By now, it's an old story: one-half of a high-profile and long-married couple — usually the man, truth be told — admits to having an affair. Sometimes, the couple's marriage can withstand the infidelity; other times, the breach of trust is too deep, and a split ensues.

CIA Director David Petraeus resigned from post over extramarital affair. — Cliff Owen/AP Photo

David and Holly Petraeus don't fit the mold, say, of Arnold Schwarzenegger and Maria Shriver, since Holly Petraeus has not been nearly as prominent as her military-hero-turned-CIA-chief husband of 38 years. And we don't know, yet, whether their marriage will survive.

But what we do know is that while questions of infidelity grab the most headlines, having an extramarital affair is not what's behind the breakup of most long-term relationships.

The AARP Sex, Romance and Relationships Survey on the sexuality of people 45 and older found that extramarital affairs happen for only a relatively small number of couples. So while infidelity is certainly the precipitating factor in some marriages failing, it's not the reason in most cases.

Why do so many long-married couples decide to split? How can people be so happy for so long, only to then have the marriage turn sour in what are supposed to be their "golden years" together?

In most cases, the reasons are far less dramatic. Some relationships have been in decline for decades and finally lose all their juice. A marriage doesn't usually just blow up. It's more like a balloon that has been seeping air for a long time. After a while, it's totally deflated.

Another possibility is that a couple's issues intensify. Most problems are manageable, but then something sends them into hyperdrive. It could be a change in jobs, health, children's lives, personal ambitions or any number of other triggers. Whatever balance had been achieved is undermined, and with it the ability to handle the issue and still have a decent marriage.

Of course, we've all heard the familiar phrase, "We grew apart." But just because it's a cliché doesn't mean it's not a common cause of divorce or separation among long-time married couples. A typical scenario is where a husband and wife live increasingly different lives: He gets more and more into his work, she gets more and more into her children, her adult children, her grandchildren. Or she gets ambitious and he wants to relax, cut down, travel, and play golf.

Next page: The reason why couples have less staying power these days. »