A University of Iowa study found that one-third of sexual relationships in the Chicago area lack exclusivity. One in 10 men and women reported that both they and their partner had slept with other people.
Lovers in "friends with benefits" situations or those "hooking up" with a stranger or acquaintance proved much more likely to have multiple partners, according to the survey of 783 heterosexual adults.
Researchers are interested in the topic because concurrent partnerships speed up the spread of sexually transmitted diseases, said Anthony Paik, a sociologist in the UI College of Liberal Arts and Sciences and author of the study published in the latest issue of the journal Perspectives on Sexual and Reproductive Health.
"The United States has seen a major shift toward nonromantic sexual partnerships -- people becoming sexually involved when they are just casually dating or not dating at all," Paik said. "A quarter of the respondents became sexually involved while casually dating and a fifth did so as friends or acquaintances."
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 nonromantic sexual relationships," he said. "We encourage people be aware of the potential for sexual concurrency and take appropriate precautions to avoid sexually transmitted infections."
Explore further: Diagnosing deafness early will help teenagers' reading development