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A supporter of gay marriage waves a rainbow flag outside the Utah State Capitol. Photo: AP

Less than 3 per cent of the US population identify themselves as gay, lesbian or bisexual, the Centres for Disease Control and Prevention reported in the first large-scale government survey measuring Americans’ sexual orientation.

The National Health Interview Survey, which is the government’s premier tool for annually assessing Americans’ health and behaviours, found that 1.6 per cent of adults self-identify as gay or lesbian, and 0.7 per cent consider themselves bisexual.

The overwhelming majority of adults, 96.6 per cent, labelled themselves as straight in the 2013 survey. An additional 1.1 per cent declined to answer, responded “I don’t know the answer” or said they were “something else”.

The figures offered a slightly smaller assessment of the size of the gay, lesbian and bisexual population than other surveys, which have pegged the overall proportion at closer to 3.5 or 4 per cent. In particular, the estimate for bisexuals was lower than in some other surveys.

The inclusion of the sexual-orientation question in an influential survey used to guide government funding and research decisions was viewed as a major victory for the gay community, which has struggled with a dearth of data about its special health needs.

“This is a major step forward in trying to remedy some of these gaps in our understanding of the role sexual orientation and gender identity play in people’s health and in their lives,” said Gary Gates, a demographer at the Williams Institute, a research centre at the University of California at Los Angeles that studies the lesbian, gay, bisexual and transgender (LGBT) population.

Begun in 1957, the US health interview survey comprises a wide range of questions, on topics including medical care, vaccinations and tobacco use. The data is collected for the CDC by the Census Bureau, which conducts interviews with thousands of Americans across the country. It is highly regarded because of its large sample size — it comprised 33,557 adults between the ages of 18 and 64 for the most recent survey — and because of its methods, which include face-to-face interviews and some follow-up telephone queries.

A few other federal surveys ask about sexual orientation but are not large enough to provide data that can be generalised to the country as a whole, government health officials have said.

The information released by the CDC offers an initial analysis through the lens of sexual orientation on measures critical to public health, such as smoking, drinking and health insurance status.

It did not find a broad pattern suggesting that one group was less healthy overall than any other group, said Brian Ward, the researcher for the report. Echoing other studies, it found that, compared with straight people, gays were more likely to smoke and to have consumed five or more drinks in one day at least once in the past year. Straight women were more likely to consider themselves in excellent or very good health than women who identified as lesbian.

But gays were more likely to have received a flu shot than straight people, and gay men were less likely to be overweight than straight men.

In some cases, the more notable disparities were experienced by bisexuals. People who identify as being attracted to both sexes are more likely to have experienced psychological distress in the past 30 days than straight people, the survey showed.

“We just don’t know much about bisexuality right now, and we’re finally starting to do some research in that area,” said Judy Bradford, director of the Centre for Population Research in LGBT Health at the Fenway Institute in Boston. The study may prompt more scrutiny of this understudied population, she said.

In their report, CDC researchers acknowledge that their estimate of the size of the bisexual population differs from those in other studies. A national estimate from the 2008 General Social Survey — which is funded by the National Science Foundation, a federal agency dedicated to the advancement of nonmedical science — estimated that 1.1 per cent of the population identified as bisexual. Other surveys suggest the number of bisexuals roughly matches the number of gays.

“There’s a variety of factors that could come into play, and we don’t have an answer right now,” Ward said. “It’s something we are looking at.”

The survey did not ask about gender identity, which is a more complicated topic than sexual orientation. Previously, officials had discussed including the question in future surveys. But Ward said Monday that there were no immediate plans to add such a question.

The difficulty stems partially from the large sample size needed, experts said. One challenge is that there are more than 200 terms used by people who identify as a different sex than the biological one they were born as, Bradford said.

Some people who have completed gender reassignment surgery may no longer consider themselves transgender but rather a member of their new sex, she said. Others may be offended by terms such as “transsexual,” which was once routinely used but in some circles is now considered pejorative.

The broader 2013 National Health Interview Survey data set was released publicly on June 30. It contains more information for researchers interested in studying other measures of health by sexual orientation, such as cancer rates or disabilities.