The new season of “Will & Grace” reunites original stars Eric McCormack, left, Debra Messing, Sean Hayes and Megan Mullally. (Chris Haston/ NBC)

While discussing marriage equality on “Meet the Press” in 2012, Vice President Joe Biden said, “I think ‘Will & Grace’ probably did more to educate the American public than almost anything anybody has ever done so far.” This assertion is as much a part of the show’s legacy as actor Megan Mullally’s squeak is. And the idea that TV can lower prejudice is backed by science, in part through the 2006 study “Can One TV Show Make a Difference?”

That’s not surprising. “Will & Grace,” which ran on NBC from 1998 to 2006, brought two charming gay men into millions of American living rooms — the first gay leads on a hit sitcom in history. But as the series returns Sept. 28, what can its impact be, in a world where gay marriage is legal, and New York City’s Pride parade sponsors include T-Mobile, Walmart and the Walt Disney Co.?

The 2006 study, conducted by Edward Schiappa, Peter Gregg and Dean Hewes, was groundbreaking not for its conclusion but because it discovered how prejudices were reduced.

The big news, according to Schiappa, was this finding, quoted from his study: “For those viewers with the fewest direct gay contacts, exposure to Will & Grace appears to have the strongest potential influence on reducing sexual prejudice, while for those with many gay friends, there is no significant relationship between levels of prejudice and their exposure to the show.” Schiappa, the head of Comparative Media Studies at MIT, explained over email, “That is what persuaded us that exposure to gay men on television was functioning in the same manner as interpersonal contact.”

He and his team drew on the work of Gordon Allport, who in 1954 solidified research proving that contact between minority groups and majority groups can reduce prejudice in the latter. (There are qualifications to the contact, however, including that people must feel of equal status and must not be opposed by an outside authority.) Basically, Schiappa’s team determined that Allport’s theory applies to mass media. Their new theory, the Parasocial Contact Hypothesis, has been cited 399 times and counting in subsequent publications.

From Season 2 of the show. (Chris Haston/NBC)

The team confirmed the theory with follow-up studies on “Queer Eye for the Straight Guy” and “Six Feet Under.” Schiappa explains: “Our work proved that viewers learned, and that learning reduced stereotypes. Will and Jack were quite different gay men, and the Fab 5 in ‘Queer Eye’ were all quite different. That taught viewers a lot about the category of gay men.” Their work appeared in Newsweek before either of the studies was published.

But minimizing homophobia was never the intention of “Will & Grace” creators Max Mutchnick and David Kohan. “Our directive was to write a romantic comedy,” Kohan says. A traditional rom-com needs an obstacle to keep the man and woman apart. “The more insurmountable the obstacle,” Kohan explains, “the longer the show can go.” Making one of the two characters gay certainly provides an obstacle.

Mutchnick adds, “If we had been trying to make social commentary, we definitely wouldn’t have lasted.” The duo was merely following the artist’s adage to write what you know. When asked whether they plan to continue tackling LGBT issues in the new version, Kohan replies, “Especially the B and T parts of those letters,” but clarifies, “not because it’s an expectation but because it makes for interesting stories, because it’s stuff that comes up in people’s lives in the writing room.”

Americans tend to be far more familiar with gay people now than they were during the show’s initial run. But even today, only 30 percent of American adults know a transgender person. The number drops to 16 percent among people older than 65.

“TV influences those who lack real-world contact,” Schiappa says. However, his original hypothesis posits that “television exposure must be repeated or sustained over time” for viewer attitudes to shift. And, although trans characters are indeed appearing in recurring or leading roles — for example, in “Transparent,” which returns Sept. 22 — self-selection is a side effect of niche TV. Most people who watch “Orange Is the New Black” had low levels of transphobia before they tuned in.

But a study out of the University of Southern California, published last month, has proved that a positive depiction in just one guest appearance can effect change. It’s based on a 2015 episode of USA Network’s “Royal Pains,” which doesn’t usually feature LGBT stories. It determined that viewers of an episode featuring an 11-minute subplot — about a trans teenager who suffers medical complications from hormone treatments — had more positive attitudes toward transgender people and policies than did “Royal Pains” viewers who missed that episode.

When the results of the study came in, the lead author, USC doctoral candidate Traci Gillig, also discovered, as she writes in the paper, "neither exposure to news stories about transgender issues nor the highly visible Caitlyn Jenner story were associated with attitudes toward transgender people or policy issues." So an 11-
minute scripted subplot shifted attitudes, but Jenner's 2015 interview on "20/20," which nearly 17 million people watched, did not.

This was somewhat surprising because documentary can typically have just as large an impact as a fictional narrative, says one of the study’s co-authors, Erica Rosenthal of the Norman Lear Center’s Hollywood, Health and Society program: “As long as it’s perceived as realistic, either can be transporting into the story line.” So why didn’t Jenner’s story transport viewers? In a phone conversation, Gillig hypothesizes, “There was a lot of polarization in reaction to Jenner’s coming out.”

Regardless of their effects on viewers, positive depictions of trans people are popping up in reality TV. During a taping of “Survivor,” in a scene that aired in April, even the show’s producers were surprised when contestant Jeff Varner outed fellow contestant Zeke Smith as transgender. The remaining players criticized the action as dangerous and unfair, and unanimously voted off Varner (who posted an apology on Twitter after the episode’s airing). Contestant Sarah Lacina, a police officer from a conservative background in Iowa, tearfully told Zeke that the news did not change the positive way she felt about him and that she had grown as a result of the experience. Lacina went on to win the season.

Before the “Survivor” episode aired, CBS contacted GLAAD for advice. “ ‘Survivor’ and CBS did a phenomenal job of making it educational around why outing someone who is trans is not safe,” explains GLAAD president Sarah Kate Ellis. According to a 2010 survey, “Survivor” is the eighth most popular network show among Republicans (among Democrats, it didn’t rank in the top 25). Last season’s premiere reached 7.7 million viewers.

The cast of "Will & Grace" at NBC’s presentation to advertisers in May. (Heidi Gutman)

“Will & Grace” could potentially be a more polarizing show. Just before last fall’s election, the gang reunited to make a one-off webisode, which not-so-shyly endorsed Hillary Clinton. (The short’s success sparked plans for the return of the series.) How will the show cross the current minefield of partisan America? “Not lightly,” Kohan says.

“It’s not like we are saying, ‘We have to be careful because a lot of people are triggered easily on both sides,’ ” he explains. “Our attitude is, no, people will watch because it reminds them of a time when people weren’t triggered so easily, when things were less polarized.”