Couples who live together before they get married are less likely to stay married, a new study has found. But their chances improve if they were already engaged when they began living together.

The likelihood that a marriage would last for a decade or more decreased by 6 percentage points if the couple had cohabited first, the study found.

The study of men and women ages 15 to 44 was done by the National Center for Health Statistics using data from the National Survey of Family Growth conducted in 2002. The authors define cohabitation as people who live with a sexual partner of the opposite sex.

“;From the perspective of many young adults, marrying without living together first seems quite foolish,”; said Pamela J. Smock, a research professor at the Population Studies Center at the University of Michigan, Ann Arbor. “;Just because some academic studies have shown that living together may increase the chance of divorce somewhat, young adults themselves don't believe that.”;

The authors found that the proportion of women in their late 30s who had ever cohabited had doubled in 15 years, to 61 percent.

Half of couples who cohabit marry within three years, the study found. If both partners are college graduates, the chances improve that they will marry and that their marriage will last at least 10 years.

“;The figures suggest to me that cohabitation is still a pathway to marriage for many college graduates, while it may be an end in itself for many less educated women,”; said Kelly A. Musick, a professor of policy analysis and management at Cornell.

Couples who marry after age 26 or have a baby eight months or more after marrying are also more likely to stay married for more than a decade.

“;Cohabitation is increasingly becoming the first co-residential union formed among young adults,”; the study said. “;As a result of the growing prevalence of cohabitation, the number of children born to unmarried cohabiting parents has also increased.”;

By the beginning of the last decade, a majority of births to unmarried women were to mothers who were living with the child's father. Just two decades earlier, only a third of those births were to cohabiting couples.

The study found that, overall, 62 percent of women ages 25 to 44 were married and 8 percent were cohabiting. Among men, the comparable figures were 59 percent and 10 percent.

In general, one in five marriages will dissolve within five years. One in three will last less than 10 years. Those figures varied by race, ethnicity and sex. The likelihood of black men and women remaining married for 10 years or more was 50 percent. The probability for Hispanic men was the highest, 75 percent. Among women, the odds are 50-50 that their marriage will last less than 20 years.

The survey found that about 28 percent of men and women had cohabitated before their first marriage and that about 7 percent lived together and never married. About 23 percent of women and 18 percent of men married without having lived together.

Women who were not living with both of their biological or adoptive parents at 14 were less likely to be married and more likely to be cohabiting than those who grew up with both parents.

The share who had ever married varied markedly by race and ethnicity: 63 percent of white women, 39 percent of black women and 58 percent of Hispanic women. Among men in that age group, the differences were less striking. Fifty-three percent of white men, 42 percent of black men and 50 percent of Hispanic men were married or had been previously married at the time of the survey.

By their early 40s, most white and Hispanic men and women were married, but only 44 percent of black women were.