While monogamy is often touted as a way to protect against disease, young couples who say they have discussed monogamy can't seem to agree on what they decided. And a significant percentage of those couples who at least agreed that they would be monogamous weren't.

A new study of 434 young heterosexual couples ages 18-25 found that, in 40 percent of couples, only one partner says the couple agreed to be sexually exclusive. The other partner said there was no agreement.

Public health researchers Jocelyn Warren and Marie Harvey of Oregon State University looked at data from the PARTNERS Project, a Center for Disease Control-funded study conducted by Harvey. The researchers said this study showed that many couples are misjudging their partners' risk behaviors.

The results are in a forthcoming article published online in the Journal of Sex Research.

"Other studies have looked at perceptions related to monogamy, but this is really the first one that explores the discussions that heterosexual couples are -- or aren't -- having about monogamy," Warren said. "Miscommunication and misunderstandings about sexual exclusivity appear to be common."

Previous research has shown that condom use tends to decline as relationships become more intimate and steady over time. Yet Warren and Harvey's study shows that some couples may not be communicating effectively on the terms of their relationship. Even among those who agreed they had an explicit agreement to be monogamous, almost 30 percent had broken the agreement, with at least one partner having had sex outside the relationship.

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Harvey, a leading researcher in the field of sexual and reproductive health, said this study adds to a growing body of research on safer sex communication.

"Couples have a hard time talking about these sorts of issues, and I would imagine for young people it's even more difficult," she said. "Monogamy comes up quite a bit as a way to protect against sexually transmitted diseases. But you can see that agreement on whether one is monogamous or not is fraught with issues."

The couples surveyed included both married and non-married couples. Interestingly, couples with children were less likely to have a monogamy agreement in place. Married couples were no more likely to have an explicit monogamy agreement in place than other couples.

Only commitment was related to sustained monogamy. Relationship commitment was assessed using an accepted measurement scale where participants rated themselves from one to five (five being highest) on questions such as "You view your relationship as permanent." With every unit increase in the commitment scale, the odds that the couple had a sustained monogamy agreement increased almost three-fold.

"Relationship variables appear to be related to monogamy," Harvey said. "But factors such as marriage and children did not increase the likelihood that the couple had agreed to monogamy."

Warren said couples become monogamous generally for emotional reasons, to show love and trust in a relationship. Yet the concern is that a lack of communication between heterosexual couples is leading to unintended risks.

Harvey said the sexual behavior and protection of young couples is ripe for intervention. She recommends that those who work with young people in clinical and community settings ask what kind of protection they are using. "And if they answer that their partner is monogamous, they may want to think about advising that young person to use protection," Harvey said.

Warren is a research associate at OSU and Harvey is a professor of public health at OSU. Christopher Agnew from Purdue University contributed to this study.