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…