David Thorpe knew he was gay by the time he was a teenager. In his hometown of Columbia, South Carolina, though, he felt the need to hide.

Locals regularly preached about the dangers of homosexuality — some cited religion, others traditional values. The "why" became less important over time. Thorpe stopped trying to understand it; instead, he just felt mute.

"It was the '80s in the Bible Belt. 'Fag' was a common word everyone used. There was this widespread understanding that being gay was a sin," he says.

Over time, his insecurity turned to shame: "I didn't know a single gay person to turn to. So I kept my mouth shut."

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Thorpe eventually came out during his freshman year at Harvard University. Now, at 45, he's producing a film called Do I Sound Gay?, which aims to de-stigmatize the stereotypes surrounding men with "gay"-sounding voices.

And while progress for LGBT equality has undoubtably been made since his childhood, he says, there are still plenty of issues left unaddressed.

One in particular: internalized homophobia.

"It's something that's really well-known to anyone who's gay," he says, "but not many people in the mainstream world think about it. Most people don't even know what it is — and that's dangerous."

So, what is internalized homophobia?

If it sounds like a fancy clinical term, it's because it is.

Definitions vary between therapists, but it generally refers to the internalization of a society's homophobic attitudes within lesbian, gay or bisexual people.

Researchers say it can correlate with a number of psychological and medical outcomes, including depression, shame, substance abuse and dangerous sexual behaviors.

For Thorpe, his environment shaped his internalized homophobia. The shame he felt, almost subconsciously, was a direct result of the "facts" against homosexuality he had been taught while growing up.

"You're impressionable when you're young. When I came to terms with my sexuality, I was trying to accept who I was," he says.

"But I still had all these notions I'd been taught while growing up. It's easy to put aside those prejudices and acknowledge that that's all they are — but still, there's this lingering default feeling that sticks around. You can't just put that aside."

Matthew Weissman, Ph.D., a clinical psychologist in Washington, D.C., works predominately with gay teens and adults. Most of his patients struggle with some form of internalized homophobia. Like Thorpe, the root cause is typically the home environment.

"People hear disparaging comments from friends, parents, political leaders," Weissman says. "The church, of course, creates a huge stigma against it — that one still affects a lot of people."

Geography plays a big role, too. For gay people living in communities where those stigmas still exist, he says, it's best to just move to a more progressive area. But it's not always an option — especially for teens and high schoolers.

"When you're a social minority, it's so important to make contact with communities to avoid feeling isolated," he says. "That can't be stressed enough."

Bottom line: It's not a new phenomenon. But thanks to the increasing number of states legalizing same-sex marriage, it's now an issue that's tiptoeing its way toward the spotlight. Nobody fighting for LGBT equality is saying, "OK, we've just about legalized same-sex marriage everywhere. I think we're done." There's more work to be done.

It's just a check off the list. But, Weissman adds, it's a long list. Internalized homophobia is, and should be, next up.

Michelle Birkett, Ph.D., is a faculty member at Northwestern University's IMPACT Program. Over the past few months, she's co-authored 17 papers with the American Journal of Public Health, all examining the health of LGBT high school students.

The studies used data from the Youth Risk Behavior Survey (YRBS) Pooling Project, which polled a random sample of high school students from 14 states. The surveys only recently added a "sexual orientation" option for students to check, which Birkett says has allowed her team to examine the health of LGBT students in greater detail than ever before.

More than 6,400 students identified as LGBT (or "sexual minority") in the survey; 70,552 identified as straight ("sexual majority"). The questions addressed specific social problems, such as drug use, mental health and family life.

Those who identified as LGBT showed significantly more troubling results in every category.

"This is a subconscious problem, which is part of the reason why it's often difficult to address," she tells Mashable. "In this study, the numbers are clear: Those kids [who] identified as sexual minorities were much more prone to drinking, getting worse grades, feeling depressed."

Some of the other results: Gay, lesbian and bisexual students had between 1.8 and 4.7 times the odds of using drugs (like cocaine, heroin and methamphetamine) than their heterosexual counterparts. They were also four times as likely to attempt suicide.

"This is one of the strongest ways this type of thing has been looked at before," Birkett says. "It's clear that LGBT kids experience more risky outcomes earlier on — and minority stress, or internalized homophobia, plays a very large role."

What can we do?

The solution is both easy and extremely challenging at the same time. For a social issue to change, the overall mindset needs a makeover. It's a tall order.

"The best thing is to identify the source of the stigma. I tell my patients to think: 'Who, or what, was it that made me feel this way for the first time?'" Weissman says. "Then go from there. If it's a certain group that you're still associated with, leave; if it's an organization, get out of it."

The Internet has definitely helped, he adds. It's easier to build communities online — an especially helpful tool for anyone who feels geographically constricted.

Thorpe agrees that the legalization of same-sex marriage is an encouraging step — but even that's not fully solved. And when it is, he says, it won't be enough. The fight for equality is a multidimensional battle. Passing legislation is one thing; mending mental scar tissue is another.

When asked about long-term solutions, Birkett pauses. She exhales after a few seconds, then laughs as if she'd just been asked the most obvious question in the world.

"If we want to end internalized homophobia, we need to first end homophobia," she says. "It's as simple as that."