I agree changing attitudes towards divorce was the main factor. I was imprecise in my wording, I was fishing for phraseology that would encompass both.
My siblings in law come from one of those middle/lower middle class primarily white towns you sometimes read about, where two-parent families have declined tremendously. The devastation to the kids is real. Comfortable finances dissolve into financial strain. Lack of stability affects kids’ performance in school. Boys, in particular, suffer growing up without fathers to keep them in line. (It wasn’t until I had boys of my own that I realized how differently they often relate to their mothers versus their fathers. And trust me, we’re not modeling traditional gender roles at my house—my wife’s a scary trial lawyer who pays our mortgage.)