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Some of these terms may require a little explanation:
1. Tax-free health insurance contributions. The tax exclusion for employer-provided health benefits is the single largest tax break -- it alone will cost the government $1 trillion in foregone taxes over the next five fiscal years. This huge tax expenditure massively subsidizes the nation's employer-based health insurance system. It also provides an incentive to employers to overspend in health benefits (which are tax free) and pay less in salary (from which, of course, the IRS takes a bite). This tax break only helps families with at least one member employed by an employer who offers them health benefits. Others have to buy health insurance with after-tax dollars.
2. The mortgage interest deduction. The second-largest tax break is essentially the nation's largest housing program. By letting taxpayers who itemize deduct the interest they pay on their home mortgages, the government massively subsidizes home ownership. The more expensive the home -- and the higher the homeowner's tax bracket -- the bigger that subsidy is.
3. Treatment of capital gains at death. When you die, the government forgives your capital gains tax on appreciated assets that you pass on to your heirs. In accounting terms, this is the "step up in basis" on death. From the heirs' perspective, it means that the "basis" going forward (the amount above which anything is considered taxable capital gains) is the value of the asset at the time they received it. So if you buy stock at $1,000 and it's worth $10,000 when you die, your heir gets $10,000 as the basis. No one ever pays taxes on the $9,000 in appreciation. Now imagine a multi-million-dollar stock portfolio and multi-million-dollar homes -- and you're talking real money.
4. Tax-free contributions to 401(k)s. Federal government policy encourages savings for retirement by allowing employees and employers to make tax-free contributions to retirement plans, the most common of which is the 401(k). This break is a big gift to the financial securities industry, which is where most of this money goes, and to the very wealthy. Indeed, the bulk of benefits go to high-income households, while little goes to the lower and moderate income households. There are limits to how much can be contributed tax-free, but the amount of tax foregone through contributions to 401(k) plans, along with employer plans, when combined still make tax-deferred retirement savings the second largest tax expenditure.
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http://www.huffingtonpost.com/2011/04/18/the-top-10-tax-breaks-_n_850534.html