Adjustments to income are specific deductions that directly reduce your total income to arrive at your AGI. The types of adjustments that you can deduct are subject to change each year, but a number of them consistently show up on tax returns year after year. Some of these adjustments include half of the self-employment taxes you pay, alimony payments made to a former spouse, contributions to certain retirement accounts (such as a traditional IRA) and the deduction for tuition and fees.
The 1040 is the only form that allows you to deduct every possible adjustment. Using form 1040A significantly reduces the number of available adjustments you can take. And if you're filing on the 1040EZ, your AGI is the same as your total income, since the form doesn’t allow you to deduct any adjustments to income.