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Cliff Owen/AP Photo CIA Director David Petraeus resigned from post over extramarital affair. En español | 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 or

divorce ensues. 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 or divorce 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. SEX & CHEATING * The #1 reason people over 50 cheat (It's not what you think!) * Photos: 11 famous cheating scandals * Quiz: Relationship

myths * More from sex expert, Pepper Schwartz * More sex & intimacy articles JOIN OR RENEW WITH AARP TODAY — Receive access to exclusive information, benefits and discounts 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. Lack of communication and loss of trust are also issues that can seriously push a marriage toward divorce. I suspect that it

wasn’t so much an affair that sent Maria Shriver heading for the door, but more the fact that her husband had deceived her for so long. On top of that, she is dealing with public

humiliation — as well as the destabilizing presence of a child. It is a rare relationship, of any length, that could face these factors and continue on. Fortunately, the overwhelming

majority of marriages are not presented with such mega challenges. Still, plenty of breakups occur after a relationship of many years. Although some people are able to negotiate the

inevitable bumps in the road, for others those bumps turn into a sinkhole — something that they cannot seem to climb out of. Sadly, and often with great affection for each other, the couple

say "enough." And, yes, couples are saying that more often these days. Why? The answer is longevity. We live so much longer now. Half a century ago, an unhappy couple in their

mid-60s might have stayed together because they thought it wasn't worth divorcing if they had only a few years left to live. Now, 65-year-olds can easily envision at least 20 more

active years — and they don't want them to be loveless, or full of frustration or disappointment. And then, of course, we're now looking at the aging of the boomers. They're

different from the 50-year-olds who lived before them. In previous eras, couples soldiered on even if they were very unhappy. But boomers gave up on the concept of the dutiful-but-unhappy

spouse a long time ago. They were the originators of a higher divorce rate, and while that divorce rate has slowed, we may be seeing a spike as people ponder whether or not they will stay

with their spouses into extreme old age. So, yes, there are plenty of reasons why a couple who have been married for 30, 40, even 50 years might break up. And although we don't

celebrate divorce in this country, we are not afraid of it, either. This now extends to our golden years, as well. ------------------------- DISCUSS: WHY DO COUPLES DIVORCE AFTER 25 YEARS OR

MORE? Did you or someone you know divorce after a long-term marriage? What were the contributing factors and what advice would you give other couples struggling to hold it together? Leave a

comment below or discuss in Late-Life Divorce discussion in the AARP online community. -------------------------