Pros
1. Simplicity. When you simply multiply your income by a percentage, there is no need for thousands of forms, expensive software, and tax accountants.
2. Lack of loopholes. The simpler the system, the harder it is to evade it.
3. Prevents
moral hazard. Since everyone is hit by the income tax proportionally, everyone has skin in each new government program created.
4. It increases the motivation of entrepreneurs to start small businesses. The more you penalize people who "make it," the fewer people will even bother to try.
Cons
1. It retards income redistribution. Our current system is designed to take from the rich, and give to the poor. A flat tax would stop this, making everyone pay the same percentage of their income.
2. It is not as fair. Any economist will tell you, the less money you have, the more you personally value each dollar. A person making only $20k a year will be dramatically affected by a $5k tax, while a person making $200k a year won't miss the $50k as much.
3. It may not generate as much revenue as the current system, without significantly increasing taxes on a majority of Americans. This will likely sour several voting blocks to the proposal (principally, those who benefit from the current system).
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I support the proposal, primarily for pro #2.
Moral hazard occurs when one benefitting from risk, is not exposed to that risk. It is akin to taking a stranger's money to Vegas, but personally keeping all the winnings.
Moral hazard is, fundamentally, what is currently wrong with the leadership of Financial Sector, and is also the reason why our political leaders continue to forsake our economic future with debt. CEOs are immune to the consequences of their actions, politicians will be retiring on their islands even if America collapses, and people benefiting from government regulation/handouts/loopholes don't give a damn about the health of the overall economy.
The Flat Tax, while imperfect, would go a long way in eliminating the moral hazard that is dooming our country.