I've linked to an article from a reputable source on the matter below. However, I would say that having lived in Bay Area, France and the UK, despite the fact that the headline figures place France and the UK below California, France and the UK feel wealthier than those numbers would suggest. I suspect the difference lies in better public amenities and provisions in Europe.

http://www.csmonitor.com/USA/DC-Decoder/Decoder-Voices/2014/...