Marthe*, an event planner for a New York City nonprofit, ended a relationship because of a laugh. “I met him on the tennis courts. He was smart, pleasant, and a good player who loved the game as much as I did,” she says.

Bonded by their shared lust for the sport, a romance was born. But after about a year, she found a trait she couldn’t tolerate off the courts.

“If we were at a party or out with friends and anyone told a joke, he’d burst out laughing—always louder than anybody else,” she says. “He’d bray, snort, and wheeze like some asthmatic animal. Everyone’s eyes would go wide.”

Marthe hoped the quirk would fade in time. No such luck. Soon the intensity of her tennis partner’s guffaws began to affect his sex appeal. “I became repulsed; I had to break it off,” she says. “I never even told him my lame reason for ditching him.”

Modern-Day Dealbreaker
If Marthe’s story sounds funny, it may be because the things we tend to end relationships over usually aren’t [1]. Typically, they’re big, dramatic romantic wrecking balls: infidelity, an addiction, or differences in opinion about a major life decision, like whether to get married or have kids.

Few of us think that little things—loud chewing or, say, singing the lyrics from that Kit Kat commercial incessantly—could actually sabotage romance. Yet new research on such annoyances shows they regularly erode, and frequently end, relationships.

Michael Cunningham, a University of Louisville psychologist, began studying the phenomenon in the context of intimate relationships and found reactions so intense he likened them to physical allergies.

In fact, the behaviors could become so irritating they would cause stomachaches, rashes—even fevers. He named them “social allergens,” identifying four distinct behaviors: Uncouth [2] (forgoing deodorant, peeing with the door open); intrusive [3] (peeking in his inbox, criticizing her hair or clothes); egocentric [4] (always insisting on picking the movie, or being right); and norm-violating (drunken partying, shoplifting).

The allergens also proved a reliable litmus test for romantic success—or lack thereof.

Too Close For Comfort [5]
So if allergens can spell the end, how do they begin? It turns out that pivotal moment when a couple settles into everyday love (when sexy, witty, and pretty take a backseat to easy, cozy, and lazy) is when they’re most likely to emerge.

Cunningham describes this phase as being a transition from “front stage behavior”—when we’re always on, trying to make an impression—to “back stage behavior,” when we drop all pretense and share our “true” selves.

Sometimes, all too true: After a year or two, he says, we tend to act freely—and that begets trouble. While an allergen may start as a mere irritation, over time it becomes “symbolic of larger things,” says Cunningham.

Consider, for example, smelly feet guy: Each night he takes off his socks before bed, leaving them on the floor, where the odor wafts up to his partner.

“She’s disgusted. She wants them in the hamper,” Cunningham says. “He’s tired. He wants to go to sleep.”The real danger, he says, is that “neither of [them] see the ‘sock issue’ as jeopardizing the relationship.”

Yet this is when a couple first flirts with disaster: She complains; he dismisses her reaction as irrational and moody. The more he dismisses, the more she sees him as selfish and uncaring. Suddenly, stinky socks are grounds for couples therapy.

“You do a slow burn,” says Robin M. Kowalski, psychology professor at Clemson University. “Anger accumulates over time and you wonder, ‘Why doesn’t he have more respect for me?’” What may have seemed cure and innocent during the first few months of dating starts to grate, until a tic is interpreted as a personal affront—and battle lines are drawn.

The Rx for Allergens
While, as of yet, there’s no Social Al-Anon, many relationships do survive these annoyances, Cunningham says, because the perks of intimacy outweigh the quirks. Still, learning to cope requires effort and finesse.

When confronting an allergen, the first step, says Kowalski, is early, clear communication. A friendly intervention, if you will. “Tell the other person how you’ve been feeling, without being accusatory, and see how they respond.” Do it early enough, and your partner may be able to curb the behavior before a bonafide allergy evolves.

But the way you communicate your request for change is key, Cunningham adds. A sock in the hamper should be rewarded, yet a stray on the floor shouldn’t elicit an angry response. If you explode or act cold and aloof, he’ll have no incentive to change.And worse, that sock becomes a fight tripwire.

Women, especially, should take note: Kowalski found these habits may be bigger dealbreakers for us than for men because we’re more sensitive to social allergens...and more likely to be physically turned off by them, too.

But what if a mate just can’t change? You have two choices, says Cunningham: “Honey, you left the cap off again. I want a divorce.” Run screaming—or learn to accept.

“If you’re getting a lot of good things from the relationship, too, you can make the choice to accept the person, including that behavior, as part of your life.” And love is all-or-nothing: Chances are, when it’s for good, you’ll take the good—with the laugh.

Men vs. Women: A Battle of the Irks
He hates when you call him “honeypie [6]” in front of his friends. You go nuts when he scrapes his fork against his teeth. Why? According to Cunningham’s research, particular allergens cause the different genders to bristle more. Below, the his-and-hers divide:

Men can’t stand it when she:
• Morphs into a Primp-a-Donna who obsesses about having every hair in place.
• Croons to him in baby talk.
• Criticizes him constantly.
• Gives him the silent treatment.
• Issues ultimatums: “If we don’t visit my best friend in Florida, we’re never going on vacation again!”

Women cringe when he:
• Appears to lack ambition.
• Stares at or flirts with other women.
• Starts arguments over seemingly insignificant issues.
• Forgets important dates, like birthdays and anniversaries.
• Chooses “the guys” over her: “No, I can’t go see the play. We moved fantasy football to Thursdays.”
A bane to both sexes? Cursing, belching, and gas.

*Names have been changed to protect the innocent.