In the survey of 783 heterosexual adults, one in 10 men and women reported that both they and their partner had slept with other people.
Anthony Paik, a sociologist in the UI College of Liberal Arts and Sciences and author of the study, said the study is important as concurrent partnerships speed up the spread of sexually transmitted diseases.
"The United States has seen a major shift toward non-romantic sexual partnerships -- people becoming sexually involved when they are just casually dating or not dating at all. A quarter of the respondents became sexually involved while casually dating and a fifth did so as friends or acquaintances," Paik said.
Respondents, ranging in age from 18 to 60, were asked how many people they had been with during their most recent relationship.
They also estimated how many partners their partner had during that time. Sexual involvement was defined as genital contact.
Overall, 17 percent of men and 5 percent of women acknowledged that they had been with someone else. Another group -- 17 percent of women and 8 percent of men -- said they''d been exclusive but their partner had not. Twelve percent of women and 10 percent of men said neither of them had been monogamous.
Being involved with a friend increased the likelihood of non-monogamy by 44 percent for women and 25 percent for men. Involvement with an acquaintance or stranger increased the odds by 30 percent for women and 43 percent for men.
The study also found that respondents who got along with each other''s parents were less likely to have multiple sex partners. Paik said people are less likely to risk a relationship when they take family stakeholders into consideration.
Paik said the research does not lead to the conclusion that efforts should be made to revive dating.
"People can make their own choices, but we hope this information will be useful as they weigh the risks and rewards of non-romantic sexual relationships. We encourage people be aware of the potential for sexual concurrency and take appropriate precautions to avoid sexually transmitted infections," he said.
The study has been published in the latest issue of the journal Perspectives on Sexual and Reproductive Health.