The rules for the Best Shirtless Performance category would seem to be pretty cut and dry: if you're wearing a shirt, you're not qualified. No sleeves? No problem.

However, the inclusion of Kate Upton and her dazzling white bikini in "The Other Woman" in the category raises a few questions. She's not topless, but is she shirtless? Is a bikini a shirt now? Maybe being shirtless is not quite as simple as wearing a shirt or not wearing a shirt. (We here at MTV News are devoted to answering life's big questions.)

Desperate to gain expert insight into the situation, we sought the help of Professor Beth Charleston, a fashion historian at Parsons The New School For Design in New York City. As we suspected, being shirtless doesn't just depend on whether your torso is totally covered or not, according to Charleston. It's a state of mind.

"If you think about European explorers going to South America or Africa and seeing indigenous people who do not wear shirts, are they shirtless before the European explorers get there, or do they become shirtless because a European explorer from a culture that is very heavily dressed and always wears shirts in public is viewing them?" Charleston asked. "Is that what makes them shirtless? I definitely think it's a relative thing."

It's also a situational term, according to Charleston. If you're in the shower, you're supposed to not have a shirt on. You wouldn't say, wow, that person is in the shower, and WOW, they're not wearing a shirt!

"If you're in a situation where it's appropriate to have a shirt on, then you can be shirtless," Charleston said. "If a person's in a shower, they don't have a shirt on, but I wouldn't necessarily use the word shirtless or topless for them. So I think to me it has something to do with the situation and exposing yourself in a situation that is possibly other people will be more dressed than you. It's a position of being more vulnerable and very aware of your body, as opposed to when you're in a situation where you shouldn't have your shirt on, like swimming or in the bedroom or in the shower, that sort of thing."

Get it? Good. Let's see who Charleston thinks who is -- and isn't -- shirtless, of this year's Best Shirtless Performance nominees.

Channing Tatum, "Foxcatcher"

"I think Channing Tatum in that scene, being next to Steve Carell with the tuxedo on, even though [Tatum]'s in his wrestling singlet and it's pulled down, because Steve Carell is so dressed, it makes him definitely shirtless," Charleston said. "I think it's a situational thing as well."

Verdict: Shirtless!

Ansel Elgort, "The Fault in Our Stars"

"He's not supposed to have a shirt on there," Charleston said, in what may be a controversial ruling on Elgort's shirtlessness. "Yes, you and I both see he doesn't have a shirt on, but I think it's appropriate for the situation. But if he left that room and went to answer the door, then he would be shirtless."

Verdict: Not shirtless!

Kate Upton, "The Other Woman"

"I don't think Kate Upton is shirtless because she has on an amount of material that is socially acceptable," Charleston said. "She's on the beach. She's beautiful and she's Kate Upton, but she's in a bikini. She could be topless! She's on a beach, she could take it a step further. I'm not uncomfortable with her looking like that. I would look and I would stare and I would be interested, she's beautiful and it's provocative, but I'm not uncomfortable."

Verdict: Not shirtless!

Chris Pratt, "Guardians of the Galaxy"

"This is not their jail uniform, so he's definitely stripped down. But he's being showered!" Charleston said, debating how appropriate Pratt's nudity was to his situation. "This is a hard one. He's being stripped down, but it also looks like he's being humiliated. There's definitely a sense of power and it has to do with being vulnerable and not being dressed properly for the situation. He's not choosing to be showered. I think shirtless."

Verdict: Shirtless!

Zac Efron, "Neighbors"

"Yes, absolutely," Charleston said of Efron's shirtless Abercrombie modeling. "It's inappropriate! The other person is uncomfortable."

Verdict: Shirtless!

And, of course, after all these academic explanations of why a person could be deemed not shirtless even while wearing no clothing on their top half, we had to ask Charleston who she was rooting for to win the big award at Sunday night's Movie Awards.

"Zac," she said decisively of the "Neighbors" star's topless turn. "That is hysterical. It's such a riff on Abercrombie too. I love how it makes the other character uncomfortable and how comfortable Zac Efron's character is dressed like that, being shirtless in the mall. It says a lot about being shirtless. Making other people uncomfortable and showing off a little bit and being an exhibitionist. He's very aware of it, but he's very comfortable with it too."

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