In an attempt to encourage more research into the health and well-being of gay people, a California demographer has estimated that more than 9 million Americans are gay or bisexual, a number equivalent to the population of New Jersey.

Gary Gates, who studies the demographics of the gay community for the Williams Institute at the UCLA School of Law, said in a report released Thursday that about 3.5 percent of Americans identify themselves in surveys as being gay or bisexual.

But those who said they have had same-sex encounters are almost evenly divided between gay people and bisexuals, leaving just 1.7 percent who say they are gay. The percentage more than doubles, however, if it includes people who say they have engaged in same-sex sexual behavior at some point.

Gates arrived at the estimates by averaging the results of five surveys conducted in the United States between 2004 and 2009. In the survey results, the range of people who say they are gay or bisexual varied widely, from a low of 1.7 percent to a high of 5.6 percent. In contrast, studies conducted in other countries have made much smaller estimates — as low as 1.2 percent in a 2010 survey in Norway.

In a telephone interview, Gates said that only recently have researchers doing surveys on health and social characteristics started asking respondents for their sexual orientation. Gates has argued that people should be asked their sexual orientation on many surveys that have nothing to do with sexual behavior, such as those involving crime, for example. He says only then will it be possible to more accurately gauge the size of the gay community.

“The number matters,” he said. “An unfortunate part of our political system is that you don’t really count unless you’re counted. LGBT Americans still are not routinely counted. That allows legislators and policymakers to say they really don’t matter much, because if they did, we’d have this data.”

In Maryland, the Gates numbers were seized upon by advocates and opponents of efforts to legalize same-sex marriage.

Del. Donald H. Dwyer Jr. (R-Anne Arundel) noted that the number was considerably smaller than estimates made by advocates of same-sex marriage.

“Delegates in Maryland heard from their constituents,” he said. “That’s why same-sex marriage failed this year. Until there’s a wide acceptance of it across the state of Maryland, I don’t think those bills are going to be successful.”

Officials with Equality Maryland, which has lobbied hard for same-sex marriage to be legalized in the state, cited previous studies estimating that Maryland’s gay community has 178,000 people, including 15,000 couples.

“Because folks are self-identified, the number is probably always a little lower than expected,” said Linsey Pecikonis, a spokeswoman for the group. “But I think having 9 million Americans be members of the LGBT community, that’s a fairly significant community.”

Estimates of the size of the gay community remain elusive four decades after the gay-activist movement was born in the wake of the Stonewall riots in New York City during the summer of 1969.

Gates said many people have heard of estimates as high as 10 percent of the population, a figure that stems from a 1948 study of male prisoners done by Alfred Kinsey. He wrote, in carefully parsed prose, that one in 10 men was “more or less, exclusively homosexual for at least three years between the ages of 16 and 55.”

“The number stuck,” Gates said. “Here we are, decades later, and it’s still the most predominant number cited.”

morelloc@washpost.com