Ever wonder what a successful marriage looks like? Well, according to José-Manuel Rey of the Complutense University in Spain, it looks something like this:

Rey has identified a number of variables which, if calculated together, seem to predict why some marriages last and others fail. He has published his work in the journal PLOS ONE.

The formula is based on something called the second law of thermodynamics for sentimental relationships. In physics, the second law of thermodynamics states that in any closed energy system, order will gradually deteriorate into disorder (as entropy) over time. The second law as applied to relationships works under a similar principle: that relationships tend to deteriorate over time unless effort is consistently introduced to sustain them. In other words, love isn't enough. Relationships take effort. And, according to Rey, relationships can be put on predictable trajectories when levels of effort are properly quantified.

The purpose of the formula is to deal with an omnipresent concern in most western societies: high divorce rates. Divorce isn't just stressful for those involved, but it also poses a dilemma for sociologists and therapists looking to diagnose the problem. Of course, some failed marriages seem easy to predict from the beginning: the couple was too young, they married too quick, they had opposing values, etc. But Rey isn't concerned with these kinds of failed marriages. His formula is more ambitious than that. He aims to predict why divorce still happens among seemingly stable marriages, when both partners have similar emotional attributes.

In other words, marriage is difficult to maintain even for those who have the best of intentions. This is what marriage researchers occasionally refer to as the failure paradox. Rey has identified some key variables that help to predict the outcome even in these situations.

Basically, all relationships have something called an effort gap. According to Rey, "a remarkable finding of the model is that the level of effort which keeps a happy relationship going is always greater than the effort level that would be chosen optimally a priori [or logically]." The effort gap is therefore the difference between these two levels of effort.

Instability is commonly introduced into a relationship through the effort gap, mainly because putting out the extra effort on a consistent basis isn't typically much fun. (That's what makes it an effort.) So whether a relationship can be maintained depends in large part on whether the effort gap is tolerable or not. If it's not, then even well-intentioned couples will begin to feel worn out and the relationship trajectory will rapidly deteriorate.

Rey's research inserts each of these factors into a number of equations and charts, which you can view at his article linked above.

So what does all of this mean about the odds of your relationship lasting? Well, it means what you probably already knew: it's complicated. The good news for marriage counselors is that they're unlikely to be replaced by calculators anytime soon. The research, while fascinating, still has a lot more to assess.

The method offers more insight to sociologists, who are burdened with having to explain complicated phenomena such as divorce rates, than it does to lovers contemplating marriage. But the findings do reinforce a cold fact that lovers would do well to remember: when it comes to relationships, love alone isn't enough. Relationships are destined to deteriorate without effort.