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* If the utility gained from income exhibits diminishing marginal returns, as many psychologists assert (see Weber-Fechner law), then for the tax burden to be shared in a utilitarian way the tax-bill must increase non-linearly with income.

* As income levels rise, levels of consumption tend to rise. Thus it is often argued that economic demand can be stimulated by reducing the tax burden on lower incomes while raising the burden on higher incomes.

* It is also argued that people with higher income tend to have a higher percentage of that in disposable income, and can thus afford a greater tax burden (this is the “vertical equity” argument). Some would claim that a person earning exactly enough money to pay for food and housing cannot afford to pay any taxes without it causing material damage, while someone earning twice as much can afford to pay up to half their income in taxes.

* Some believe that the wealthy have a disproportionately greater interest in maintaining societal goods typically supported by taxation such as security of property rights, defense and infrastructure, as they have much more to lose if these fail than do the poor. Public investments in defense and foreign aid often support assets abroad whose expropriation is a far greater risk than is the risk involving domestic investments.

* Some supporters of progressive taxation favor increasing taxes on middle class tax-payers, who have inelastic household budgets.[citation needed]

* A progressive tax is an automatic stabilizer in the sense that if a person were to suffer a decrease in wages due to a recession then the money regained by being in a lower tax bracket lessens this blow.

* It is inherent in tax policy that it implements economic and social policy. People who are concerned about a runaway, cancerous character in the global economy, greenhouse gases, etc., see benefits in progressive taxation, both in its braking effect on the economy and in helping shape economic activities towards necessities more effectively than purely monetary or fiscal policies.

* As long as after-tax income is a strictly increasing function of gross income, there is a monetary incentive to increase compensation received. Indeed, for any particular income goal, the higher the tax rate, more compensation one must receive to reach that income goal. For this reason, progressive income tax may increase the incentive to produce among the largest producers (if higher production is truly associated with higher compensation).

Copy and pasted from the Wikipedia page on the reasons for implementation of progressive taxes.