Final answer:
The social security tax is an example of a regressive tax structure, where as the tax base increases, the taxes paid increase, but the marginal tax rate decreases.
Step-by-step explanation:
The social security tax is an example of a regressive tax structure. As the tax base increases, the taxes paid increase, but the marginal tax rate decreases. This means that individuals with higher incomes pay a smaller share of their income in taxes compared to those with lower incomes.