Marriage may be overrated, according to researchers who compared the pros and cons of wedlock and cohabitation.
The idea that marriage has health and happiness advantages over living together is largely a myth, it is claimed.
Even the "honeymoon period" of well-being that sets married and cohabiting couples apart from singletons is short-lived, say scientists.
They also suggest that time and money devoted to promoting marriage might be better spent elsewhere.
Previous research has linked marriage to happiness and health, arguing that couples who wed tend to live longer, more contented lives. But these studies largely focused on comparisons with being single, or relied on "snapshots" of how people fared at specific points in time.
For the new research, US experts analysed data from the National Survey of Families and Households that followed the long-term progress of people's relationships. The sample included 2,737 single men and women, 896 of whom married or moved in with a partner over the course of six years.
Lead author Dr Kelly Musick, from the College of Human Ecology at Cornell University in New York, said: "Marriage has long been an important social institution, but in recent decades western societies have experienced increases in cohabitation, before or instead of marriage, and increases in children born outside of marriage.
"These changes have blurred the boundaries of marriage, leading to questions about what difference marriage makes in comparison to alternatives."
The findings, published in the Journal of Marriage and Family, showed that both married and cohabiting couples reduced their contact with parents and friends - and this appeared to be a lasting effect.
On the other hand, entering into any kind of committed union improved psychological well-being. The scientists concluded: "We are certainly not saying that marriage is irrelevant for individual well-being. What we have found is simply that, once individual differences are taken into account, it is far from a blanket prescription for individual well-being."