There’s a good explanation as to why most of us stay in dead end relationships.

Why we continue to invest time, money and effort into people who have already gobbled a lion’s share of our lives.

Researchers believe it might be because of the “sunk cost effect”.

A “sunk cost” is a term generally applied in business or economics. It’s when a cost that has been incurred cannot be recovered.

Psychologists say the “sunk cost effect” occurs “when a prior investment in one option leads to a continuous investment in that option, despite not being the best decision.”

Think of it like this: you buy a ticket to see a film. Halfway through you realise the film is rubbish but you've already spent money on the ticket, invested time in going and devoted effort to the whole shebang.

It's unlikely you're going to walk out. Now try applying that principle to a shitty relationship.

Psychologists at the University of Minho in Portugal did just that and found individuals were more likely to stay in failing relationships if they had previously invested time, money and effort.

Two studies were conducted that presented individuals with hypothetical scenarios involving decision-making in relationships.

The first study divided 951 participants into four different groups: time, money, effort and a control group.

Each group were given a different scenario about a 10 year marriage that was only getting worse. Individuals would listen to their scenario before deciding whether they would stay or leave.

The money group were told they had bought a house together. The time group were told they had been in the marriage for only one year. The effort group were told they continuously tried to fix the relationship and the fourth group, the control, were told nothing.

It was found that around 35 per cent of those in the effort and money groups would stay in the relationship compared to 25 per cent of those in the time or control group.

Listen to the story of how a divorce boosted one woman's career.

The second study also asked participants (275) to say how long they would stay in a failing marriage. Except this time participants were divided into two groups, one containing 150 individuals and one containing 125.

The group of 150 individuals were told they had been in the marriage for one year and the group of 125 individuals were told they had been in it for 10 years.

Participants were asked to say how long they would stay in the relationship, by either pointing to a mark on a ruler (where one end says 'no time' and the other says 'a lot of time') and by saying how long in days.

It was found that both groups pointed to a similar area on the ruler but differed on the amount of days they would invest.

Those who had been in the marriage for a year would invest, on average, 289 days while those who had been in it for 10 years would invest 583 days.

Researchers were therefore able to show that investing time in a relationship significantly affects how long individuals stay in them.

Psychologist Daniel Kahneman offers another explanation as to why the sunk cost effect is so influential.

In his book, Thinking Fast And Slow, Kahneman says during evolutionary times, beings that prioritised avoiding threats over maximising opportunities were more likely to pass on their genes.

Here are the moments that told us our partners were "the one".

If humans are biologically trained to "avoid threats" then one can see why people would be motivated to avoid being alone by staying together.

Research aside, it's important to remember that negative relationships, long or short, can hugely impact our lives.

Let's use these findings to help ourselves and our friends know when to invest and when to cut your losses.