Here is an interesting Pew Report on the changes in incomes of individuals over the past decade. It claims that while the number of people living on less than $2/day has been greatly reduced, most of the gains have been in the low income group (between $2/day and $10/day). The number of "middle class" people, or those earning between $10 and $20/day, has undergone a substantial increase, but this growth was concentrated primarily in China, Eastern Europe, and South America. The vast majority of high income people (over $50/day) continue to be located in North America and Europe.

You can read the full report here: