A statistic from 2008 before we surpassed 7 billion people was as follows:

The top 1 billion people on earth make over $20,000 per year.
The next 1 billion people on earth make between $2,000 and $20,000 per year.
The bottom 4 billion people on earth make less than $2,000 per year.

According to this statistic, you can apply basic math and determine that making over $20,000 per year puts you in the top 83% of world earners. At the same time you earn more than 10 times more than 67% of the world.

You can Wikipedia something called 'the power law' and apply it to wealth disparity. By this law, for every person with $1,000,000, there must be 1000 people with only $1,000. It should also be noted that with a large sum of money to invest, you will not gain using basic addition, rather, you will gain in proportion to you percentage share of total capital. By this logic, and that of the power law, the rich do get richer and the poor do get poorer. Obviously, I have vastly oversimplified but the concepts I am presenting are straight forward. The feudal system serves as a good example of unchecked wealth disparity.