Key Points:

Couples who live together won't necessarily stay together, according to new research.

Ruth Weston and Lixia Qu, research fellows at the Australian Institute of Family Studies, conducted a study into the percentage of cohabiting couples who end up marrying one another.

They found that marriage rates had fallen dramatically since the beginning of last century and those couples who did tie the knot were doing so at a later age.

Weston told the Sydney Morning Herald that many young people thought moving in together was "a fun thing to do".

But she said cohabiting couples often lingered for years in unsuitable relationships and had trouble finding new partners when they eventually did split.

"In the old days people might go 'steady' but there was still opportunity to meet others," she said.

"Now once you are living with someone you are cheating if you see someone else. When you cohabit it adds a sense of commitment to a relationship that might be going nowhere."

Weston and Qu's research found that 63 per cent of couples who began living together in the early 1970s ended up marrying, but only 43 per cent of couples who began living together since the 1970s ended up married.

Of those who first moved in together in the early 1990s, there was only a slight difference between the number who were married within five years (43 per cent) and the number who had separated during the same time period (38 per cent).

And while marriage rates have been declining since the 1970s, the rate of cohabitation has risen for all age groups, the research, published in Family Relationships Quarterly shows.

Rates of separation have also increased over the same period, indicating that although more people may be living together, the relationships are not necessarily stable.

Weston told the Sydney Morning Herald that cohabiting couples in the 1980s had tended to treat the relationship as a trial marriage and usually went on to marry each other quickly. "Some might have separated later," she said.

Now, many people enter relationships before they are committed, and without having discussed their future.

"They enter prematurely but can linger on and waste their time," she said.