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. 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 (forgoing deodorant, peeing with the door open); intrusive (peeking in his inbox, criticizing her hair or clothes); egocentric (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
So if allergens can spell the end, how do they begin?