Cohabitation before marriage increases the risk of divorce, according to a recent study from Stanford:

(I can't say I've read all that, but it agrees with what Dr. Harley has been saying other studies demonstrate, for years and years.)

I found the link to the study in this slightly more readable article about the association between religion and marriage, and early marriage versus later marriage:

It is commonly believed that postponing marriage until the late-20s or early-30s reduces the odds of divorce, because greater maturity results in a wiser choice of a partner, and there’s some truth to this. However, there’s a significant complexity under the surface: the life orientations associated with delayed marriage are often also associated with (and even causal of) greater acceptance of premarital cohabitation, which is also linked to a higher risk of divorce. The net result of this is that life orientations that motivate earlier marriage, like religiosity, do not necessarily create the higher likelihoods of divorce usually associated with early marriage because they discourage cohabitation. Yes, very young marriage still has risks (as does very late marriage), but religious upbringings seem to partly compensate for those risks, especially among women marrying in their 20s.

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