But considering that even countries like Iceland that do culturally were having big families, and living in big houses including split houses where the parents and grand-parents live in connected houses also is suffering from a similar fate I might not be as simple as that.

I would say it's a "natural" development of of current life styles, the US managed to delay it maybe because for a long long time and especially after the war the concept of the American dream, a big house, a hot housewife, and 3 kids was in full gear.

Now with the American Dream being less relevant, and much less attainable to many Americans we see a switch to less children, less marriages, and more adults living alone.

Compare to 1.4 in Italy (and I thought they were Catholics!) or 1.38 in Germany.

Actually now I look it up the US is only 1.88 in the last stats that come up, so Iceland is out-breeding the US.

Edit: and while I don't know why I bother researching and typing these things since I inevitably wind up with negative karma the moment I suggest something not directly in line with HN herd opinion, I wish to add that the top-selling car in Iceland is the respectably mid sized Skoda Octavia.

And it's also quite a sharp decline since the 80's and 70's for Iceland, just like for any other Western country.