Undoubtedly, the most stressful time for any couple is parenthood. Carolyn and Philip Cowan, psychologists with the University of California, Berkeley, found that 92 percent of new parents report more conflict and lower satisfaction. Pennsylvania State psychologist Jay Belsky, who has just completed a seven-year study of 250 sets of new parents, finds that only 19 percent say their marriages improved after the birth of a child. Couples usually look forward to the birth of a baby as a time of closeness, but Belsky found that nearly all new parents grew more polarized and self-centered in response to the fatigue and strain.

Difficult transitions like parenthood are also the times when spouses are most vulnerable to an extramarital affair, find psychologists Tom Wright and Shirley Glass. But more often than not, Glass and Wright find, having an affair says more about the individual than the marriage. Spouses with loving marriages but with an excessive need for admiration or thrills are notorious for extramarital dalliances. But even for more regular folks, taking on new roles makes one ripe for philandering. "Even given a rich, happy marriage, it's often easier to form a new image in the eyes of someone new," says Glass. "Trying to change your identity inside a marriage is akin to the new CEO of a major company visiting his parents, only to find they still see him as the baby of the family."

An affair is arguably the most shocking blow to a marriage. Yet study after study finds that wayward spouses are quite happy with their love life at home, both the quantity and quality as happy, in fact, as their faithful counterparts. Psychologists are divided about the ramifications of an affair. "I liken an affair to the shattering of a Waterford crystal vase," says Gootman. "You can glue it back together, but it will never sing again." But Glass and Wright, currently studying couples recovering from affairs, find that not only do two thirds decide to stay together, but many report a newfound richness and closeness gained through conquering the ordeal together.

Perhaps the best ideas about what keeps a marriage alive through thick and thin come from couples who, after decades of marriage, bask in blissful unions. Berkeley psychologist Robert Levenson is now in the process of studying pairs who have been together 40 years or more. So far, reports from the front indicate that these couples are masters in soothing one another and preventing each other's distress during conflict. These enduring couples also display a distinctly mellowed approach to marital differences, with far less conflict and far more pleasure than younger couples. And as a couple ages, gender differences appear to fade away, replaced by a more unified view of marriage and life. A nice ending to a bumpy ride.