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.
Race, Behaviors May Contribute to Trend
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.
"We found that concurrent relationships were more common among men who were black, Hispanic, or had been incarcerated in the last year," she said, adding that this correlation could be due to a number of factors, including higher death rates among men in certain racial groups and higher rates of incarceration among black men.
But Eli Coleman, professor and director of the Program in Human Sexuality at the University of Minnesota Medical School in Minneapolis, says the underlying factors go further than race alone.
"Certain groups are more at risk for HIV infection than others," he conceded. "However, racial differences obscure the underlying factors which lead to poor sexual health.
"Poverty, stigma and discrimination, lack of access to health care, education might be more of a common denominator among white, black and Hispanic groups -- these factors deserve more investigation."
Adimora agrees that other factors could be at play, as men who engaged in concurrent sexual relationships also seemed to have other behaviors in common.
"Men who did have concurrent relationships were more likely to be intoxicated on drugs and alcohol, to have relationships with women who had multiple partners, and to have had sexual relationships with men in the past," she said.
Solving the HIV Epidemic
Coleman says that in order to stem the spread of sexually-transmitted infections, public health experts must develop a holistic approach -- one which takes into account a number of factors that seem to increase the chances of a spectrum of unhealthy behaviors.
"We need approaches that will remove health disparities caused by poverty, stigma and discrimination, poor access to health care and education," Coleman said. "We need to develop a sexual health approach to HIV infection which will provide sexuality education, access to sexual health care, all which is culturally sensitive and relevant."
Central among these factors, he says, is poverty.
"HIV is probably more a risk among poor and disenfranchised groups," Coleman said. "Prevention programming needs to better target these groups ... This is a vicious cycle that needs to be broken."
And Kate Wachs, a clinical psychologist in Chicago, Ill. and author of the book "Relationships for Dummies," says women can take an important cue from the research.
"Hopefully, the general public will start to be more cautious," she says. "Women need to start thinking, 'How many partners do I have? How many partners have my partners had? How many of those were men? How many wore condoms?'
"People are not as safe as they thought they were, and women have to be careful here."