What does it mean to be in a monogamous relationship? It depends who you ask. In the modern world, it has become increasingly difficult to define “monogamy” in any kind of universally agreed upon way. Just think about it: what “counts” as infidelity to you? For example, does sexting someone other than your romantic partner constitute cheating? What about using an interactive pornography site?

It turns out that different people answer these questions in very different ways. As some evidence of this, consider a study of college undergraduates who were given a list of 27 behaviors that could potentially constitute infidelity. It turned out that there wasn’t 100% agreement that any behavior was or wasn’t cheating!

As you might imagine, this has important implications for relationship happiness. A lot of people simply assume that their partners will have the same definitions of monogamy and cheating as they do; however, this assumption may be setting them up for major heartache, as psychotherapist Esther Perel explains in the wonderfully informative and engaging Fresh Air segment below.

Perel argues that we shouldn’t simply assume monogamy in our relationships. Instead, she suggests that we communicate about our expectations. We need to negotiate and define our rules and boundaries.

Perel has much more to say than this, though; she also goes on to discuss why people cheat in the first place, arguing that infidelity has complex motives and isn’t just about experiencing a physical act. Further, she explains what couples can do to move on when cheating is discovered and how she helps clients with this in her practice.

Check out the full interview below. For more, check out Perel’s new book, The State of Affairs: Rethinking Infidelity. It’s a fascinating and worthwhile read.