Sometimes referred to as the rule of money. I learned of this from an audio cassette by Robert Kiyosaki. The 90-10 principle states that 10% of the people control 90% of the money. For example, in a particular industry, 10% of the companies in that industry account for 90% of the total revenue generated. It can also be seen as 10% of the population of people make 90% of the money that is earned every year. This is similar to the Pareto Principle, where 80% of the results comes from 20% of the effort put into work.