Open marriage incidence
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 narrower terms than others. 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.
Researchers need clear definitions of open marriage in order to accurately estimate the incidence of open marriage. Unfortunately, a number of issues can arise that make the definition of open marriage less clear. Four of these issues, and how they impact the estimated incidence of open marriage, are discussed below.
One issue is that study participants may claim to have open marriages when their spouses would not agree. Blumstein and Schwartz asked more than 6,000 couples whether or not they had an understanding allowing sex outside their relationship. Interviewed individually, the partners in some couples gave very different responses to this question. For example, the husband of one couple responded:
"Sure we have an understanding. It's: 'You do what you want. Never go back to the same one.' See, that's where it's going to screw your mind up, to go back the second time to the same person." (Blumstein & Schwartz, 1983, page 286) 
The wife responded quite differently, saying:
"We've never spoken about cheating, but neither of us believe in it. I don't think I'd ever forgive him. I don't think I'd be able to. I don't know. I haven't met up with that situation." (Blumstein & Schwartz, 1983, page 287) 
This couple illustrates that partners can disagree about whether or not they have an open marriage. Studies that collect data from only one partner of a marriage may not get accurate information about the openness of the marriage. Inaccurate information about the openness of marriages will make estimates of the incidence of open marriage less reliable.
A second issue is that couples may have an open marriage in principle but never actually engage in extramarital sex. For example, Spaniel and Cole found that 7 percent of couples would consider participating in an open marriage, but only 1.7 percent of couples reported having open marriages that actually included extramarital sex.  Blumstein and Schwartz found that 15 percent of married couples share an agreement that allows extramarital sex, but only about 24 percent of men and 22 percent of women (or 6 percent and 5 percent of the total, respectively) who had such an agreement actually engaged in extramarital sex during the prior year.  Discrepancies between what couples consider doing and what they actually do raise the possibility that some couples agree to have an open marriage but never actually engage in extramarital sex. Studies that define open marriage by agreement alone will tend to report a higher incidence than studies that define open marriage by agreement and behavior.
A third issue is that researchers sometimes define open marriages in overly narrow terms. For example, Hunt defined open marriage as swinging couples who meet with other swinging couples to swap mates.  Many couples in open marriages may never engage in this particular form of extramarital sex. Studies that define open marriage in overly narrow terms will underestimate the number of people involved in open marriages.
A fourth issue is that open marriage is usually defined in terms of legally married, opposite-sex partners. Data collected from these kinds of open marriages may not generalize to other kinds of open relationships. For example, cohabiting couples tend to show higher levels of involvement in extra-couples sexual relationships compared to married couples.    Gay male couples show very high levels of open relationships compared to straight couples.  Focusing on open marriages with legally married, opposite-sex couples will result in estimates that are not accurate for open relationships in other groups.
The percentage of men and women actively involved in open marriages may be determined from data reported by Blumstein and Schwartz.  Out of 3,498 married men, 903 had an agreement with their spouses allowing extramarital sex; out of these 903 married men with an agreement allowing extramarital sex, 24 percent (or 217 men) actually engaged in extramarital sex during the previous year. This means about 6 percent (i.e., 217 / 3498) of married men were actively involved in open marriages during the previous year. The number is only slightly less for married women. Out of 3,520 married women, 801 had an agreement with their spouses allowing extramarital sex; out of these 801 married women with an agreement allowing extramarital sex, 22 percent (or 176 women) actually engaged in extramarital sex during the previous year. This means about 5 percent (i.e., 176 / 3520) of married women were actively involved in open marriages during the previous year.
The estimates based on the Blumstein and Schwartz study are slightly higher than estimates provided by other researchers. Hunt, based on interviews from a national study of sexual behavior, has estimated that 2-4 percent of the married population is involved in open marriages.  Bartell has estimated that 2 percent of the married population is involved in open marriages.  The lowest estimate comes from a study conducted by Spanier and Cole of several hundred people living in the midwestern United States. This study found just 1.7 percent of married people involved in open marriages. 
It appears that somewhere between 1.7 and 6 percent of married people are involved in open marriages at any given time.
Growing or shrinking?
Following the 1972 publication of the bestselling book, "Open Marriage," by George and Nena O'Neill, individuals in the popular media expressed a belief that open marriages were on the rise. This belief turned out to be incorrect. Comparing data from the earlier Kinsey studies with his own data, Hunt concluded the incidence of extramarital sex had remained about the same for many years:
"In sharp contrast to most of our findings thus far, our data in this area suggest that in the past generation there has been almost no measurable increase in the number of American husbands who ever have extramarital experience, and only a limited increase in the number of American wives who do so. The overall incidence for our sample of married men of all ages appears to be basically unchanged from that of a generation ago. Only among men under 25 do we find any significant increase, but even that increase is of modest proportions. As for our sample of married women, there is no evidence of any overall increase in incidence compared to a generation ago. Among wives under 25, however, there is a very large increase, but even this has only brought the incidence of extramarital behavior for these young women close to--but not yet on par with--the incidence of extramarital behavior among under-25 husbands." (Hunt, 1974, page 254) 
Hunt attributed the mistaken impression of increasing open marriages to a barrage of books, articles, and television shows dealing with the topic. He also notes that speculative comments about increases in open marriage would sometimes be repeated often enough that people cited them as evidence.
Nearly twenty years later, in a national study of sexual behavior, Janus and Janus likewise denied that open marriages were on the rise. In fact, they suggested the number of open marriages may have declined:
"Despite popularization in a book of that title in the early 1970s, open marriage has never become as prevalent as nonconsensual extramarital activities, and its popularity seems to be waning even further today." (Janus & Janus, 1993, pages 197-198) 
Unfortunately, their comments must be taken as anecdotal, since they did not report statistical data to support a decline in open marriages.
Open marriage remains a controversial topic capable of generating much media interest. A large amount of media interest can mislead people into thinking the incidence of open marriage is on the rise. Conversely, recent media attention given to the marriage movement can mislead people into thinking the incidence of open marriage is declining. Weiss notes:
"Despite the vast attention given to these alternative lifestyles in the 1970s, and despite the more recent claims that Americans are 'returning to traditional models of monogamous marriage,' there is no scientific basis for concluding that these patterns increased in popularity earlier or that they have become less common in the 1980s and 1990s." (Weiss, 1997) 
Investigators have found no reliable evidence that open marriage has either increased or decreased substantially over the last two generations. Further studies would be needed to assess increases or decreases in the incidence of open marriages.
- Philip Blumstein and Pepper Schwartz (1983). American Couples: Money, Work, Sex, New York, NY: William Morrow and Company, ISBN 0-688-03772-0
- Spanier G.B., & Cole C.L. (1975). Mate swapping: Perceptions, value orientations, and participation in a midwestern community. Archives of Sexual Behavior, 4, 143-159.
- Hunt, M. (1974). Sexual Behavior in the 1970s. Chicago, IL: Playboy Press.
- Forste, R., & Tanfer, K. (1996). Sexual exclusivity among dating, cohabitating, and married women. Journal of Marriage and the Family, 58, 33–47.
- Treas, J., & Giesen, D. (2000). Sexual infidelity among married and cohabitating Americans. Journal of Marriage and the Family, 62, 48–60.
- Bartell, G.D. (1971). Group Sex. New York, NY: New American Library.
- Janus, S.S., & Janus, C.L. (1993). The Janus Report on Sexual Behavior. New York, NY: John Wiley & Sons.
- Weis, D.L. (1997). The United States of America: Interpersonal heterosexual behaviors. In R.T. Francoeur (Ed.), The International Encyclopedia of Sexuality. New York, NY: Continuum Publishing. Retrieved July 19, 2006, from http://www2.hu-berlin.de/sexology/GESUND/ARCHIV/IES/USA08.HTM#5.%20INTERPERSONAL%20HETEROSEXUAL%20BEHAVIORS.
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