Is he lovable? Check out his hair.

TV’s leading men are defined by their hair. If you doubt that, just ask yourself if The Office’s John Krasinski would be considered as adorable with normal, non-tousled hair, or if Kiefer Sutherland would scare the terrorists with the Billy Idol hairdo he sported in The Lost Boys. Women may get a bigger share of a show’s hair and makeup budget, but TV guys need the most distinctive coiffures. Tena Parker, department head hairstylist for How I Met Your Mother, says, “We have three fabulous men on our show, and three different looks.”

Maybe we should blame David Schwimmer. The Friends actor was the first TV star in years to be associated with a messy-looking haircut, which came to be known simply as “the Schwimmer.” Schwimmer’s sartorial influence is all over TV, with a parade of soft-spoken, puppy-dog leading men and their strange hair. Josh Radnor’s on How I Met Your Mother is half normal and half hairsprayed and upright; Parker adds that this look was Radnor’s own, and that he “loves pulling on his own hair and works with the stylist to design that.” Zachary Levi of Chuck has hair curving up at the sides and hanging down, mullet-like, at the back of his head.

Parker explains that it’s hair that is made up to look like they didn’t put too much effort into it. “Everybody wants to look like, ‘Hey, I just rolled out of bed and look how fabulous I look!’ And usually it takes some effort to get us there.” It’s a style that’s invading even the sombre world of crime procedurals; on The Mentalist, Simon Baker plays a crime-solving genius with a tragic past, but wears his mop of curly blond hair in what looks like a hand-knitted pattern. The look establishes Baker as a charming rogue with no regard for police procedure, even before the scripts do.

These hairstyles aren’t just meant to make guys look cute; like Kojak’s bald head or William Shatner’s toupée, they have a dramatic purpose. All that wavy, unkempt hair tells the audience these characters don’t have a woman to take care of them; they want to look good but can’t get their hair quite right. It’s the perfect single-man look, and one that gets us rooting for them to find love. And that look has the same effect no matter what country you’re in. Krasinski may have a different character name from his counterpart in the original British version of The Office, but his hairstyle is almost the same, and it allows us to know immediately that Jim/Tim is the nicest guy in the room. Plus, let’s face it, it’s also a little important to make them look cute. “I think subliminally, it’s a bed-head look, and that’s sexy,” Parker says.

On the other hand, if producers want a character to be an action hero or at least a tough guy, they’d better make sure he doesn’t have too much hair; it makes a character look as if he could never win in a fight. In the 24 TV movie, Redemption, the most villainous character was a spineless UN peacekeeper (Sean Cameron Michael) who had long hair hanging down in spikes over his forehead; Jack Bauer, the man of action with short, unfussy hair to prove it, contemptuously ordered him to “hide in the shelter with the other children.” Dr. House (Hugh Laurie) has an unspectacular, short haircut that never calls attention to itself; hairspray and long locks are reserved for his wimpy underlings. On Chuck, one of the main conflicts is between Levi’s character and Adam Baldwin’s tough, short-haired NSA spy. If a TV man wants to be authoritative, he has to look as if he doesn’t care about minor matters like bald spots or the natural effects of aging; Mark Harmon has allowed his grey hair to show on NCIS, and it makes him look like the no-nonsense marine he’s playing. It would be hard for him to order his subordinates around if his head were soaked in brown hair dye.

But despite the popularity of these greying, thin-haired leading men, they seem to be fighting a rearguard action; the future belongs to the ample-haired guys who bring out our maternal instincts. Even actors who don’t usually act adorable are starting to adopt that kind of haircut. Parker says that Neil Patrick Harris, who plays the amoral Barney on How I Met Your Mother, “is wanting to go a little more dishevelled in his hair.” And you can’t escape it by switching to the news: Meet the Press will replace the late Tim Russert with David Gregory, whose shaggy, puffy hair suggests eternal boyishness, even though he’s almost 40. We may be looking at a future full of network TV males who could be beaten up by Jack Bauer. If, that is, he could find them under all that hair.