Bracket creep

Most progressive tax systems are not adjusted for inflation. As wages and salaries rise in nominal terms under the influence of inflation they become more highly taxed, even though in real terms the value of the wages and salaries has not increased at all. The net effect is that in real terms taxes rise unless the tax rates or brackets are adjusted to compensate.

Examples

Suppose a person earns $20,000 per year and is liable to 20% tax on earnings above a threshold of $5,000 per year. Then they pay ($20,000–$5,000)*0.2 = $3,000 in taxes, or 15% of their income. Now suppose that due to inflation, their wage goes up by 5%, but the government does not increase the tax threshold. They must now pay ($21,000–$5,000)*0.2 = $3,200 or 15.2% of their income as tax. Thus the proportion of the person's income that is paid as tax has increased. The average tax rate went up even though the tax payer remained in the 20% tax bracket. That is, the marginal tax rate did not change. Since the wage increase was due to inflation, the person's net real income declined.

In the United States, the Alternative Minimum Tax originally (1971) targeted 155 high-income households; based on 2004 law, it would affect 20% of households by 2010.