Part of the debate over how to avert the “fiscal cliff” focuses on the possibility of reining in certain tax loopholes and deductions.

The number of tax deductions — or tax breaks — has nearly doubled since the last significant overhaul of the tax code nearly a quarter century ago, thanks to a host of tax benefits added in the years since for children, college tuition, retirement savings, job hiring and the families of victims of terrorist attacks, among others.

As the number of tax breaks has increased, so has the value of some of the most popular deductions, including those for mortgage interest and the tax-free treatment of health-insurance premiums paid by employers.

How many tax breaks are there? Depends on whom you ask. The congressional Joint Committee on Taxation counts more than 300; the Treasury Department counts more than 170. Only about 8 percent of tax benefits go to corporations, on average, with most enjoyed by middle-class households.

So which tax breaks are the most popular? Here’s a quick look at the 10 most-popular tax breaks for individuals from 2011, based on number of people who take them and the cost:

1. Exclusion of employer contributions for medical insurance premiums and medical care: Worth $173.7 billion in 2011.

2. Mortgage interest deduction: $88.8 billion.

3. 401(k) plans: $62.9 billion.

4. Earned income tax credit: $62.5 billion.

5. Step-up basis of capital gains at death: $50.9 billion.

6. Exclusion of net imputed rental income: $47 billion.

7. “Making Work Pay” tax credit: $44 billion.

8. Child credit: $42.5 billion.

9. Employer plans: $42.2 billion.

10. Deduction of charitable contributions, other than education and health: $38.2 billion.

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