Here's an ABC News story out of Australia about "types of sex workers."
Huh, color me skeptical. The trouble with these kinds of studies is that the numbers are based on women who've been arrested or, in this case, women who have STDs. It's not really a representative sample, so the study is invariably flawed.
It's certainly interesting to hear that professional dominance is "unique to wealthy countries". Presuming one accepts that as true, it raises the question: Is that because men in poorer countries don't feel the need for kink? Or is it just that the women there find it better to be generalists than to specialize in one area?
It may have to do with the legal structures of different countries. One reason why a Mistress in the US might stick to domination exclusively is because it's more-or-less legal in many (although not all) areas. If one lived in a poorer nation, where laws against prostitution were not in place, or were not enforced, one might offer a range of services in order to maximize one's client base.
And in a different but related study: get out your rulers, it's the average sizes of men's penises...