A record 4.5 percent of American adults identify as lesbian, gay, bisexual or transgender, according to a new Gallup estimate. The percentage, which works out to more than 11 million U.S. adults, is up from 4.1 percent in 2016 and 3.5 percent in 2012, the year Gallup first started tracking LGBT identification.

The increase was driven primarily by millennials, defined as those born between 1980 and 1999, according to the report’s findings. In 2012, 5.8 percent of this cohort answered “yes” when asked, "Do you, personally, identify as lesbian, gay, bisexual or transgender?” In the 2017 estimate, that numbered jumped 40 percent to 8.2 percent of millennials.

"Social acceptance of the LGBT population has increased substantially over the last decade, and those changes have been more pronounced in younger populations." Gary Gates, LGBT Demographer

In contrast, Gallup found the LGBT-identification rate of older generations is steady. In this latest survey, 3.5 percent of Generation X respondents (those born between 1965 and 1979) identified as lesbian, gay, bisexual or transgender; 2.4 percent of baby boomers (born between 1946 and 1964); and 1.4 percent of traditionalists (born prior to 1946).

Gary Gates, an LGBT demographics expert and the former director of the Williams Institute at UCLA Law School, said it’s “not surprising” that a great proportion of millennials “are comfortable being more open about their LGBT status.”

“Social acceptance of the LGBT population has increased substantially over the last decade, and those changes have been more pronounced in younger populations,” Gates told NBC News via email. “Today's youth have peers and social networks that are more supportive of LGBT people and issues when compared to older generations.”

Millennials have been found to be significantly more accepting of lesbian, gay, bisexual and transgender rights than their older counterparts. According to a 2017 NBC News/Wall Street Journal poll, 75 percent of people aged 18-34 support same-sex unions, compared to just 42 percent of people over 65.

In its latest estimate, Gallup also found women account for a greater share of America’s growing LGBT population. Between 2012 and 2017, the share of women calling themselves LGBT went from 3.5 percent to 5.1 percent. For men, the rate ticked up only a half point, from 3.4 percent to 3.9 percent.

Gates said while this trend has been observed in several large surveys, there’s not much research to explain what’s behind the gender disparity.

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