What research has been done into the maintenance of relationships?

I'm stuck on an A level past paper question. Outline and evaluate research into the maintenance of relationships.


My teacher said not to use theories e.g. social exchange and equity, but I can't find any studies.What research has been done into the maintenance of relationships?
There must be masses. That's what psychology, sociology, anthropology and probably a lot of other ologies are all about. I am sure that if you just googled the term, you'd be overwhelmed with information. Good luck!