marriage, open-relationships, polyamoryAs TV and the Internet have educated people to “new” possibilities in the last few decades, terms like “swinger” and “open relationship” have reached a new level of public consciousness.
I’m wondering how widespread these sorts of behavior really are.
Is there any data on what percentage of married couples are “open”?
I realize there are other possible definitions and degrees of “open relationship”, but I’m interested specifically in committed married couples, who engage in extra-marital sex, with the knowledge of their spouse.
I’d be interested in information from any western society countries, such as the USA or Europe.
TLDR: Around 4%-5%
The closest to a definite answer I've been able to find comes from The Fewer the Merrier?: Assessing Stigma Surrounding Consensually Non-monogamous Romantic Relationships
In that paper they state (CNM = Consensually Non Monogamous):
Consistent with the cross-cultural findings of Schmitt (2005), CNM does not seem to be as rare as some might expect. Although we know of no representative research that has addressed the question of the prevalence of CNM in the population, it appears from samples that we have recruited generally (i.e., cases in which recruitment was not targeted for CNM individuals), that approximately 4% of participants identify themselves as CNM (Conley et al., unpublished data).
This is of course a very rough estimate, since their purpose was not to measure what percentage of the population is CNM, but to measure the stigma these relationships suffer. It also depends on the size of the samples they have, which is not clear (they do of course give detailed numbers for the actual studies they present in the paper but they do not specify if this estimate is restricted only to this population).
I looked at the cited Schmitt paper. It is a monster (65 pages) so I've not been able to study it in detail. However its scope goes a bit beyond that of the question, since it studies cultural sociosexual leanings (which is a form of measuring how monogamous the different cultures are in general), indepedent of marital status.
EDIT:
In a slightly more recent paper (unfortunately behind a paywall) the same authoers reassert this estimate, and put it in some stronger terms:
Schmitt’s research and related studies have addressed monogamy as opposed to other forms of non-monogamy (i.e., infidelity). However, we are finding that a surprising number of partnered individuals are not even striving to maintain a monogamous relationship. Our recent studies with American samples have demonstrated that approximately 4% to 5% of people are currently involved in consensually non-monogamous (CNM) relationships—that is, relationships in which both partners have openly agreed that they and/or their partners will have other sexual or romantic partners (Conley, Moors, Matsick, & Ziegler, 2011, 2012a; Moors, Edelstein, & Conley, 2012).
The citations are the paper above, unpublished data and a conference talk they gave in Chicago. While it is probably the same dataset, I find this second citation a bit stronger (note the demonstrated vs it apperas on the previous paper), so I decided to add it to the answer.
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