Adverse Impact vs. Disparate Impact: The Four-Fifths Rule Explained

Adverse impact and disparate impact describe two halves of the same discrimination claim. Disparate impact is the legal theory under Title VII of the Civil Rights Act of 1964: the argument that a facially neutral employment policy disproportionately harms people in a protected class. Adverse impact is the statistical evidence used to prove it, most often a measurable gap in selection rates between demographic groups. In the debate over adverse impact vs. disparate impact, the short answer is that one is the claim and the other is the proof, and neither works without the other.

Disparate Impact as a Legal Theory

Title VII prohibits employment discrimination based on race, color, religion, sex, and national origin, and it covers employers with fifteen or more employees for at least twenty weeks in the current or preceding calendar year.¹ Most people picture discrimination as deliberate mistreatment, but the statute also reaches policies that look neutral yet produce discriminatory results.²

The Supreme Court set out this principle in Griggs v. Duke Power Co. in 1971. Duke Power required a high school diploma and passing scores on two aptitude tests before an employee could transfer out of its lowest-paying department. The requirements applied to everyone. There was no evidence the company adopted them to exclude anyone on purpose. The Court struck them down anyway, holding that practices “fair in form, but discriminatory in operation” violate Title VII when the employer cannot show they relate to job performance.³ Intent was irrelevant. Outcome was everything.

Congress codified the doctrine in the Civil Rights Act of 1991, adding Section 703(k) to Title VII. That provision now governs who has to prove what at each stage of a case.⁴

Adverse Impact as Statistical Evidence

Adverse impact is the numerical side of the same problem. It refers to a measurable gap in selection rates — hiring, promotion, termination, or any other employment decision — between demographic groups. HR professionals and the EEOC use adverse impact data to spot issues before a complaint is filed, and plaintiffs use the same kind of data to build cases when disputes reach the agency or the courts.

A general sense that a policy feels unfair will not carry a disparate impact claim. The theory requires numbers: how many applicants came from each group, how many were selected, and whether the difference between those rates is large enough to matter. Without that foundation, there is no case to bring.

The Four-Fifths Rule

Federal enforcement agencies use a specific benchmark. Under the Uniform Guidelines on Employee Selection Procedures, a selection rate for any race, sex, or ethnic group that falls below four-fifths (80 percent) of the rate for the group with the highest selection rate is generally treated as evidence of adverse impact.⁵

The arithmetic is easier than it sounds. Say a company hires 50 out of 100 male applicants, a selection rate of 50 percent. To clear the four-fifths threshold, the female selection rate needs to reach at least 40 percent (50% × 0.80 = 40%). If the company hires 30 out of 100 female applicants, that 30 percent rate falls below the threshold and raises a red flag.

The rule is a starting point, not a verdict. The guidelines themselves acknowledge that small sample sizes can produce misleading results. A gap that looks dramatic across 12 applicants may disappear across 200. Courts often supplement the four-fifths calculation with standard deviation analysis and other statistical tests when numbers are small or the context is complex.⁵ Even so, it is the first calculation most auditors and EEOC investigators reach for.

How the Two Concepts Work Together in a Case

A disparate impact case moves through three stages, and adverse impact evidence is the fuel for the first one.

The plaintiff must identify a specific employment practice and show, through adverse impact data, that it causes a significant disparity based on race, color, religion, sex, or national origin. Vague complaints about a company’s overall culture are not enough.⁴
If the plaintiff makes that showing, the burden shifts to the employer to prove the challenged practice is job-related for the position and consistent with business necessity. That is an actual burden of proof, not just a duty to offer a plausible explanation.⁴
Even if the employer successfully defends the practice, the plaintiff can still win by showing an alternative practice would serve the employer’s legitimate needs with less discriminatory impact, and the employer refused to adopt it.⁴

The employer’s defense at step two usually turns on validation: documented evidence that a test or requirement actually predicts or measures job performance. A warehouse can likely justify a strength test for workers who lift heavy loads. A height requirement for a desk job cannot survive, because height has nothing to do with administrative work.

How This Differs from Disparate Treatment

Searchers comparing adverse impact and disparate impact often mix in a third term: disparate treatment. The distinction is worth pinning down because the two theories require entirely different proof.

Disparate treatment is intentional discrimination. An employer refuses to promote women, steers applicants of one race into certain positions, or fires someone because of their religion. The plaintiff has to show a discriminatory motive, usually through direct statements, inconsistent explanations, or a clear pattern of treating similarly situated employees differently.

Disparate impact carries no intent requirement. The employer may have adopted the policy in complete good faith. The legal question is about effect, not motive, and whether the employer can justify the effect through business necessity. Griggs made that split explicit: good intentions do not save a policy that freezes out qualified candidates from protected classes.³

The remedies differ as well. Compensatory and punitive damages are available for intentional discrimination but generally not for pure disparate impact claims. Back pay is the main monetary remedy in disparate impact cases, alongside injunctive relief such as changes to the employer’s policies or reinstatement of affected employees.⁶

Filing Deadlines If You Suspect Adverse Impact

Spotting a disparity is only useful if you act on it in time. You generally have 180 calendar days from the discriminatory event to file a charge with the EEOC. That window extends to 300 days if a state or local agency enforces a law prohibiting the same type of discrimination.⁷ Weekends and holidays count toward the total, but if the deadline lands on a weekend or holiday, you have until the next business day.

Before filing a Title VII lawsuit in federal court, you must obtain a Notice of Right to Sue from the EEOC. The agency generally takes 180 days to investigate before issuing that notice, though it may agree to issue one earlier in some circumstances.⁸ Federal employees face a much shorter clock: they must contact their agency’s EEO counselor within 45 days of the discriminatory event.⁷ Missing any of these deadlines can bar the claim entirely, no matter how strong the underlying evidence.

1
U.S. Equal Employment Opportunity Commission. Title VII of the Civil Rights Act of 1964
2
Civil Rights Division. Laws We Enforce – Section: Title VII of the Civil Rights Act of 1964
3
Justia. Griggs v. Duke Power Co., 401 U.S. 424
4
Office of the Law Revision Counsel. 42 U.S. Code 2000e-2 – Unlawful Employment Practices
5
eCFR. 29 CFR 1607.4 – Information on Impact
6
U.S. Equal Employment Opportunity Commission. Remedies For Employment Discrimination
7
U.S. Equal Employment Opportunity Commission. Time Limits For Filing A Charge
8
U.S. Equal Employment Opportunity Commission. What You Can Expect After You File a Charge