Income is becoming more distributed around the world. A comparison on 1970 to 2006 shows vast improvements in income distribution. According to this report – the income distribution trend is great. (Note, this income not wealth, which may be a different matter – but income is a start.)

Parametric estimations of the world distribution of income | vox – Research-based policy analysis and commentary from leading economists.

I would guess this would be caused by more countries becoming more free. In a free country, I would think that we would see a bell curve in income distribution. This is because intelligence is distributed “normally”- and I would expect other characteristics like interests, passion, and work ethic to be characteristics that are probably normally distributed as well. And, in a society that is free – income would probably more closely match these characteristics than in a more centrally planned economy.