An in-depth look at one of the biggest reasons relationships fall apart—and the latest advice from doctors on how make sure your relationship isn’t next.

The hot sexual encounters in a new relationship are indelible. Think back to the best sex you ever had with your current partner. Maybe you ripped her panties off, or you had sex in a stairwell, or she went down on you and you’ll never forget how it felt—or how she felt. There was still an element of the forbidden, and the unpredictability of what you’d do next was off the charts. It was like the movies, except better, because was happening to you.

And then … things wind down. The frequency of sex divides itself by two, and then divides itself in half again. Pretty soon, between your stressful jobs, social obligations, and fights about money/the dogs/your in-laws, you’re left with whatever sex you can get. And it isn’t much—your sex life is basically dead.

Sound familiar? It should. Because it happens to all of us. But why? And how do we break out of it?

♦◊♦

If there’s one thing you should know about why sex dwindles, it’s that it’s normal.

“You have so much sex in the early phases of a relationship that it will inevitably decline,” says Pamela Regan, Ph.D., professor of psychology at California State University–Los Angeles. You really can’t maintain the five to seven to 10 times a week that sometimes characterizes newlyweds.”

For comparison’s sake, check out these stats: Studies have shown that most couples have sex one to two times a week, and as you age, the number will likely decrease. Relationships that are considered “non-sexual” are couples that have sex fewer than 10 times a year.

A man who wanted to be identified only as “Lorne” told me he and his wife of 12 years sometimes go for a year at a time without having sex.

“It’s not by choice, but I can say it is partly late work hours and not working out—I ballooned up to 220 from 193 pounds, so I’m tired,” he said. “She and I have grown apart because we don’t see eye to eye on much. I haven’t talked to anyone about it, as we just keep up appearances.”

Don’t like ads? Become a supporter and enjoy The Good Men Project ad free

One out of five married couples has a non-sexual relationship.

On top of Lorne’s own physical setbacks, his wife has suffered from eating disorders, which have taken a toll on her physical appearance as well as her mind-set.

“Is it because she has a poor body image or is just controlling and more focused on appearances? Perhaps. Am I fed up, and staying at work late to avoid the situation? Probably. Is this going to end well? Probably not.”

♦◊♦

Lorne’s situation isn’t that unusual, says Barry McCarthy, Ph.D., professor of psychology at American University.

“One out of five married couples has a non-sexual relationship,” he says. “It’s one of the major causes of divorce. Sex is really paradoxical. When people are fighting about sex—but especially when they’re avoiding sex—it plays an inordinately powerful role, and it really drains a relationship and a marriage.”

For Peter Palmer, 52, of New York City, problems with his sex life crept up almost immediately after they were married. They waited to have sex until they were hitched, and soon after, they discovered they weren’t sexually compatible. He was experienced, and she wasn’t—she found sex “messy” and felt she was bad in bed, so she gave up. Now, even though Palmer thinks she is “beautiful,” they almost never have sex.

“I’m an artist and seek novelty. She’s a manager and wants to know when it’ll start, stop, and what the ROI will be,” Palmer says. “I wish I could interest her. [An] affair is looking pretty good, but there’s no going back once the deed is done. The sanctity will have been compromised. I want her, not a supplement.”

And it’s not for a lack of effort. Palmer says he has tried everything, from having sit-down discussions to crying with her, but nothing changes.

“The rest of my life is perfect,” Palmer says. “Just that. Only that. Always that.”

The hard truth is, almost no one is truly having the amount of sex they’d prefer. An Australian study published this month says that 54 percent of men and 42 percent of women in long-term relationships are unhappy with the frequency of sex they have. And an equally recent British study found that romance—and, by extension, sex—tends to get stale about 36 months, or three years, into a relationship.

If date night seems corny to you, you might try this: become interesting to your partner by doing something new on your own (like joining a boxing gym or taking mixology classes) and then coming back and talking about it with her later.

“This is where all those adorable quirks turn out to be annoying habits,” says Judi James, author of the study. “And if we’re stressed at work, our irritability levels will be heightened, meaning we’re far more likely to argue over those irritations and see them as a deal-breaker.”

If you’ve ever broken up with a girlfriend after about three years, that should sound familiar. But if you’ve stuck things out, chances are the romance has not.

The physical reasons for why the decline in frequency of sex happens—even in couples who have been together far longer than three years—are myriad. Side effects from antidepressants or heart medication are a major inhibitor of sexual function, says McCarthy. Sometimes women have a negative reaction to the hormones in birth control pills. Sleep disorder is another big one—anything that makes you feel bad physically is going to impact how you feel sexually. That goes for body-image issues like weight gain as well.