A good analogy of this fact from the Wall Street Journal:
Mr. Rangel and House Democrats are also banking on the idea that raising tax rates by 20% will raise 20% more tax revenue, but that’s like telling Wal-Mart it can raise prices by 20% and get 20% more profit. When taxes on the rich rise, their reported income tends to decline. The last time the top federal income tax rate was 50%, the richest 1% paid only about 25% of all income taxes. Today, at a 35% rate they pay nearly 40%.