Democratic presidential nominee Hillary Clinton laughs in the direction of Republican presidential nominee Donald Trump during the presidential debate at Hofstra University in Hempstead, N.Y. on Sept. 26. (Julio Cortez/Associated Press)

If you are running for president, it is important that you smile. But not too much. Smile for the proper duration of time and make sure it is a good smile. Especially if you are Hillary Clinton.

Commentary about Clinton’s grin from prominent media talking heads during Monday night’s presidential debate ignited social media, prompting critiques that such feedback is a double-standard applied to women in politics.

In the past, the Democratic nominee has been criticized for not smiling enough. During Monday night’s debate, she was criticized for doing the opposite.

“Who told Hillary Clinton to keep smiling like she’s at her granddaughter’s birthday party?” tweeted David Frum, who was a speechwriter for President George W. Bush.

Following the debate, Fox News Channel’s Brit Hume spoke about how both candidates looked when they were not speaking. “The Trump expression is one we’re all familiar with from the earlier debates. He looked annoyed, put out, uncomfortable,” Hume said. Clinton “looked composed, smug sometimes, not necessarily attractive.”

“Hillary’s condescending smile is NOT likable,” tweeted Richard Grenell, former Bush spokesman for the United Nations.

Those who take offense at such commentary view it as applying stereotypes about women — that they be friendly, polite and pleasing — to someone running for the highest office in the country.

“Prepare, but don’t be over-prepared. Smile, but not too much. Be strong, but not shrill. When interrupted, grow wings & fly to the moon,” tweeted Variety television critic Mo Ryan.

And this isn’t a new issue, especially with Clinton. It’s one that her supporters also raised during her 2008 bid for president.

Although both men and women running for any political office try to be likable, studies suggest that women’s appearances receive disproportionate attention. A Rutgers University review of studies examining media coverage of women in politics discovered that among the most consistent and persistent findings “is that women candidates receive more attention to appearance, personality, and family than men.” The paper cites examples involving both Democrats and Republicans.

The Women’s Media Center released a series of studies in 2013, based on online surveys, showing how a female candidate’s chances of winning over voters lessened when the media described her appearance, regardless of whether it was a negative, neutral or positive description.

Some who have been criticized for talking about Clinton’s smile (or lack thereof) in the past have argued that such commentary is fair game, an aspect of politics that gets at whether someone is trustworthy and received well by voters.

MSNBC’s Joe Scarborough  — who faced backlash for his advice to Clinton upon her winning some big primary states: “Smile. You just had a big night.” — wrote a defense in The Washington Post entitled “Why all presidential candidates — male or female — should smile when they win.”

“No man should say ‘smile’ to a woman walking down the street. I understand that and would never say such a thing. But Clinton is not a pedestrian on her way to work,” Scarborough wrote. “She is the most experienced politician on the national stage today who wants to be the next president of the United States. Critiquing anyone’s political skills who seeks that office is always fair game. Whether your name is Donald, Ted or Hillary.”

Clinton herself has responded to comments about her face before, including to this tweet from Republican National Committee Chairman Reince Priebus earlier this month:

“Actually, that’s just what taking the office of the President seriously looks like,” she tweeted back.

The next day, Clinton was asked about the exchange and whether there is a double standard for women running for president.

“I’m going to let all of you ponder that last question,” she told reporters. “I think there will be a lot of PhD theses and popular journalism writing on that subject for years to come.”

Last week, Clinton went on “The Tonight Show Starring Jimmy Fallon” where she was asked about balancing coming across as fun and likable while also projecting strength and toughness.

“It’s especially tricky for women. It just is,” she responded. “These are important issues that the country needs to talk about. And the Republicans were saying, ‘Oh she looks so serious.’ Well, you don’t talk about ISIS with a big grin on your face.”