NEW YORK — There was no salacious sex scandal, no prostitution ring, no mysterious trip to Argentina.

Al and Tipper Gore are a famous political couple, but their split after 40 years of marriage apparently stemmed from a more mundane cause, according to friends: They simply grew apart.

And in that, experts say, they're no different from many Americans. Such late-marriage splits are much more common than we think.

"We tend to mistakenly believe that once people reach a certain point in marriage, they just stop splitting up," says Betsey Stevenson, an economist at the University of Pennsylvania's Wharton School who studies family trends. "But that's simply not true."

In fact, Stevenson says, though marriages are more likely to fail in the first 10 years, once you get past that, "the percentage of those divorcing each year is very similar throughout the years of marriage."

Of course, there's no getting around the shock factor surrounding the separation announcement by the Gores, which came in an e-mail Tuesday to friends. Unlike many political couples, they'd spoken openly of their feelings for each other and seemed to share an easy affection, not to mention four children and three grandchildren.

That affection was apparent even without The Kiss — the go-for-broke liplock between the vice president and his wife at the 2000 Democratic convention that made so many blush, and is probably doing so again, on YouTube. Was it impulsive or calculated? Either way, it was still quite a kiss.

But now the Gores are parting, and many are asking not only "Why THEM?" but "Why NOW?"

Talk to relationship experts, though, and they point to a host of possible reasons. Perhaps the most obvious: After 40 years, children have been launched and are well into adulthood. Before that, parenting can be so all-consuming that it's a permanent distraction from one's own marriage.

The Gores also went through a painful ordeal when their then 6-year-old son, Albert, was nearly killed in a car accident.

"Even with older children, the demands are quite intense," says Elana Katz, a divorce mediator at New York's Ackerman Institute for the Family. "That can be distracting, or it can create a strong bond. But when that chapter is done, people face each other across the kitchen table and say, 'Can this be the relationship that's my primary source of enjoyment as we go forward?' "

Also, says Katz, older people now have expectations for their relationships that previous generations may not have.

It's significant that Al and Tipper Gore married in 1970, notes Stephanie Coontz, author of "Marriage, A History," and a professor of family studies at Evergreen State College in Olympia, Wash. Marriages in the late '60s and '70s are marked by higher divorce rates than those of later years, she says, partly because people still married very young.

Tipper Gore was 21 and Al Gore 22 when they wed. They're now 61 and 62, with decades of health likely ahead of them. "The idea used to be that by our 60s, life was pretty much over anyway," says Coontz. "But today, people who reach 65 are likely to have another 20 years ahead. So it makes the calculus of living in an unhappy marriage even harder to take."

Plus, with longer life expectancies, there are more potential new partners out there — "what we call a thicker remarriage market," Coontz says.

After losing the 2000 presidential race, Al Gore carved out a post-politics career that has taken him around the globe. His campaign to draw attention to climate change led to a 2007 Nobel Peace Prize and an Oscar for the documentary "An Inconvenient Truth." During those years, Tipper Gore is said to have focused on her photography.

Despite the apparent divergence in the Gores' interests and lifestyles, some might ask whether a marriage that lasted 40 years can be called a failure.

Stevenson, of the Wharton School, is one of them. "People see this as sad, but I don't see how we can look at a 40-year marriage and say it's a failure," she says. "It's really tough to make it to 70 years! The Gores obviously had a lot of successes."

Whatever made the Gores drift apart, the silver lining may be that at this stage in life, splits can often be much more amicable.

Which is a good thing. Because, as Katz says, "They're going to be showing up at the same events for years."