How the IRS Selects Tax Returns for Audit: Red Flags and Triggers

Your Return Flagged at 1:30 a.m. by an Algorithm

Every tax return filed with the IRS goes through an automated scoring system before a human ever sees it. The system assigns each return a Discriminant Inventory Function (DIF) score — a number developed from decades of audit data that estimates the probability that examining the return would yield additional tax. Returns with high DIF scores are pulled for review. The IRS does not publish the formula, but researchers and tax professionals have identified the patterns. The overall audit rate fell to 0.44% for individuals in fiscal year 2022, the lowest in decades due to staffing reductions at the IRS. But that average conceals significant variation: certain income levels and return characteristics trigger audit rates many times higher.

Audit Rates Are Not Evenly Distributed

The IRS's own statistics reveal that audit risk concentrates at the extremes of income and specific return types:

Earned Income Tax Credit claimants at low income levels face a disproportionately high audit rate relative to their income, a result criticized by tax equity researchers, because EITC audits are primarily correspondence audits (mail-based) that the IRS can conduct cheaply at scale.

The Automated Matching Program: Where Most Audits Begin

The IRS receives copies of all information returns filed on your behalf: W-2s from employers, 1099s from banks, brokers, clients, and gig platforms, mortgage interest statements, and more. The Automated Underreporter (AUR) program compares these documents to what you actually reported. If a 1099-NEC for $8,000 of freelance income arrived at the IRS but your return shows no self-employment income, the mismatch triggers an automatic CP2000 notice — not technically an audit, but a proposed tax increase with a 30-day response window. Tens of millions of these are issued every year. The IRS collects billions annually from this matching process alone.

High-Risk Return Characteristics

Beyond the DIF score and matching programs, specific return features are associated with elevated audit selection:

• Round numbers throughout: Claiming $10,000 in vehicle expenses, $5,000 in home office expenses, and $3,000 in entertainment suggests estimation rather than record-keeping.
• Business losses claimed repeatedly: A Schedule C that shows losses for multiple consecutive years raises the question of whether the activity is a genuine business or a hobby under IRC Section 183. Hobby losses are not deductible.
• Home office deduction: Historically a red flag. The deduction is legitimate when properly calculated, but it adds a layer of scrutiny because of historical overclaiming.
• Large charitable contributions relative to income: A return showing $80,000 income and $30,000 in charitable deductions is a statistical outlier that may trigger the DIF score.
• 100% business use of a vehicle: Personal vehicles are rarely used 100% for business. A vehicle deduction based on 100% use invites scrutiny.

How the IRS Conducts Audits

Three main audit types exist, and they represent very different levels of intrusion:

Most individual audits are correspondence audits. The IRS sends a letter specifying what it questions and what documentation it needs. Responding thoroughly and on time — with organized records — resolves the majority of correspondence audits without further action.

What Triggers Referrals for Criminal Investigation

Civil audits concern additional tax owed plus interest and penalties. Criminal referrals — to the IRS Criminal Investigation division — require evidence of willful tax evasion or fraud. Triggers that elevate an audit toward criminal territory include: evidence of false documents submitted to auditors, unexplained cash deposits significantly exceeding reported income, complete absence of business records for a business claiming substantial deductions, and offshore account structures that were not disclosed via FBAR or FATCA reporting requirements.

Record Retention and Self-Defense

The IRS generally has three years from the return filing date to audit a return. This extends to six years if there is a substantial understatement of income (more than 25% of gross income omitted). There is no statute of limitations for fraudulent returns or unfiled returns. Best practices for audit protection include:

• Keep all receipts, bank statements, and records supporting each deduction for at least 4 years (6 if you have unusual income items)
• Use separate bank accounts for business and personal transactions
• Document the business purpose of expenses at the time of the expense, not years later
• Report all 1099 income — the matching program will find it anyway, and a failure to report looks intentional

This article is for informational purposes only and does not constitute financial advice.