Picture this. It’s a Saturday night, and you’re queueing to get into the hottest new bar in town. Inside, you’ve heard it’s fantastic: amazing music, gorgeous décor, cheap drinks. For this, you’re prepared to pay a £15 entry fee and stand in line for 20, 30, 45 minutes – even though it’s raining outside, and a woman just walked past you saying, “It’s not that great in there, you know.”

More and more time passes, but the longer you wait in line, the more determined you become to stick it out. You will not be defeated. Besides, it would just be a waste of time if you left the queue now.

But once you finally make it inside, you’re bitterly disappointed. It’s unbearably crowded, the music is terrible – and the drinks aren’t even that cheap.

Do you:

A) Stick it out – you waited so long to get in, you may as well try to make the best of it?

or

B) Head home, knowing that things aren’t going to get better? 

If you chose option A, you’ve fallen victim to what’s known as the ‘sunk cost effect’. According to psychologist Sara Rego, this occurs when a “prior investment” in something causes you continue to invest in it, despite evidence that it’s probably not the wisest decision.

In a new study, Rego and two of her colleagues at the University of Minho in Portugal found that people often take the same approach to relationships: that is, they’re more likely to stay in an unhappy relationship if they feel like they’ve invested time, money or effort into it.  

Read more: How many of the Married At First Sight couples actually stayed married?

The study, published in the journal Current Psychology, presented over 1,000 participants with hypothetical romantic situations and asked them how they would react.

In the first experiment, 951 participants were divided into four groups. Each group was asked to imagine that they were in an increasingly sexless and unhappy marriage, and told they had to decide whether to stay in the relationship or end it.

The first group, the control, was only given the above information about their hypothetical marriage. However, the other three sets of participants were presented with slightly different relationship conditions.

Read more: Why “never go to bed angry” isn’t just an old wives’ tale

People in the first group, under the time condition, imagined that they had only been married for a year, rather than a decade. The second group, the money condition, were told that they had bought a house with the person they were unhappily married to. And the third group, the effort condition, were asked what they would do if had “made a huge effort” to improve their marriage by showing their partner more attention, giving them presents, and similar loving gestures.

Rego and her fellow researchers found that people were significantly more likely to stay in a miserable marriage if they believed they had invested effort and money in their relationship. Some 35% of people in the effort and money conditions said that despite the lack of sex and affection, they would stay with their husband or wife.

In contrast, only 25% of people in the time and control conditions said they would stay in an unhappy marriage. This suggests that people are more likely to walk away from a shorter marriage than a longer one – or if they don’t think they’ve invested a significant amount of effort or money into the relationship.

In the second experiment, 275 participants were presented with just two different scenarios, based solely on relationship length. The first group were given a scenario where they had been married for ten years, while the second were asked to imagine they’d been married for just one year. They were then asked to share how long, in days, they thought they’d stay in an unhappy marriage.

It was discovered that the ten-year group would invest about a year and a half (583 days, or a little over 19 months) into trying to save a floundering relationship. The one-year group, in contrast, would work at it for nine and a half months (289 days) before throwing in the towel – which still seems a pretty valiant effort.

“Together, both experiments confirmed the initial hypothesis that investments in terms of time, effort and money make individuals more prone to stay and invest in a relationship in which they are unhappy,” write the study’s authors.

So if you’ve been wondering why your best friend is still with that terrible boyfriend, there you have it. Mystery (kind of) solved.