This is the new finding of Scott Stanley and his colleagues. He has been doing follow-up research on the early '90s finding that married couples who cohabited first are more likely to divorce than marrieds who waited to live together. The nuance that the first study found was that people who were engaged when they cohabited end up like marrieds who waited to live together. It is the other kind, the ones who drifted into living together, who are more likely to drift in to marriage and drift out again.
The new finding in Stanley's latest study is how a big a proportion of cohabiters slide into living together without a future plan of marriage. This means that almost half of all couples cohabit without a definite marriage plan. This is a sizable group putting itself at risk willy-nilly.