Melbourne: Ruth Weston, an Australian family researcher at the Australian Institute of Family studies, said that young people think the living together before marriage is "a fun thing to do", and believe in taking "each day as it comes".

However most of these live-in relationships result in unsuitable couples hanging onto to one another.

As a result when they eventually break off, it might take them a long time to find a new partner. And, as far as women are concerned, it might also mean that they may have lost their chance of having children.

The researchers analysed 2006 census data, which revealed that 35 per cent of women, and 41 percent of men aged 30 to 34, are single.

Among women aged 35-39, it is 31 per cent and for men 35 per cent.

"This is a lot of people in their mid- and late 30s without a partner, although some would have once had a partner," Weston said.

Analysis wherein it was found that marriage rates have been falling for decades while the cohabitation rate has risen for all age groups is published in Family Relationships Quarterly.

Weston said that since the last census, the divorce rate appears to have reached a plateau, or even fallen and living together has become more unstable. More people were splitting before they married.

She also said that the fragility of live-in relationships had contributed to a 15-year decline in partnering rates - the proportion of men and women in there 30s who had neither a spouse nor live-in partner. The fall in partnering rates was precipitous between 1996 and 2001 and had since slowed down but not reversed.

"The rate is still high in relation to what we know young people want," Weston said.

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

Another researcher, Professor Janeen Baxter, of the University of Queensland, has shown that cohabitors may have at least one advantage over married couples - men do more housework than women.