Two generations ago, moving in together before marriage was scandalous in most of the Western world. One generation ago, it was controversial. Today it is the default. In the United States, roughly two-thirds of couples who marry have cohabited first. In much of Northern Europe, cohabitation is not a prelude to marriage at all — it is a substitute, a destination in its own right, often producing children and lasting decades. The institution of marriage has not collapsed; it has been preceded by a new institution that operates by different rules and serves different functions.
What is cohabitation actually doing? The polite answer, repeated in countless wedding toasts, is that it lets couples "test compatibility." This framing imports a consumer-grade logic into intimacy: try before you buy. The framing is not entirely wrong, but it is shallow. Cohabitation is doing several things at once, and the things differ across class lines, age cohorts, and national contexts. For some couples it is a deliberate trial. For others it is a financial arrangement that drifted into intimacy. For others it is a low-friction default that postpones any explicit decision about the future. The same external behavior — sharing an address — carries radically different meanings depending on what brought the couple there.
Sharon Sassler's careful interview work shows that the trial-run model is most common among college-educated couples, who tend to discuss expectations, divide labor with intention, and treat cohabitation as a phase with an expected outcome — marriage or breakup, within a defined window. Working-class couples more often slide into cohabitation. The rent is high, one partner has a lease ending, the other has a place, and the boxes get moved over a weekend. There was no conversation. There is no expected endpoint. The relationship continues until it stops. Both pathways can produce stable, loving partnerships. But the pathways have different failure modes, and the failure modes show up in the data.
The "cohabitation effect" — the older finding that couples who cohabited before marriage had higher divorce rates — has been substantially revised. When researchers control for age at coresidence and for whether the couple was already engaged when they moved in, the effect mostly disappears. What remains is a more specific finding: people who move in young, before forming clear life plans, are more likely to drift into marriages they would not have chosen deliberately. The mechanism is inertia. Once you share furniture, a lease, a pet, a routine, the cost of leaving rises. Some couples ride that inertia into a marriage that works out. Others ride it into one that doesn't.
The collective consequence is that the marital filter has weakened. Marriage used to be the first major commitment a couple made together. Now it is often the third or fourth — after moving in, merging finances, perhaps having a child. Each prior commitment has already locked in the relationship. The wedding becomes a public ratification of a private arrangement that already exists. This is not necessarily worse than the older model, but it is different, and the difference matters for how marriage functions as a sorting mechanism.
For children, cohabitation introduces new vulnerabilities. Children born to cohabiting parents are roughly twice as likely to see their parents separate before age twelve as children born to married parents, even controlling for income and education. The gap is not because cohabitation causes instability; it is because the couples who cohabit-and-stay-cohabiting tend to be those with weaker commitments. Selection, not causation. But from the child's perspective the mechanism is academic. Wendy Manning's research consistently shows that the stability of the parental union — more than its legal form — is what shapes child outcomes, and cohabiting unions are on average less stable.
Cross-nationally, the picture varies. In Sweden, cohabitation and marriage have nearly equivalent stability because the surrounding institutions — parental leave, childcare, legal recognition of cohabiting partners — treat them as functionally similar. In the United States, the legal and economic gap between marriage and cohabitation remains wide, and the stability gap tracks it. The lesson is that cohabitation is not an inherently stable or unstable form. It takes on the stability of the institutional scaffolding around it.
To think clearly about this — Law 2 — means resisting both the conservative frame that cohabitation degrades commitment and the progressive frame that the legal form of a relationship is irrelevant. Both frames flatten. The truth is that cohabitation has become a real institution with real consequences, and the consequences depend on who is doing it, why, and within what surrounding structures. The wedding has lost its monopoly on commitment. Something else is doing the work the wedding used to do, and that something else has not yet been named, scripted, or institutionalized with the clarity that marriage once had.