True, but the USA, unlike Europe, is considered a single nation-state. In European countries there are differences within national borders as well, e.g. northern part of Italy vs Sicily. That said there are definitely great commonalities in Europe that can be generalized, my point was that this metric isn't one of them.

And I am actually surprised with the quoted 1/3rd figure for the USA. Might be true but not what I'd expect from my outsiders insight into American culture.

Well, social programs can be a factor in that, but IMHO most of the difference is cultural. In Southern Europe, large, multi-generation households are traditional, much less so in the North.