How times change. I once read that in the 1950s a professor at a major state university was fired when an interview in the student newspaper quoted him as saying that it would be a good idea for students to live together before getting married. In the 21st century, cohabitation before marriage has become normative, with about 70% of couples moving in together before they marry. It is accepted by most people as axiomatic that moving in before marriage is an important way to test the relationship.

After all, how could a couple possibly know if they are compatible enough to have a successful marriage without taking a test drive? Dating compatibility is one thing, but living together compatibility is something else. The only sure-fire way to know if a man and woman are sufficiently compatible is cohabitation first to test the relationship and then marriage, right?

Wrong. The logic of cohabitation before marriage might make intuitive sense. But the data says otherwise. Cohabitation before marriage increases the risk for divorce. This is called the cohabitation effect.

But surely the data supporting the cohabitation effect are from the olden days? After all, there used to be a stigma attached to shacking up. But in our enlightened modern era, there is no stigma. The cohabitation effect must be a relic from the past. Wrong again.

Data published within the last year by Drs. Scott Stanley and Galena Rhoades of the University of Denver demonstrate that the cohabitation effect is alive and well despite the cultural acceptance. They distinguish between people who moved in together before engagement and those who moved in only after either being engaged or being married. During the time frame of the study, couples were 48% more likely to get divorced if they moved in before becoming engaged.

Understanding why they made the before and after engagement distinction helps understand why the cohabitation effect lives on. The key is the order of constraints and dedication.

Stanley and Rhoades believe that a key reason for the cohabitation effect is that because of cohabitation, some couples marry who would have broken up had they never moved in together. Moving in together creates constraints that make it harder to leave the relationship, but these constraints do not necessarily increase, or even indicate, dedication to each other and the relationship.

Constraints include things like having a lease together, sharing a cell phone plan, getting a pet, or having a child together. There is nothing wrong with constraints. It is impossible to have a successful romantic relationship without constraints. Happy couples do not feel constrained by constraints. Less-than-happy couples may feel trapped in a relationship but still progress from cohabitation to marriage because the constraints push them in that direction. Absent the constraints created by cohabitation, these less-than-happy couples would have moved on to new relationships rather than marrying each other.

Consistent with this, Stanley and Rhoades found that couples who moved in together to save money or to test the relationship had particularly unstable marriages later.

Engagement is a public declaration of a couples’ intention to marry and thus remain together until death parts them. Engaged couples have actively and thoughtfully chosen a common future. Stanley and Rhoades call this “deciding” in contrast to “sliding.” Deciding by either marrying or becoming engaged increases dedication before increasing constraints. Sliding increases constraints without increasing dedication.

Looking at this from a Christian perspective, God would have couples wait to move in together until after the wedding. Publicly vowing lifelong love before God, family, and friends is important for Christians. Being clear with the data requires acknowledgement that the public commitment of engagement meaningfully increases the odds of a successful marriage even when couples move in before the wedding. Engagement is not marriage, but engagement alone is a powerful demonstration of dedication to a common future.

Instability increases when a couple moves in together out of convenience or to share expenses. These couples have not made a thoughtful and public commitment to a common future. They are choosing to become more constrained before establishing dedication.

Thus a key reason the cohabitation effect lives on is because constraints before dedication lead to unhappy marriages and divorce.

If only couples could test compatibility and increase dedication without becoming trapped by constraints.

Dr. Joseph J. Horton is professor of psychology at Grove City College and the Working Group Coordinator for Marriage and Family with the Institute for Faith and Freedom. He is also a researcher on Positive Youth Development.