Unhappily unattached menfolk of America, that soul patch beneath your lower lip may be discouraging a potential soulmate from sticking around through the salad course of your first date. And the stakes are as high as the waist on those dad jeans.

That's right; while we've been taught not to judge a book by its cover, experts say that when it comes to calibrating long-term compatibility, the reality is that what men wear is the first — and often most crucial — criterion that women use to judge. "And research shows that's usually within the first 15 to 20 seconds," says Pepper Schwartz, a professor of sociology at the University of Washington in Seattle and author of 15 books on relationships. "After that, the door starts to close."

Which makes the forthcoming book, "Undateable: 311 Things Guys Do That Guarantee They Won't Be Dating or Having Sex" (by Ellen Rakieten and Anne Coyle, due in stores next month), a kind of fascinating field manual of future-foreclosing fashion faux pas that ranks infractions such as high-waisted dad jeans (we're looking at you, POTUS), novelty belt buckles, "mandannas" and Ed Hardy gear, on a scale from "Red Flag" (tube socks) to "Kiss of Death" (Speedos).

While Rakieten and Coyle catalog some of the cringe-worthy things men say ("Come to poppa," "Booyah!") and do (bring a baseball glove to a professional game, pop the collar on a polo shirt), the first half of the book focuses on the laundry list of wardrobe malfunctions that threaten future connubial bliss.

"We put in a lot of those because they're so easily fixable," Rakieten said. "And what woman doesn't enjoy a little makeover? At the same time, there's a range (of infractions). If you're a suburban white guy who decides to dress like Lil Wayne, it says a whole lot more about you than just choosing the wrong color sneakers."

Co-author Coyle thinks that because women have traditionally invested more time in — and paid more attention to — the way they look, "in those first five minutes after meeting someone, we're more attuned to it. I don't think a guy is going on a date and saying: 'Oh, my God, this girl is wearing this queer Cynthia Rowley blouse.' "

Coyle, who is divorced, explains the reasoning behind "undateable" behavior with a story from her own recent dating life.

"My friends set me up with this guy who came very highly recommended. We had fun together, we had the same political views. Everything was perfect, except he had this gross soul patch beneath his lower lip. I just couldn't do it. ... My friends said, 'Well, maybe he can shave it off,' and my response was, 'Yes, but he can't shave off the part of his brain that thought growing it was a good idea.' That's the real problem."

"Undateable" isn't the only upcoming tome to tackle the topic. Dave Horwitz and Marisa Pinson, a once romantically linked couple who have spent the past year of their lives compiling hilarious instances of relationship-ruining behavior (entries include "You're an American Apparel Model" and "You Still Wear Your High School Class Ring") on their Dealbreaker blog, recently inked a deal for a book that's due out sometime before next Valentine's Day.

"It's not so much about what someone's wearing or if they slip up and say something that makes you want to write them off," Pinson explained. "It's a general example of where they're coming from." Or as Horwitz puts it: "It's usually indicative of a much larger problem. ... You rarely find a scintillating, fascinating, brilliant sexy person wearing Crocs."

Schwartz, who created the compatibility matching system for the online dating site Perfectmatch.com, said these snap judgments function as a kind of protective mechanism. "People are nervous, they're trying to figure out how to avoid pain and not waste their time, so they're looking for cracks in the onstage performance," she explained. Their behavior "may be triggered by seemingly superficial signals, but they're not generally superficial concerns."

While this behavior is common to both genders, Schwartz thinks women are much pickier. "When I lecture on dating, I tell women to shorten their damn lists. They have a list of things they need, a list of things they don't want. ... One woman told me her deal breaker was that a guy didn't floss. A guy's list might have 10 things — and if the woman is good-looking enough, nine of them won't matter."

Which brings up an interesting question: If Crocs footwear, pleated-front khakis and belt-clipped cell phones can cause men to become so much radioactive relationship road kill, does it work the other way around?

Can wearing Birkenstock sandals, men's neckties or eyeliner out to your ears send the wrong message? Does referring to your breasts as "the boys," speaking in baby talk to other adults or continuing to sit on your dad's lap (well into your late 20s) discourage dates?

Rakieten doesn't think so. "Let's be honest," she said. "Men traditionally will overlook many, many undateables if there's sex at the end of the night in front of them. ... It's not a question of 'undateable' but 'unmateable.' "

Coyle agrees. "I think men are willing to tolerate a lot more if there is sex involved. Men will date anything. The real question there is not whether you want to go home with the girl, but do you want her to be the mother of your children? For women, it's much more of a mountain to climb."

And if you happen to recognize a little bit of yourself as Exhibit A for the offenses chronicled in "Undateable" or at Dealbreaker, there's still hope. "First, if you're under 21, you get a free pass," Rakieten said. "And after that, swagger and confidence can overcome a lot."