Personal Income Taxation: Gross Income, Adjustments, and Deductions
The architecture of the individual return
Personal income taxation is best understood as a pipeline rather than a single calculation. Start with gross income - all income from whatever source derived unless a statute excludes it. From gross income subtract adjustments (above-the-line deductions such as certain retirement contributions, self-employment tax, and student loan interest) to reach adjusted gross income (AGI). AGI is the hinge of the return: most limitations, phase-outs, and credit thresholds are keyed to it. From AGI, subtract the greater of the standard deduction or itemized deductions (medical, state and local taxes subject to a cap, mortgage interest, charitable contributions), then subtract qualified business income deductions where applicable, to arrive at taxable income. Apply the rate schedule, then subtract credits, which reduce tax dollar-for-dollar, unlike deductions which reduce income.
Concepts candidates must know
Distinguish exclusions (not income at all), adjustments (reduce gross income), deductions (reduce income after AGI), and credits (reduce tax). Know the difference between for AGI and from AGI treatment, and between refundable and nonrefundable credits. Understand basis and amount realized for computing gain, the treatment of capital gains as short-term or long-term, and the character rules that make holding period matter.
Traps
Questions often test placement: an item that looks like a deduction may actually be an adjustment, or a credit may be described as a deduction. Watch for items that are excluded rather than deducted, and for deductions subject to AGI floors or ceilings. A frequent trap is confusing a standard deduction amount with an exemption concept, or treating a phase-out as a flat disallowance.
Rule of thumb
When unsure where an item belongs, ask: does it reduce gross income (adjustment), reduce income after AGI (deduction), or reduce the tax itself (credit)? Placement determines the tax effect.