By Jonathan Zimmerman

Here's an old cliché, tired but true: You never know what goes on inside somebody else's marriage.

But in the case of Al and Tipper Gore, why would you want to know? That's the real question raised by the Gores' separation, which they announced last week after 40 years of marriage.

The sad answer lies in the erosion of privacy throughout American politics. Today we think we have a God-given right to know every detail about our leaders' intimate lives. And that impoverishes our democracy.

Consider the expressions of shock and surprise that reverberated across the newspapers and the Internet following the Gores' announcement. If, say, Bill and Hillary Clinton were splitting up, we'd understand. But Al and Tipper? The romance-novel couple, straight out of Love Story? Say it ain't so!

We felt confused and crestfallen, deceived and disappointed. And then, like jilted lovers, we started to speculate.

Was Al still depressed about his controversial loss to George W. Bush in 2000? Was he too consumed by a fight against global warming that eventually cooled his marriage? Did one of the Gores take a "walk on the Appalachian Trail"?

That new euphemism for marital infidelity came from South Carolina Gov. Mark Sanford, whose staff told reporters he was out hiking when he was really out with his Argentine mistress. Ironically, the race to replace Sanford has generated its own sex scandal. State Rep. Nikki Haley was leading the pack for the Republican nomination, with Sarah Palin's endorsement, when a former employee alleged that he and Haley had an adulterous affair.

During a gubernatorial debate last week, reporters asked Haley if issues in her "personal life" would diminish her effectiveness in office. Haley said no, of course, but everyone knew the right answer. The more we pry into our leaders' private lives, the less able they are to serve and govern the public.

The best way to see that is to look backward. Imagine, for example, if Franklin D. Roosevelt had been subjected to a similar scrutiny of his intimate life. It's hard to imagine that he could have become president, let alone led us through the Great Depression and World War II.

That's because his personal life was, to put it mildly, a mess. Following the birth of her last child, in 1916, Eleanor Roosevelt decided she would no longer have sex with Franklin. Two years later, she found a cache of love letters from his mistress, Lucy Mercer.

After that, one of the Roosevelts' sons recalled, his parents "agreed to go on for the sake of appearances, the children, and the future, but as business partners, not as husband and wife." Journalists knew this, too, but they kept it quiet.

And lucky for us. If the couple's private peccadilloes had been exposed for all to see, the Roosevelts couldn't have made their enormous contributions to American public life.

Or consider John F. Kennedy, whose personal indiscretions went well beyond his dalliance with Marilyn Monroe. Kennedy also had sexual relationships with two staffers, his wife's press secretary, and a 19-year-old White House intern. But the public didn't find out, because the press didn't report it.

Bill Clinton had his own fling with a young intern. But this time the whole world knew, Clinton was impeached, and his entire presidency was stalled. We'll never know what he could have accomplished if we hadn't pried into his private life.

Here you might reply that Clinton's intimate behavior revealed his "character," which in turn reflected on his capacity to govern. The argument is that the best human beings make the best leaders. So we need to know every personal detail about them to decide if they can or should lead us.

That's hogwash. And it's pernicious hogwash to boot.

Did the bedroom betrayals of FDR and JFK illustrate deficits in their character? Definitely. But did these private antics render them unfit for public office? Hardly. They were inadequate spouses and inspired leaders. You can be both.

That brings us back to Al and Tipper Gore, both dedicated public servants. Their marriage seems to be over, and we don't know why. And we shouldn't want to know, either.

Let's back off the Gores and everybody else, letting our public leaders have their private lives. We'll all be better for it.

Jonathan Zimmerman teaches history at New York University and lives in Narberth. He is the author, most recently, of "Small Wonder: The Little Red Schoolhouse in History and Memory" (Yale University Press). He can be reached at jlzimm@aol.com.