Even though more than half of couples now do it, compared with only 10 percent 30 years ago, living together before marriage still is linked to higher rates of troubled unions, divorce and separation, according to a new study.

A team of researchers from Pennsylvania State University compared data on 1,425 people married between 1964 and 1980, when cohabitation was less common and between 1981 and 1997, when cohabitation was more common. They found that, in both groups, cohabiters reported less happiness and more marital conflict than noncohabiters. Also, in both groups, couples who lived together before marriage were more likely to divorce.

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"It had been consistently shown in the past that, contrary to the popular belief, that living together will improve a person's ability to choose a marriage partner and stay married, the opposite is actually the case," said lead researcher Claire M. Kamp Dush, a doctoral candidate in human development and family studies.

The study's findings were published in the August issue of the Journal of Marriage and the Family.

Researchers said that although the reason behind the link between living together and resulting troubled unions is unknown, they said personal characteristics and the experience of cohabitation play important roles.

The study found that people choose riskier partners when cohabiting because they think living together will be easier to break up than marriage. But once a couple is living together, the fact that they share possessions, pets, and children and have invested time in their relationship may propel them to marry.