For decades, the common-sense assumption was that living together before marriage would reduce divorce rates — that couples who tested their compatibility under the same roof before making a legal commitment would be better prepared for marriage. The data, at least until recently, told a different story.

Numerous studies conducted through the 1990s and 2000s found that couples who cohabited before marriage divorced at higher rates than those who did not — a finding so consistent it was dubbed the "cohabitation effect" by researchers. The counterintuitive result generated significant debate and extensive efforts to understand the mechanism behind it.

Several explanations were proposed. The "sliding vs. deciding" theory, developed by psychologist Scott Stanley, argued that many cohabiting couples didn't so much choose to get married as slide into it — gradually accumulating shared furniture, leases, and social entanglement until the inertia of the relationship made marriage the path of least resistance rather than a deliberate choice. Couples who marry without clearly choosing each other, the theory went, are less committed to working through difficulty when it arises.

A second explanation focused on selection effects: people who cohabit tend to have somewhat different attitudes toward marriage and commitment than those who don't, and those attitudinal differences — not the cohabitation itself — might explain the higher divorce rates.

More recent research has complicated the picture significantly. Studies controlling for age at cohabitation find that the cohabitation effect largely disappears when couples begin living together at older ages — suggesting that it may be youthful decision-making, not cohabitation per se, that predicts worse outcomes.

The current consensus is nuanced: intentional, committed cohabitation — where both partners have explicitly discussed and agreed on the path toward marriage — does not appear to raise divorce risk, while the "slide in" variety may.