If the government punishes you more for earning more income, you will figure out ways to reduce the amount of money you report on your tax return.
This sometimes means that people will choose to be less productive. Why bust your derrière, after all, if government confiscates a big chunk of your additional earnings? Why make the sacrifice to set aside some of your income when the government imposes extra layers of tax on saving and investment? And why allocate your money on the basis of economic efficiency when you can reduce your taxable income by dumping your investments into something like municipal bonds that escape the extra layers of tax?
Or people can decide to hide some of the money they earn from the grasping claws of the IRS. Contractors can work off the books. Workers can take wages under the table. Business owners can overstate their expenses in order to reduce taxable income.
To reiterate, people respond to incentives.