THE sudden breakup of Al and Tipper Gore’s seemingly idyllic marriage was the latest and among the sharpest reminders that the only two people who know what’s going on in a marriage are the two people who are in it.

The truth is that most marriages, even our own, are something of a mystery to outsiders.

Several years ago, a marriage researcher  Robert W. Levenson, director of the psychophysiology laboratory at the University of California, Berkeley  and his colleagues produced a video of 10 couples talking and bickering. Dr. Levenson knew at the time that five of the couples had been in troubled relationships and eventually divorced. He showed the video to 200 people, including pastors, marriage therapists and relationship scientists, asking them to spot the doomed marriages. They guessed wrong half the time.

“People on the outside aren’t very good at telling how marriages are really working,” he said.

Even so, academic researchers have become increasingly fascinated with the inner workings of long-married couples, subjecting them to a battery of laboratory tests and even brain scans to unravel the mystery of lasting love.

Bianca Acevedo, a postdoctoral researcher at the University of California, Santa Barbara, studies the neuroscience of relationships and began a search for long-married couples who were still madly in love. Through a phone survey, she collected data on 274 men and women in committed relationships, and used relationship scales to measure marital happiness and passionate love.