More than a century ago, Russian novelist Leo Tolstoy wrote, “Happy families are all alike; every unhappy family is unhappy in its own way.”

The words have become immortalized, and the unhappy story of “Anna Karenina” is considered one of the greatest novels ever written. Recently, however, psychologists and sociologists are starting to question the observation.

“I think Tolstoy was totally wrong,” said John Gottman, a professor of psychology at the University of Washington in Seattle. “Unhappy families are really similar to one another—there’s much more variability among happy families.”

As couples clink wine glasses over candlelit Valentine’s Day dinners this week and exchange vows of undying love, Gottman and others are trying to understand why as many as one in two marriages end in divorce, and why so many couples seem to fall out of love and break apart.

Some of the most revealing answers, it turns out, come from the couples who stay together. While conventional wisdom holds that conflicts in a relationship slowly erode the bonds that hold partners together, couples who are happy in the long term turn out to have plenty of conflicts, too. Fights and disagreements are apparently intrinsic to all relationships—couples who stay together over the long haul are those who don’t let the fighting contaminate the other parts of the relationship, experts say.

“Why do people get married in the first place?” asked Thomas Bradbury, a professor of psychology at the University of California at Los Angeles. “To have someone to listen to—to have a friend, to share life’s ups and downs. We want to try to draw attention to what's valuable in their relationship.”

Researchers are finding that it is those other parts of relationships—the positive factors—that are potent predictors of whether couples feel committed to relationships, and whether they weather storms and stick together. As long as those factors are intact, conflicts don’t drive people apart.

“What we’ve discovered is surprising and contrary to what most people think,” said Gottman, the author of “The Mathematics of Marriage.” “Most books say it’s important for couples to fight fair — but 69 percent of all marital conflicts never get resolved because they are about personality differences between couples. What's critical is not whether they resolve conflicts but whether they can cope with them.”