Married couples - straight and gay - more likely to stay together, US researcher finds.

Relationships that don't work out are bizarre things, miniature lives that burn out like stars. We all have our regrets - the one(s) that got away, the one(s) that never should have been.

But how often do things fizzle out? How frequently do two people go their separate ways? And how do the chances of breaking up change over time?

These are some of the questions sociologist Michael Rosenfeld, of California's Stanford University, has been asking as part of a longitudinal study he started in 2009.

"We know a lot more about the relationships that worked out than the ones that didn't," said Rosenfeld.

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"The way the Census and other surveys tend to collect data just doesn't produce a very good picture. People also don't recall failed relationships too well."

Rosenfeld, who has been tracking more than 3000 people, is helping to fix that. And the answers he has mustered so far are pretty revealing.

There are obvious patterns, of course. Marriage, for instance, is a strong binder. Both straight and gay married couples are far less likely to separate than non-married ones.

For same-sex married couples, the break-up rate falls from roughly 8 per cent for those who have been together for five years to under 1 per cent for those who have been together for at least 20 years. For heterosexual married couples, the rate falls from a shade over 3 per cent to less than 1 per cent over the same period. (If you're wondering why the break-up rate is so low, given divorce rates, these are cumulative - the percentages compound over the years, creating an overall probability that is higher).

Unmarried couples on the other hand, both straight and gay, have much higher break-up rates - even when they have been together for more than 20 years.

It's not that surprising. Marriages, after all, are a necessarily more binding agreement. Annulling a marriage involves far more hurdles.

Where things get interesting is when one zeroes in on Rosenfeld's data for non-married couples, which offers a rare window into the trajectory of modern relationships.

Broadly, it seems that time really helps reduce the likelihood that two people go their separate ways.

Sixty per cent of the unmarried couples who had been together for less than two months during the first wave of Rosenfeld's study were no longer together when he checked up again the following year. But once a relationship lasts a year, the likelihood that it ends begins to drop precipitously. Over the first five years, the rate falls by roughly 10 percentage points, reaching about 20 per cent for both straight and gay couples. And the rate continues to fall until about 15 years in, when it levels off for both - at just over 10 per cent for gay couples and roughly 5 per cent for straight couples.

Why? Well, as Rosenfeld notes in a 2014 study, "the longer a couple stays together, the more hurdles they cross together, the more time and effort they have jointly invested into the relationship, and the more bound together they are".