In her first book, Mating in Captivity: Reconciling the Erotic & the Domestic, New York psychotherapist Perel offers alternative ways of understanding the relationship between intimacy and sex in long-term relationships.

A lot of the issues you discuss—dwindling eroticism in otherwise healthy long-term couples, for instance—seem to stem from unrealistic expectations of marriage or commitment.

We have invested marriage with an enormous amount of differing needs. We still want what we used to have: security, respectability, reproduction, social status, companionship. And now we want confidants, best friends and passionate lovers to boot. It's not always so easy to experience excitement and security in the same place. People who have multiple nurturing relationships with friends and family often do better. Maybe marriage, or any committed relationship for that matter, isn't for everything.

You were born in Belgium and lived in Israel and England, among other places, before moving to New York. Is this kind of erotic deterioration particularly an American problem?

Whatever happens in America slowly moves like a tectonic plate over to the rest of the world, but in many places—especially societies where the individual is so central—individual freedom comes with greater individual isolation. Those are often societies where people are less connected.

The book is a mix of theory, cultural criticism and practical advice. Many of your ideas, such as the notion that couples can benefit from understanding that each partner may fantasize about other people, require fairly sophisticated ideas of love and commitment. Who is your ideal audience for the book?

The educated reader, and men as much as women. I also want the book to represent the hybrid that I am between Europe and America. It's being published in nine countries, Germany, Holland, France, and Taiwan among them—places where this kind of book is more rare, especially one by an author working in America. One idea that may seem foreign to an American audience is that I've written from the perspective that not all problems have a solution, which is really what America believes in.

Do you have any advice for couples in the early stages of their relationships, in terms of heading off the decline of their erotic lives?

A certain amount of uncertainty feeds desire. Don't assume you know your partner that well; don't turn him or her into a familiar sofa. Good intimacy does not necessarily guarantee good sex, despite everything we have been told.