The four-fifths rule (or "80% rule") comes from the EEOC's Uniform Guidelines on Employee Selection Procedures, 29 C.F.R. § 1607.4(D). It provides a practical screen for adverse impact: if the selection rate for any protected group is less than four-fifths (80%) of the rate for the highest-selected group, that gap is generally regarded as evidence of adverse impact.
It rests on Griggs v. Duke Power Co. (1971), where the Supreme Court held that employment practices neutral on their face but "discriminatory in operation" are unlawful. Intent is not required — effect is what matters.
How to read the ratio: divide each group's selection rate by the highest group's rate. 1.00 is parity; 0.80 is the threshold; below 0.80 flags the practice for scrutiny.
What it does NOT mean: a ratio under 0.80 does not by itself prove illegal discrimination — the employer may show the practice is job-related and consistent with business necessity. And a ratio above 0.80 does not guarantee lawfulness. It is a starting flag, not a verdict.