"All marriages are happy," Canadian playwright Raymond Hull once said. "It's the living together afterward that causes all the trouble."
In recent decades, more people have tried to avoid wedded trouble by living together before tying the knot. People say they want to see if things work out before committing to a ring.
But a new government study suggests that cohabitation can actually hurt the chances the chances of having a long marriage.
Among women between the age of 15 to 44 who lived with their first husband before getting married, the study found their marriages had a 61 percent chance of surviving 10 years. Women who had not cohabited with their first husband before marriage had a 66 percent chance that their marriage would last 10 years.
The Centers for Disease Control and Prevention study was based on a 2002 survey of men and women for the National Survey of Family Growth.
New Yorkers Gladys and Otto Mond have had a successful marriage by any measure. They've been married nearly 51 years, and have four children and seven grandchildren.
For them, the secret to a great marriage comes down to simple things like trust, mutual respect and compromise. Those are things they say you can't learn just by living with another person. You've got to be married.
"You might know the other person, but you know them in a relationship. You don't know them in a marriage relationship," Otto said.
Younger couples often see it differently. Ashley Moore and Mike Batistick live together in New York City and are now engaged to to be married.
"I think there is the fear that you might move in too soon and rush into the marriage," Moore said. "We'd been dating for two years, and I actually made the conscious choice to move in."
To the couple, it was important that moving in with each other meant commitment, that they were "not just moving in to save a grand a month," Moore said.
The sense of purpose is important according to the study, which concluded it made a big difference whether couples are engaged when they move in together. Women who were engaged before cohabitating had a 10 percent better chance of having a 10-year marriage, and engaged men had 18 percent better odds than those who were not committed before moving in. Engaged men who lived with their fiances actually had a slightly higher chance of a 10-year marriage than men who had never cohabitated before getting married.
New Yorkers Shirley and Errol Harry have been married for 30 years, and they said the key to a successful marriage can't be discerned in numbers.
"With me, I made a commitment, and I take my vows very seriously," Shirley Harry said.
"I think trust, communication, honesty, values, there's no formula, but there's a lot of moral criteria that needs to be implemented," said her husband.
The Harrys don't think it's a good idea to move in before getting married. It's better to "make a commitment and make a go of it."
More than half of couples who live together marry within five years, the study found, and a greater percentage of college-educated people use cohabitation as a stepping stone to marriage than people with less education.
But ultimately, said Shirley Harry, a successful marriage depends on one thing.
"Love," she said. "I don't know what my life would be without him. I mean, I truly love Errol."
ABC's Sharyn Alfonsi and Ursula Fahy contributed to this report.