily while together (group sex).

  • Other CNM relationship is used here as a catch-all category for relationships where people have agreed to engage in some form of romantic and/or sexual engagement with multiple concurrent partners and do not label their relationships with any of the above terms

It is also important to note that the term monogamy itself is often variably used and understood. In general, monogamy refers to a long-term exclusive or committed relationship between two partners. However, many behavioral scientists distinguish between two specific forms of monogamy: social monogamy and sexual monogamy [19,20,21]. Thus, when evolutionary biologists refer to a species as having a monogamous mating system, they are specifically referring to social monogamy, which is the tendency to form pair-bond relationships, characterized by an intense biobehavioral bond with one conspecific, generally leading to mutual home and family construction (biparental care and mutual territory defense). This is distinct from sexual monogamy, which is characterized by sexual exclusivity with one particular partner (sole genetic contributions to breeding). While socially monogamous individuals may often attempt to exert sexual exclusivity over their mating partners, social monogamy does not necessarily indicate sexual monogamy. This distinction is not always recognized or considered when applied to human research by scholars or clinicians in other fields or by the general public. This potential communication gap is likely exacerbated by the “halo effect” surrounding monogamy in humans, which leads monogamous relationship structures to be idealized while consensually non-monogamous ones are stigmatized and dehumanized [3, 22, 23].

How Many People in North America Engage in Consensual Non-Monogamy?

Few studies have attempted to assess the prevalence of CNM in the general population. To date, most research has utilized relatively small convenience samples with recruitment methodology that specifically targeted people engaged in CNM relationships. While these studies can provide some information about the experiences of people who engage in CNM, they cannot answer questions about the general prevalence of this type of relationship. Recently, there have been three notable exceptions, all based on large general population samples which did not specifically target people engaged in CNM relationships.

Rubin, Moors, Matsick, and Conley [24•] collected two large online convenience samples (i.e., from Facebook.com and Craigslist.org, largely based in North America) for general relationship studies. Notably, they did not specifically recruit people in CNM relationships in any way. For those participating in the study, engagement in CNM relationships was defined in two ways. In the first study, participants currently in a romantic relationship indicated whether they were monogamous (exclusively dating one person), casually dating (dating one or more people), or consensually non-monogamous (dating one or more people and your romantic partners agree/know about it; for example, open relationship, polyamorous relationship, swinging relationship). In the second study, participants completed a 6-item behavioral measure of participation in various types of CNM relationships, with items like “You and your partner may have sex with others, but never the same person more than once” and “You and your partner take on a third partner to join you in your relationship on equal terms,” with those who indicated current engagement in any of the six CNM relationship types classified as being in CNM relationships. Among the 2395 total participants, 120 (5.3%) identified as currently in a CNM relationship, and this percentage did not differ across the two studies. The similarity of the prevalence results from these studies, which were collected at different times and using different operational definitions of CNM relationships, and not specific to particular geographical regions, makes this estimate of current prevalence the most robust currently available (though both drawing from convenience samples).

Recent research using similar convenience sampling techniques in Canada further corroborate the earlier findings of Rubin and colleagues [24•]. Seguin and colleagues [25•] recruited a total of 6449 Canadian participants to take part in a study of relationship quality among people engaged in monogamy, polyamory, and open relationships, with special efforts made to recruit people in CNM relationships. Of these, 237 (3.7%) were currently in polyamorous relationships (defined as having more than one romantic/loving partner and explicitly labeling their relationship as polyamorous), while 468 (7.3%) indicated that they were currently involved in open relationships (defined as having only one romantic/loving partner and an open sexual agreement, i.e., an explicit agreement that sex with outside partners was permitted).

Complementary data on the lifetime prevalence of CNM relationships come from Haupert, Gesselman, Moors, Fisher, and Garcia [26•]. In two large demographically representative samples of single Americans (n = 3905 and n = 4813, respectively) drawn from established opt-in panels, they found that slightly more than 21% of participants (826 and 1056, respectively) reported ever having engaged in a CNM relationship (defined as “an agreed-upon, sexually non-exclusive relationship”). Again, this estimate did not differ across studies, although they were collected at different times and used slightly different inclusion criteria, making this the strongest evidence currently available about the lifetime prevalence of CNM relationships.

Taken together, while not nationally representative data, these studies suggest that about 3–7% of the North American population is currently in a CNM relationship of some type, while about one fifth have ever had a CNM relationship. Notably, these estimates are consistent across multiple samples and a variety of highly distinct operational definitions of CNM relationships.

Who Engages in CNM?: Social Identities and Individual Difference Factors

Gender and Sexual Orientation

Previous research has commonly suggested that men and non-heterosexual people (especially gay men and bisexual people) have a high frequency of engagement in CNM relationships (see [6] for review). While this notion has since been critiqued, it was based on evidence available at the time regarding sexual health and behavior among sexual minorities. Indeed, the first large-scale study to report prevalence data for non-monogamous relationships found that 61% of their gay male participants had an open relationship agreement and acted on it in the past year, compared to only 7% of heterosexual men in the sample [27]. This formative assumption, coupled with the emergence of the HIV/AIDS epidemic in the 1980s that disproportionately affected gay men, led to an explosion of research on the relationship agreements of gay men, a trend which has continued to the present day. For instance, of the 16 papers with demographic data about CNM relationships that we identified in the present literature review, seven (39%) sampled only gay men or men who have sex with men.

Research specifically targeting gay men found extremely varied prevalence of CNM relationships. For example, Parsons and colleagues [28•] found that only 7–8% of the gay men they recruited through street interception in New York City and Los Angeles were currently in CNM relationships. In another study, Perry and colleagues [29•] found that 45% of the gay men they recruited through community outreach to LGBT centers in San Francisco were currently in a CNM relationship. In contrast, the large nationwide samples described earlier found smaller numbers of gay men had engaged in CNM relationships. For example, Haupert and colleagues [26•] found that 31.6% of gay men reported ever having engaged in any form of CNM relationship in their lifetime.

Other research casts doubt on the notion that gay men are disproportionately engaging in CNM relationships (compared to other sexual minorities or heterosexual people). For instance, Moors, Rubin, Matsick, and Conley [30] found no difference in expressed desire to engage in CNM (e.g., going to swinger parties, taking on a third partner) between sexual minority men and women recruited as part of a large online convenience sample. These results suggest that sexual minorities, regardless of gender identity, indicate higher levels of interest in CNM relationships than do heterosexuals; it is not just gay men who are interested in such relationships. Indeed, Currin and colleagues [31] found that even among heterosexually identified men and women, both sexual attraction to and sexual behavior with members of the same sex predicted openness to CNM relationships. While none of these studies can establish causation, it is possible that any departure from the heteronormative model of relationships encourages people to consider alternatives to the mononormative model of relationships as well. Regardless of the origin of these demographic differences, they have emerged quite consistently across various large studies of the general population.

Rubin and colleagues [24•] found that men were more likely than women, and sexual minorities were more likely than heterosexual people, to currently be in CNM relationships. Likewise, Seguin and colleagues [25•] also found that non-heterosexual people were more likely than heterosexual people to be in polyamorous or open relationships. Finally, in their demographically representative samples of single Americans, Haupert and colleagues [26•] found that a greater portion of men than women reported ever engaging in a CNM relationship—a pattern which held true across sexual orientations—and those who were not heterosexual were more likely to have engaged in CNM relationships. Specifically, a greater proportion of gay men reported previous engagement in a CNM relationship (31.6%) than did straight men (24.6%), and a greater portion of bisexual men reported previous engagement in a CNM relationship (44.6%) than did either straight or gay men. This same pattern held true for women as well; a greater proportion of bisexual women reported previous engagement in a CNM relationship (35.0%) than did lesbian women (21.3%) or straight women (16.3%). Moreover, lesbian women were more likely to have previously engaged in a CNM relationship than straight women. Overall, bisexual people were more likely than gay or lesbian people to have previously engaged in a CNM relationship at some point during their life, and gay/lesbian people were more likely than heterosexual people to have had this kind of relationship.

In sum, recent research corroborates the early assumption that sexual minorities are more likely to engage in CNM relationships than heterosexuals. However, this growing body of scholarship raises questions about the size of this discrepancy. It is also important to note that, in the studies to date, the vast majority of people in CNM relationships are heterosexual. For example, Haupert and colleagues [26•] found that 78.6% of the people who had ever engaged in a CNM relationship identified as heterosexual. Thus, researchers interested in CNM relationships should recruit participants from groups other than gay men if they wish to have data that can speak to broader swaths of the population. Additionally, this point should be noted by practitioners, as they may falsely assume their patients/clients are, or are not, engaged in monogamous relationships based on whether they match one’s presumptions about the demographics of those engaging in CNM relationships.

Race and Ethnicity

Existing literature on CNM relationships has not yet fully addressed the influence of race and ethnicity. If race and ethnicity is discussed in the CNM literature, the discussion typically focuses on the overwhelming Whiteness of polyamorous communities and discourses, as well as of consensual non-monogamy researchers [14, 32, 33]. For instance, Sheff and Hammers [33] reviewed studies of kink and polyamory and found that the percent of people of color in each study ranged from zero to a high of 48% (in a study of lesbian and bisexual women conducted in New York), leading the authors to conclude that these communities are part of a “largely homogeneous universe populated with highly educated, white, middle- and upper-middle-class professionals” (p. 205). However, Sheff and Hammers [33] also draw an important distinction between polyamorous communities and polyamorous practices:

“Although it is quite unlikely that these samples are representative of the actual range of kink and poly people, they are certainly representative of the range of people involved in mainstream poly and kink communities” (p. 209).

This distinction between CNM practices and specific communities appears to have been apt; while smaller non-representative samples of people in CNM relationships have tended to find that they are more common among white people, Rubin et al. [24•] and Haupert et al. [26•] both found that people of color were equally likely to engage in CNM as white people. This finding suggests that researchers need to reconsider their recruitment techniques, study designs, and terminology, to ensure that they are not excluding racial and ethnic minorities. For example, terms such as “polyamory” are stereotypically perceived as referring to white people, and as such people of color may not identify with them even if they engage in behaviors which researchers might classify as polyamory [14]. Researchers may more accurately identify people of color in CNM relationships by using measures that ask what types of romantic and sexual behaviors participants engage in and what relationship philosophies they endorse (as in [24•], for example).

Other Demographic Factors

Beyond gender and sexual orientation, it appears that most other demographic factors assessed so far are unrelated to engagement in CNM relationships. For instance, across their large national studies, Haupert and colleagues [26•] found only gender and sexual orientation to be consistently associated with lifetime engagement in CNM relationships. Other factors they assessed, including education level, income, religious affiliation, political affiliation, and geographical region, were unassociated with previous engagement in CNM. However, other studies have generally not included this demographic information analyzed by relationship type; for instance, neither Rubin et al. [24•] nor Seguin et al. [25•] included measures of such demographic characteristics. In the few other cases where these data are available, they have been included in Table 1.

Table 1 Sociodemographic descriptive results from research on consensual non-monogamy; 16 studies reviewed
Full size table

Future Directions: What Should Be Next for Research on CNM Relationships?

Researchers have only recently begun to distinguish between behavior, identity, and attitudes toward CNM. As such, there is disagreement over how to “best” conceptualize subtypes and experiences with CNM. While some research collects lifetime prevalence data of participants, other research measures current participation in CNM relationships; yet other research considers one’s philosophical orientation and attitudes toward CNM relationships rather than specific engagement in various relationship types. Using a variety of measures is not inherently problematic (as different measures are employed for different research questions and methods), but it makes consistently and validly measuring occurrence rates of CNM, as well as patterns of behavior and identity, challenging. With such a range of conceptualizations and methodologies, direct comparisons of prevalence across studies are difficult or impossible, and our ability to predict outcomes relevant to sexual health (e.g., relationship and sexual satisfaction, frequency of sex with multiple partners, condom use) is limited by the generalizability of findings between studies using disparate definitions of CNM.

Perhaps the most important problems for CNM research to confront in the years to come are to develop theoretically based, valid measures of relationship status and/or relationship orientations that are inclusive of consensually non-monogamous relationships, and to apply these measures consistently to collect data from a wide variety of participants, including in clinical settings and among representative probability samples. Such measures should also consider other characteristics of these relationships, such as relationship duration and commitment level, which might enable future researchers to more meaningfully understand subtypes of CNM relationships. For example, in instances of different relationship histories among partners, or where partner social/relationship hierarchies exist, primary partners in CNM relationships may face different levels of stigma and engage in different types of communication and sexual behavior than secondary partners [43]. Only after such rigorous assessment can we begin to understand the true prevalence of CNM relationships and gain a more accurate perspective on the diversity of people’s romantic and sexual lives and their consequences for sexual health.

Conclusions: How Might CNM Impact Current Sexual Health Research and Practice?

Recent estimates of current and previous engagement in CNM relationships reveal that few social identities are associated with engagement. Specifically, sexual minorities are more likely to have previously engaged in or are currently in a CNM relationship compared to heterosexual people [24•, 26•]. A similar pattern was found for men compared to women (regardless of sexual orientation). Other than sexuality and gender, few differences based on social identities emerge. In fact, factors such as political affiliation, race and ethnicity, geographic region, education level, income, and religion were unrelated to previous engagement in CNM [26•].

These results illuminate an important take-home message for researchers and practitioners: engagement in CNM is quite common—and a wide variety of people are engaging in these “non-traditional” relationships. Thus, engaging in open, swinging, or polyamorous relationships appears to be more of a part of the diverse range of human development than once thought. Although previous engagement in CNM is common, societal norms and monogamy ideals often filter people’s perceptions of health and well-being. For instance, when people in one study were asked “what are the benefits of monogamy?”, more than one half of participants cited that monogamy affords protection from sexually transmitted diseases [3]. This belief is predicated on the notion that those who are in socially monogamous relationships are also sexually monogamous with each other, which research on infidelity has repeatedly shown to not always be true. For instance, recent research with college and community samples found that approximately one quarter of adults reported having been sexually unfaithful to their monogamous partner [4•, 5•, 44, 45]. Even among those who are sexually faithful, most people practice serial social and sexual monogamy rather than lifelong sexual monogamy, which can similarly increase STI risk [7]. Thus, there is a discrepancy between sexual monogamy in theory and sexual monogamy in practice—and this protective fallacy can jeopardize people’s health and well-being. For a comparison of the presumed benefits of monogamy to the empirical evidence, see [7].

Unlike those who are unknowingly exposed to STIs when their partners are unfaithful, many people who engage in consensual non-monogamy relationships employ a wide range of safer sex strategies, including explicit rules about routine STI/HIV testing and consistent use of barrier methods such as condoms [5•, 13]. In fact, while people in CNM relationships report a greater lifetime number of sexual partners than people in monogamous relationships, people in both types of relationships have similar rates of STI diagnoses [4•]. The protective fallacy of monogamy, however, contributes to making decisions that do not necessarily promote sexual health. That is, to maintain an identity as monogamous—despite having concurrent sexual partners—presumably to avoid stigma, those people who are sexually unfaithful to their socially monogamous partners are unlikely to tell their partner about an extra-dyadic sexual encounter, to use protective measures with the extra-dyadic partner, or to spontaneously resume condom use with their primary partner (if no longer used) following an extra-dyadic sexual encounter [5•, 45].

In sum, we are not suggesting that merely being in a CNM relationship is linked with protective sexual health factors. We do believe, however, that future research should focus on relationship processes that appear to be prominent among people engaged in CNM, such as open and honest conversations with partners about sexual needs and risks [e.g., 5•, 38•, 45, 46]. While much of the prior research on CNM relationships has focused on how these relationships differ from monogamous relationships, having a better understanding of the protective factors associated with CNM engagement improves understanding of relationships generally and could benefit partners in all contexts. That is, identifying interpersonal communication techniques and sexual health promotion strategies among people with consensually agreed upon multiple concurrent partners could shed light on ways to improve health for all people—regardless of their relationship configuration.