Americans may seem filled with cynicism and nervousness when it comes to marriage, but underneath it all, they wish they could get it right.

So says author Judith Wallerstein. After 25 years of studying the fallout from divorce, the Marin-based clinical psychologist has turned her mind and pen to unions that are successful. Her new book, "The Good Marriage, How and Why Love Lasts," has been enormously popular since its release in June. It is now a bestseller in the Bay Area.

According to Wallerstein, widespread interest in the book — feature stories have appeared in publications from The New York Times to USA Today — reflects a growing interest in coupling.

"People are profoundly interested in marriage. And the importance of marriage has risen, not gotten less. People are lonely. They don't think living alone does it. Dating at 40 gets old quickly, and divorce looks not so good," says Wallerstein, the daughter of Jewish community professionals, who was raised in New York and Israel.

In an interview from her home in Belvedere, the author's high, dignified voice often breaks into laughter. She comes across like a sage grandmother — with wisdom and experience gained from a 48-year marriage, and a Ph.D.

The more stressful modern life becomes, she says, the more central marriage becomes. Along with the "commute, the workplace, the dating scene" comes a pervasive loneliness. A good marriage can be an antidote, she suggests.

So the question becomes, what is a good marriage?

Why do some marriages flourish while others crash and burn? Wallerstein has been considered one of the world's experts on the crash-and-burn kind since co-authoring the landmark book "Second Chances: A Decade After Divorce," with reporter Sandra Blakeslee, with whom she also collaborated on "Good Marriage."

Five years ago, however, she decided to "change the level of discourse from what do we do about divorce, to how we can build a happy marriage. What do we know?"

She posed the question to 50 happily married Northern California couples, predominately white, middle class and well-educated. Though 40 percent of her subjects were Jewish, Wallerstein says "Jewish divorce is no different, Jewish marriage is no different."

She does, however, use a Jewish symbol — the chuppah — to explain the first of nine tasks she found to be crucial in creating a strong marriage: separating from the family of origin, changing the focus from the parents to the new spouse.

The change, she writes, "is symbolized by the chupa, a canopy held aloft on four poles by family members. Under it the bride and groom stand symbolically beneath their own roof. The first task of marriage is to provide walls for that chupa with gateways over which the young couple exercise reasonable control."

Other tasks for couples include coping with crises, sharing laughter and keeping interests alive, exploring sexual love and intimacy and making a safe place for conflict. Among the 50 couples, these tasks were commonalties. Wallerstein says they are crucial building blocks for a loving union.

Among the couples, she was also able to pinpoint four types of successful marriages: the companionate marriage, based on friendship and equality; the rescue marriage, which serves to heal traumatic childhood experiences; the traditional marriage, in which the woman takes charge of home and family; and the romantic marriage, which revolves around sexual passion.

In her research, she also found the conventional wisdom about marriage stands up under scrutiny: a shared sense of humor is the most important thing. Not only does humor create a low-key, spontaneous banter that keeps couples emotionally connected, it also serves a more serious purpose: defusing anger. As Wallerstein writes, a joke can be the best way to extinguish marital brush fires.

"There is no better tool for putting the little annoyances of life in their place."