As awareness about bisexuality has grown over the years, it could be getting easier for people to label themselves as bisexual, said Debby Herbenick, associate professor at Indiana University and author of the book "Sex Made Easy," who was not involved in the study.
Researchers asked more than 9,000 people in the United States age 18 to 44 about the types of sexual experiences they have had, whether they are attracted to the same or opposite sex and whether they identify as being straight, gay/lesbian or bisexual. Interviews were conducted between 2011 and 2013 as part of the CDC's National Survey of Family Growth.
Many of the findings about sexual behavior, attraction and orientation were similar between the current survey and the previous (2006-2010) family growth survey. Similar to previous surveys the group conducted, 1.3% of women and 1.9% of men said they were homosexual.
However a few trends stood out. More women reported having had sexual contact with other women: 17.4% in the current survey compared with 14.2% in the 2006-2010 survey. And higher numbers of both women and men identified as bisexual, 5.5% of women and 2% of men, compared with 3.9% and 1.2% respectively in the last survey.
"It's certainly not a new idea that women and men may be attracted to more than gender," Herbenick said. "But that doesn't mean it's an easy orientation to adopt. Women and men who self-identify as bisexual experience stigma not just from heterosexuals but also homosexuals," she said.
The finding that women were more likely than men to say they were bisexual is consistent with what previous studies have found, said Casey E. Copen, demographer at the CDC National Center for Health Statistics and lead author of the study, which was published on Thursday.
Women were also more likely than men to report having same-sex sexual contact. Compared with 17.4% of women, only 6.2% of men said they had ever had this activity.
However, as Copen noted, the survey could have given women more opportunity than men to report same-sex sexual contact. For example, women were asked if they have engaged in oral sex or any other sexual experience with another woman, whereas men were asked specifically whether they have engaged in oral or anal sex with another man.
The wording of questions in the survey could also be part of the reason for the low number of men who said they were gay, Copen said. Other surveys have found that closer to 4% to 6% say they are gay, a higher proportion than the 1.9% in the current survey.
Among women who reported being lesbian, the rate of 1.3% is consistent with other surveys. Over the last several decades, fewer women have been saying they are lesbian and more report being bisexual, similar to what the current study found, Herbenick said.
There is high correspondence between how survey participants identified themselves -- whether straight, gay/lesbian or bisexual -- and the sexual attractions and behaviors they reported, Copen said. For example, among those who labeled themselves heterosexual, 12.6% of women and 2.8% of men had had sexual contact with the same sex.
"You do expect some differences, because for some people ... they may or may not have had the experiences they're contemplating, [especially] if they're younger," Copen said.
The survey found some differences between women of different racial groups. Only 11.2% of Hispanic women have engaged in same-sex sexual contact compared with 19.6% of white women and 19.4% of black women.
The next survey, covering 2014 and 2015, will be coming out this fall, Copen said. These surveys are important to allow researchers "to separate out and study these categories, like lesbian and bisexual women and gay and bisexual men, because they all have different health outcomes and different levels of access to health care," she said.
Understanding trends in sexual behavior and orientation can help health groups and programs reach at-risk populations, Herbenick said. For example, putting information about sexually transmitted infections in a gay bar may only reach men who identify as being gay, and miss men who have sex with men but do not identify as being gay.
"There are real effects when you find out what people are doing sexually that can translate into safer sex, sex education, (and) informing doctors and nurses (about) what people are doing so they can talk with them in more informed and compassionate ways," Herbenick said.