Using an implicit task, researchers monitored how people automatically responded to words – in this case, whether they find it easier to link words referring to their partner to words with pleasant or unpleasant meanings - and this told them how likely couples were to split up.

Most research on relationship success has focused on how the people in the relationship feel about each other and  this is usually done by the obvious route - asking them.

But people may not really know how happy they are, says Ronald D. Rogge, of the University of Rochester, and "to make things worse, a lot of people don't want to tell you if they're starting to feel less happy in their relationship."

So he and his colleagues Soonhee Lee and Harry T. Reis turned to a technique often used to assess racism and bias, other feelings people have trouble admitting to themselves and to researchers.

The 222 volunteers in their study were all involved in a romantic relationship. Each volunteer supplied the partner's first name and two other words that related to the partner, like a pet name or a distinctive characteristic. Then they watched a monitor as three types of words were presented one at a time – good words (like peace, vacation, or sharing), bad words (such as death, tragedy, and criticizing), and partner-related words (names or traits).

There were two different kinds of tests: one where the volunteer was supposed to press the space bar whenever they saw either good words or partner-related words, and one where the combination was bad words and partner words. The idea is to get at people's automatic reactions to the words – if they have generally good associations with their partners, they should be able to do the first task more easily than the second.

The researchers found that volunteers who found it easy to associate their partner with bad things and difficult to associate the partner with good things were more likely to separate over the next year. The researchers also asked volunteers to report on the strength of their relationships at the start of the study – and found that the new test did a much better job of predicting breakup.

"It really is giving us a unique glimpse into how people were feeling about their partners – giving us information that they were unable or unwilling to report," says Rogge. The research is published in Psychological Science, a journal of the Association for Psychological Science.