Why are people jealous?

Found on Reddit, rewritten a bit to fix grammar issues.  Interesting comment.

Jealousy, as every emotion, has an evolutionary background.

We all feel this emotion. Let me repeat myself: We all do feel this emotion. It is only question do we care enough to trigger the sensation.

What is the purpose of this emotion? It main purpose is to protect the relationship we have with our partner. The fact is that we all feel it. Our fathers and their fathers felt it long before us. People who didn't felt this emotion died long time ago. This emotion gives you a huge evolutionary advantage.

Now, the punch line. Scientific studies show that this emotion is very accurate. In fact, in the most popular study they talked with people in marriage who went to couples counseling, and had problems with jealousy. They all felt that it was a sort of burden for the relationship and that it was a problem. A problem they couldn't do anything about. They just felt it.

Now, at the and of each interview they would take aside the suspected cheater and ask the question: Do you cheat on your partner? People would answer truthfully as the interview was guaranteed to be discreet. How many people did, in fact, cheat? ... an amazing 90%.

Just think about it. The sensation of jealousy is 90% accurate*. So, don't fight it. If you feel it, you have the reason to feel it. Trust your emotions. 

* a faithful reader made me notice, that jealousy is 90% accurate only in the cases presented in the study, that is, people who were going to counseling because of jealousy problems.