The No-Argument Couple
By not fighting, you're not engaging each other, says Harville Hendrix, PhD, author of Getting the Love You Want and co-founder of Imago Relationship Therapy, and that may be due to a fear of intimacy. These relationships can last a long time while you function well as parents without any hint of problems, but you often become more buddies than lovers. "It's a category we call the parallel marriage," he says, which tends to turn flat and colorless. Some couples, however, maintain perfectly healthy relationships without quarreling, according to John Gottman, PhD, executive director of the Relationship Research Institute in Seattle and author of Why Marriages Succeed or Fail. In his years of research, he's observed various types of marriages: validating, in which partners pick their battles and fight fair; volatile, in which they fight all the time; and conflict avoiding—they rarely fight. All three are equally stable, Gottman has found, as long as it's working for both partners and there's a minimum of criticism, defensiveness, contempt, and stonewalling.

The Attached-at-the-Hip Couple
By all accounts, you get along famously. But "fused" pairs, Hendrix says, may harbor a fear of separation and can blend together with such strong dependence that they lack any kind of individual identity.

This is especially true when you're with each other to the exclusion of everyone else, says Peggy Papp, editor of Couples on the Fault Line and a therapist at the Ackerman Institute for the Family in New York. "One partner can end up feeling trapped, smothered by the relationship, and that they can't possibly express a need for independence without the other feeling totally betrayed," she says. "So they stay and then suddenly they can't tolerate it and they're gone." One warning sign of a split is a mate who seems newly distracted or "just not there anymore," she says.

The Two Mega-Paycheck Couple
Ultrabusy partners "need to schedule time together—set it aside, have it weekly—in a way that both are declaring that they hold their relationship precious and are giving it priority," Papp says: Love requires nurturing. In Gottman's studies, there was a group who ended up divorcing an average of 16 years after the wedding. "They were distinguished from couples who stayed married longer by not having had much 'purely positive affect,' by which we mean interest in one another, affection, humor, empathy, joy, adventure, pure fun together," he says. "They looked great to outsiders, who were usually shocked by their divorce. They just didn't enjoy their time together." Actually, according to Gottman's research, you should have at least five times as many positive moments together as negative if your relationship is to be stable. Translation: Just don't forget to have a good time.