Caroline Freund and Sarah Oliver have this interesting paper on superrich across world regions:
This working paper presents a new dataset on the sources of billionaire wealth and uses it to describe changes in extreme wealth in the United States, Europe, and other advanced countries. The data classify wealth as either self-made or inherited and identify the company and industry from which it comes. Among self-made billionaires, individuals are further classifi ed as company founders, executives, politically-connected, or in finance.
Data analysis shows that the superrich in the United States are more dynamic than in Europe. Just over half of European billionaires inherited their fortunes, as compared with one-third in the United States. The median age of a company of a European billionaire is nearly 20 years older than that of an American billionaire. Traditional sectors explain more than half of the rise in wealth in Europe; the financial sector and technology-related sectors together are largely responsible for the rise in US wealth.
There is some evidence that rents are higher in the United States than Europe, as not only is the number of US billionaires expanding rapidly, but US billionaires are also getting richer on average over time, especially when wealth is connected to resources, nontradables, or finance.