The split of Al and Tipper Gore isn't as surprising as you might think, relationship experts say. (AP)

Given the outwardly sunny appearance of the 40-year marriage of former Vice President Al Gore and his wife, Tipper, the announcement of their separation Tuesday seemed shocking.

But a Delray Beach psychologist, author and relationship expert says that while it’s impossible to know the insides of a private relationship, partners in long-time marriages do indeed grow apart, as friends of the Gores have noted was the case with them.

And in a very public marriage like the Gores’, in which the public stature of one of the partners continues to climb, separations and divorce can be all the more common.

“Some people get divorced because one partner changes and the other doesn’t, or because they both do, but not in the same way,” says David Eigen, author of Men: The Gods of Love and Women: The Goddesses of Wisdom, and a frequent guest on local and national broadcasts about celebrity relationships.

“In a political marriage, the idea is that you stay until it hits the fan, so to speak. But with Tipper and Al, it’s a little different — you get the sense that he went from being vice president to the presidential candidate to movie star and now this popular environmental hero who is larger than life to everyone, and maybe to himself,” Eigen says.

Statistics show that few people still in a first marriage at the Gores’ ages get divorced. Of course, there are many reasons that people stay in unhappy marriages for years, like children or fear of the unknown, just as there are many reasons one suddenly ends such a relationship.

Eigen says that decision in men is often referred to as a “midlife crisis” or what he calls a spiritual awakening. “Al Gore is past the standard time for a midlife crisis, but he might be waking up to his own self, and to a god-like ego,” Eigen said.

The Gores’ friends have said that marital infidelity is not a factor in the couple’s separation, and cited the increasing time they have spent apart in recent years.

Eigen says he wouldn’t be surprised that the attention that Al Gore has gotten in recent years as a leader in the environmental movement may have triggered “adulation from other people. He may not have been getting that same adulation from her, and (she) got left out of that and had enough.”