An interesting phenomenon seen in several European countries is an increase in the number of couples choosing to not marry before or during the process of raising children. One thing that often gets ignored on this subject is how stable these pairings are compared to more traditional ones based on marriage. Findings from the Brookings institute show that:

But it turns out that even in Europe, cohabitation is markedly less stable for children than marriage. Analyzing data from 16 countries across Europe, we find that children born to cohabiting couples are about 90 percent more likely to see their parents break up by the time they turn 12, compared to children born to married parents.

Indeed, across much of Europe, we found that parents’ marital status was generally a better predictor of family stability than their educational status. So much for the argument that it is education, not marriage, that matters for family stability.

Our new report from the Social Trends Institute and the Institute for Family Studies, The Cohabitation-Go-Round: Cohabitation and Family Instability Across the Globe, shows that the “stability premium” associated with marriage holds even among highly educated families. Almost half of cohabiting college-educated mothers will break up with their partner before their child turns 12, compared to less than one-fifth of mothers who were married when the child was born.

So it seems that marriage is a much better predictor of relationship stability then other factors such as education or income level - although these are often correlated with marriage to begin with. I would be interested in hearing more thoughts on this matter

Source:

https://www.brookings.edu/blog/social-mobility-memos/2017/03/27/in-europe-cohabitation-is-stable-right/