Ending a romantic relationship is never easy, and there's something about a bad relationship that makes it especially hard to break off.

Researchers are still not entirely sure what that "something" is, but a new study suggests it may be a curious sign of altruism.

You might assume people who stay in an unhappy relationship are lazy. And when examining close romantic relationships, most research has found that breaking up with someone is driven predominantly by self-interest.

Traditional thinking puts the reasons for staying in a relationship into two categories. The person either thinks they have invested too much time and emotion to give up now, or they think that being single would leave them worse off.

But when a person is deciding whether to break off a relationship or not, their pro/con list may not be as wholly self-interested as these studies have led us to believe.

Even when people are not particularly satisfied with their relationship, concern for their partner's feelings may prompt them to stay.

"When people perceived that the partner was highly committed to the relationship they were less likely to initiate a break up," says lead author Samantha Joel, who studies decision making in romantic relationships at Western University in Ontario, Canada.

"This is true even for people who weren't really committed to the relationship themselves or who were personally unsatisfied with the relationship. Generally, we don't want to hurt our partners and we care about what they want."

Joel's research is the first to directly test the idea that people take their partners' feelings into consideration when deciding whether to stay in a relationship.

The research tracked 1,348 participants who were in romantic relationship over a 10-week period. A second study looked at 500 participants who were contemplating a breakup and followed them for another two months.

The findings add to a growing body of research that shows human behaviour is inherently prosocial - a type of voluntary behaviour that aims to help others.

Both studies have revealed that the more dependent a person believed their partner was, the less likely they were to initiate a breakup.

While it's difficult to rule out every possibility, this prosocial behaviour doesn't appear to be masking any selfish considerations. These findings were not attributed to feelings of guilt, fears of retaliation, or concern about the fall-out from friends and family.

When considering a break up, it seems that people do honestly take into account their partner's commitment to the relationship and how distressing a breakup would be for them.

"In the present research, we consistently found that people took their partner's feelings into consideration – such that they were less likely to end a relationship with a highly dependent partner – regardless of their own commitment, satisfaction, investment, and quality of alternatives," the authors conclude.

This isn't to say that all selfish factors are thrown out the window. It just suggests that people also consider their partner's feelings.

Nevertheless, it still remains unclear whether this is an example of "true" prosocial behaviour. The researchers acknowledge that there may be selfish considerations hiding among the results that they haven't managed to drive out.

For instance: are committed individuals truly more prosocial towards their partners, or are these efforts to benefit the partner part of some long-term, self-interested strategy to maintain a valued relationship?

Even if this is truly an example of prosocial behaviour, staying in a relationship for the sake of another may not lead to better outcomes.

For starters, people could be reading their partners completely wrong, keeping both themselves and their significant others in a relationship that neither is happy about.

"One thing we don't know is how accurate people's perceptions are," Joel says.

"It could be the person is overestimating how committed the other partner is and how painful the break up would be."

Prosocial behaviour may also lead to poor individual outcomes, and this is especially true when an unfulfilling relationship leads to personal unhappiness and poor mental health.

So while pro-social behaviour may seem more virtuous, it doesn't necessarily lead to better outcomes, and it can hurt both people involved.

After all, as Joel points out, "Who wants a partner who doesn't really want to be in the relationship?"

The authors are calling for further research to figure out the pros and cons of staying in a romantic relationship for someone else's sake.

This study has been published in the Journal of Personality and Social Psychology.