If you go back 70 years, the share of women and men living together before marriage was under 1%. If you go back 50 years, it was less than 10%. Now, about 70% of men and women live together before marriage.
Arielle Kuperberg digs into some of the patterns behind this trend in “From Countercultural Trend to Strategy for the Financially Insecure: Premarital Cohabitation and Premarital Cohabitors, 1956-2015,”written as a briefing paper for the Council on Contemporary Families (October 8, 2018). The briefing paper draws on her article, To cite this article “Premarital Cohabitation and Direct Marriage in the United States: 1956–2015, just published in the Marriage & Family Review (but not freely available online).
Here’s the overall trend in cohabitation before first marriage over time.
More information here: http://conversableeconomist.blogspot.com/2018/10/insights-into-dramatic-rise-in-pre.html