A research study published this year in the Journal of Social and Personal Relationships answered a burning question of ours: can exes be friends? (Note: If you want to see this question answered by a variety of fascinating people, check out our #howimend interview series.)
Researchers looked at a racially representative sample of over 100 people between the ages of 18 and 30 who had gone through a breakup. They hypothesized that the greater the level of commitment between two people in a relationship, the greater the chance that they would have some sort of relationship after the breakup.
The researchers assumed for the sake of analysis that commitment is a result of three things: satisfaction in the relationship, feeling that the alternatives to being in that relationship are less attractive than being in it, and having already invested in it. These three factors form what's known in the literature as the "Investment Model" of relationships. The researchers found that the more of these three factors participants claimed to feel before the split, the more likely those participants were to be close to their partner after a breakup. It's worth mentioning that the average amount of time that had passed between the breakup and the research collection was about four months.
Other older research has found that the more time couples spend together, the longer it takes them to move on, and the more likely they are to try to get back together.
The bottom line: it looks like there's a sunk cost effect at play; the more commitment that went in to a relationship, the more likely we are to hang on to it in some way, even after it ends. Whether or not that's a rational thing to do - well, that's an article for another day.