Researchers have provided proof of how cohabitation contributes to the number of long-term relationships lasting eight years or longer.
Audrey Light, co-author of the study and professor of economics at The Ohio State University, said that when you take this longer view, it turns out that cohabitation plays "a major role" in the total number of couples whose relationships last eight or more years.
She said that there are so many couples that start out cohabiting, and enough of their relationships last that they end up making a significant contribution to the total number of long-term relationships.
Light noted that couples who start out cohabiting usually do end up marrying at some point if their relationship lasts eight or more years.
Light conducted the study with Yoshiaki Omori of Yokohama National University in Japan.
The study found that a representative 18-year-old woman with no prior unions (defined as a marriage or cohabiting relationship) has a 16 percent chance of marrying by age 22 and remaining with her husband for at least 12 years.
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But what happens if you also include the possibility of cohabitation? In that case, the same woman has a 22 percent chance of marrying or cohabiting by age 22 and remaining with the same partner for at least 12 years.
The researchers used data from 2,761 women born 1960 to 1964 who participated in the 1979 National Longitudinal Survey of Youth.
The results have been published in the journal Population Research and Policy Review.