Self-Employment Tax

• Social Security self-employment tax reduces a taxpayer's AGI. A taxpayer paying the full 13.3 percent of Social Security tax on his self--employment income can claim a deduction of half of that amount on his federal tax form 1040. This deduction is not itemized and is reported under the adjustments to income section on the front page of the main tax return.
  
  

Deductible Retirement Plan Contributions

• Contributions that a taxpayer makes to a traditional IRA account are tax deductible, and they are deducted as part of the adjustments to income section of the tax return, meaning that these deductions reduce a taxpayer's AGI. Contributions to employer-sponsored plans, such as 401(k) or 403(b) plans, also reduce the AGI, because the money is deducted from the earnings that employers report for employees on W-2s.
  
  

Alimony Payments

• Spousal support, or alimony, paid as part of a court order in a divorce are tax deductible. Such payments are also considered adjustments to income, so they reduce the AGI. Child support is not a deductible expense and has no affect on the taxpayer's AGI.
  
  

Health Savings Accounts

• If you qualify for a Health Savings Account (HSA) and make contributions to the account, these contributions can also reduce your AGI. A person qualifies for HSA deductions if he is covered by a high-deductible health insurance plan, either at work or through an individual plan. A taxpayer can contribute up to $6,150 as of 2011, and can add another $1,000 to the contributions if he is over age 50. The maximum out-of-pocket expense for a policyholder must be less than $11,900 for a family to qualify.