At any given time, a significant percentage of men are engaging in multiple sexual partnerships with women -- a situation that may facilitate the spread of sexually transmitted infections, including HIV.
Researchers looked at The National Survey of Family Growth, a national database that interviewed 4,928 men in the United States.
In the survey, men reported the first and last date they had sexual intercourse with each of their sexual partners during the year before the interview.
Though the actual reported rate of such behavior in the study is 6.6 percent, the authors of the study estimate from adjusted measurements that up to 11 percent of men may have been involved with multiple sexual partners at some point during the previous year.
Concurrent sexual relationships may have huge implications when it comes to the spread of sexually transmitted disease.
"Concurrent partnerships are an important sexual network characteristic because of the way they connect people to each other," says lead study author Dr. Adaora Adimora, clinical associate professor of epidemiology at the University of North Carolina at Chapel Hill School of Public Health.
"These kinds of relationships can spread HIV through a population faster than the same number of monogamous relationships."
This quicker spread is, in many ways, a simple function of time.
"For example, three concurrent partnerships will spread HIV faster than three monogamous relationships back-to-back, because if a person only has sequential partners and he gets HIV, he won't give it to another partner until he ends his relationship and strikes up a new one," Adimora explains. "If the individual has concurrent partnerships, he can immediately give the next partner HIV without waiting to end the first relationship."
Some public health experts hope the study will offer a new direction for efforts to stem the spread of HIV.
"I think that this study is highly significant," says Bruce Dezube, associate professor of medicine at Beth Israel Deaconess Medical Center in Boston, Mass. "It's amazing to me how much progress we have made with HIV drugs -- we have over 30 drugs available and 5 different mechanisms to treat patients -- but the one thing we haven't figured out is human behavior."
Not all infectious disease experts agree, however, that the results of the study are a surprise.
"There is nothing significant about this study that was not known before it was published," says Dr. Richard Spark, associate clinical professor of medicine at Harvard Medical School in Boston, Mass.
And Dezube says that even though the research will likely help improve efforts to curb infectious disease spread, he notes that he is "not surprised that the rate of concurrent sexual relationships is so high."
"I'd like to quote the surgeon general Coop who said that 'the majority of people who get HIV get it through mechanisms that the majority of people wouldn't understand,'" he says.
Adimora says race appears to have much to do with the chances a given man is involved in concurrent relationships -- a finding that could help public health experts design strategies to combat the trend.