People want to be fully formed before they get married.

You're also seeing a social acceptability around cohabitation and delaying marriage.

CNN: In your book, it sounds like communication is a key factor for interpreting whether a relationship is headed for marriage. Seligson: I was surprised how little communication there was between couples. I heard this from many men that they want to be able to provide.

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I was "a little bit married." We use to go on each other's family vacation. I thought this was all leading to marriage, but it wasn't.

It was this confusing, new romantic right of passage. Do you think these long-term relationships without marriage will become less popular?

I don't think people will delay marriage inevitability, though, because women have a biological clock.

I’d been dating my boyfriend, let’s call him Jordan, for almost two years when we decided to move in together.

Seligson: My baseline is one year in a monogamous relationship.

Do you see this as someone you are making big sacrifices and life decisions around?

Also, part of it is complacency, and couples aren't always on the same page.

Someone is dating, and they are thinking this is a past agreement -- that we are going to get married because we've been together for X years. A man's decision to get married is often correlated to income.

For example, the day-to-day staple activities are done together.

You buy furniture together but you say, "What happens when if we broke up? " There is no definite sense this will culminate in marriage.

Seligson: It's hard to imagining it shifting back because there are so many factors that aren't slowing down, such as women entering workplace and the double-income, no-kid phenomenon.