Hillary Clinton doesn't willingly talk much about the hurdles she faces as a woman in politics that her male opponents don't have to.

But in an interview aired Monday night with Jimmy Fallon on "The Tonight Show," the first major-party female presidential candidate weighed in on what she called a "tricky" struggle to be a serious yet likable presidential candidate — the latter being a character trait female politicians have to think about more than men, research shows.

Fallon brought the topic up by asking Clinton about the duality she often has to present: "You have to go and say 'Hi' to everybody and be happy, and you do, but also people kind of want to see this Hillary," — he brought out a frame of the image below — "which is the badass Hillary. ... Is that a tricky balance for women?"

Clinton said it was. "It's especially tricky for women. It just is. Because there are a lot of serious things. The other night, I was on a show and being asked about ISIS and Iran," she said, using the acronym for the Islamic State, "and I was serious. These are important issues that the country needs to talk about. And the Republicans were saying: 'Oh, she looks so serious.'"

None other than the chair of the Republican National Committee, Reince Priebus, accused her in NBC's Sept. 7 Commander-in-Chief Forum of not smiling enough:

Clinton continued: "Well, you don't talk about ISIS with a big grin on your face. They're a barbaric, evil group that we have to defeat and wipe out. But it is a constant balancing act: How do you keep the energy and positive spirit while taking seriously what you need to?"

Research shows Clinton is right. Women in politics have to balance opposing emotions more than men. That's because voters want to elect a likable female politician, while they don't value that quality nearly as much among their male elected officeholders.

Earlier this year, the nonpartisan Barbara Lee Family Foundation studied how voters measure this hazy, intangible quality of likability among women officeholders (they didn't study Clinton or any particular presidential candidate). They came up with a dizzying list of do's and don'ts that, fairly or not, they said aspiring female politicians would be wise to follow. Among them:

  • Don't pose for a headshot. Instead, circulate more candid, informal photos of you engaging in your community — say hanging out with children on a playground. "To show likeability, a woman doing her job among constituents is effective," the study's authors say.
  • Do share personal anecdotes when explaining why you're passionate about an issue or how you've helped constituents.
  • Don't take credit all the time for your accomplishments; every once in a while share credit with your team.
  • Do recognize your hair, makeup and clothes will be scrutinized by voters much more than a man's.
  • Above all else, be confident. And you can't fake it. "Voters assessed a woman officeholder’s confidence in less than 30 seconds," the study's authors said.

Clinton's acknowledgment that it can be tricky out there for a woman in public office comes as her likability sits at the lowest levels of her career. An August Washington Post-ABC News poll found a record number of Americans view Clinton unfavorably. Her likability is now on par with Donald Trump, who is the least-liked major-party candidate in modern times.

Over the weekend, President Obama chimed in on that narrative. He said the election is close because of sexism toward Clinton, saying the country isn't yet comfortable with women in leadership roles.

That statement can — and is — being debated. But the hard truth is that being likable probably matters more to Clinton's chances of getting elected than to Trump's, simply because Clinton is a woman.

In a rare moment Monday, she acknowledged how difficult that double standard can be.