Vail Reese is the world expert on movie characters’ skin conditions. The San Francisco dermatologist can tell you anything you want to know about scars, birthmarks, tattoos, Jon Hamm’s vitiligo and the Austin Powers adversary Fat Bastard’s extra nipples, a spoof of the same condition on Christopher Lee’s assassin in “The Man With the Golden Gun.”

For two decades, Reese’s website, Skinema, has chronicled these abnormalities, which, he argues, too often appear on villains. For instance, in the movie “Grease,” he writes, “Pretty boy Travolta . . . musically drag races . . . the extensively acne-scarred ‘Crater-face.’ ”

Reese sees how such conventions affect his own patients. “It’s not just, ‘Am I going to look pretty?’ ” he says. “It’s, ‘Are people going to judge me?’ ”

His work is a peek into how Hollywood equates classical beauty with virtue, from Disney romances to James Bond bad guys to comedians making fun of Stephen Bannon’s face. Despite some progress, movies and TV still lazily perpetuate a notion we no longer believe: that looks correlate with character. When many in Hollywood are fighting for greater diversity and against stereotypes of all kinds, should that fight include types of bodies and faces?

“It’s overdue,” says Nancy Etcoff, an assistant clinical professor at Harvard Medical School who wrote the 1999 book “Survival of the Prettiest.”

“Hollywood could do a lot to move us in a direction to widen our empathy and widen our notion of what is beautiful.”

A recent study in JAMA Dermatology found that six of the American Film Institute’s top-10 villains of all time have dermatologic issues, from Hannibal Lecter’s androgenic alopecia (hair loss) to the Wicked Witch of the West’s verruca vulgaris (wart), as Travis Andrews reported recently in The Post . “Unfairly targeting dermatologic minorities may contribute to a tendency toward prejudice,” the study says.

Reese points out that good characters turn evil after they get blemishes, like Two-Face, the district attorney who becomes a “Batman Forever” nemesis after an acid attack. Sure, scars can cause people to feel alienated, the doctor says, but “I’ve never met the serial killer who said, ‘I didn’t want to kill anybody but because I got this bad skin rash. . .’ ”

“Beauty equals good” is an age-old trope in entertainment, points out Doris Bazzini, a professor of social psychology at Appalachian State University. In a 1999 study she co-authored, a panel watched 20 of the top-grossing movies in each decade from 1940 to 1989, rating characters by attractiveness and other traits. They found that the stereotype held true across all decades and genres. She discovered the same thing in a 2010 study on human characters in 21 Disney animated movies.

“Attractive characters displayed higher intelligence, lower aggressiveness and greater moral virtue,” the study said. Even animal characters show similar patterns, Bazzini points out: The villain in “The Lion King” is literally named Scar and has one across his eye. “Evil as plain as the scar on his face,” says one song in “The Lion King II.”

Albino characters have been a big concern for Reese — in 2006, an advocacy organization counted 68 albino villains since 1960.

“A movie like ‘The Da Vinci Code’ would come out and [albino] kids in high school would get comments and stares and occasionally violence,” Reese says, referring to the film’s albino character, played by Paul Bettany.

Reese does acknowledge that it has improved. The sympathetic Deadpool (played by Ryan Reynolds) has severe scarring under his mask, for instance.

Still, stereotypes persist. Christoph Waltz as Blofeld in “Spectre” and Adam Driver as Kylo Ren in “Star Wars: The Force Awakens” get scars — each perpetuating conditions from villains earlier in the series (unhelmeted Darth Vader and the earlier Blofeld, a model for Dr. Evil). In “Beauty and the Beast” — even leaving the premise aside — Reese spotted a scar on one of the wolves chasing Belle. “It just was like, ‘Okay, were we not afraid of these wolves already?’ ”

In the TV show “Goliath,” William Hurt’s character has scarring across the side of his face, and although the show calls out the stigma, “he’s a monster,” Reese says. “He arranges murders, he’s duplicitous, he’s dishonest. So they’re trying to have it both ways.”

The trope finds its way into comedy, where political cartoons exaggerate a nose, chin or mouth to imply someone is silly or sinister. And late-night comedians poke fun of their targets’ looks all the time. As Seth Meyers said recently, “Pollen, thanks to you, I am so stuffed up, my face feels the way Steve Bannon’s face looks.” President Trump’s skin tone has prompted many variations on the insult “Cheeto.”

But Trump reportedly takes medication for rosacea, a condition marked by redness that affects 16 million Americans. Such jokes aren’t just targeted at politicians — they imply a broader attitude about what faces mean. Shouldn’t we avoid spreading the stigma of any perceived appearance quirk, regardless of how detestable that person may be?

Heightening the issue is how, as Etcoff writes, “beauty conveys modest but real social and economic advantages.” Attractive people have been found to get better wages and shorter prison sentences, for instance. In one of Bazzini’s studies, subjects watched movies with different levels of the “beauty is good” trope and then rated job applicants. “People who had just watched strongly biased movies were more likely to use attractiveness in their judgment of an applicant,” she says.

Some might argue that Hollywood is a business — let them put the pretty people on screen and let the people pay to see them. Our preference for classical beauty is to some extent socially influenced, but it’s also hard-wired, dating to when clear skin meant no parasites, for instance. But Etcoff compares this attitude to our desire for fast food — another remnant of evolution that’s unhealthy.

Hollywood activism on this issue could be impractical, and it might appear ridiculous, even dangerous, to try to relate it to gender and race, which are deeper aspects of identity.

Still, Etcoff argues, all types of diversity on-screen should be part of the same fight.

“It does come back to . . . where am I in this culture? Why does no one [on screen] look like me? Is it because I’m short? Because I’m above average weight? Because of my skin tone? Maybe I have some sort of disability. It makes it feel like those [traits] are being hidden in some way, or devalued,” she says.

“There are a lot of people who are interesting and fun and moving to look at who are not there. All people in some ways should be visible.”