Discrimination without a discriminatory intent - a neutral rule, test or practice that falls much harder on a protected group and cannot be justified by business necessity - and the statutes under which that theory is and is not available.
Most discrimination claims allege disparate treatment: the employer, landlord or lender treated the person differently because of who they are. Disparate impact is the other theory. It targets a practice that is neutral on its face and applied to everyone - a strength test, a high-school-diploma requirement, a criminal-record screen, a minimum credit score, a zoning rule - but that disproportionately excludes members of a protected group, where the practice cannot be shown to be necessary for the job or the legitimate purpose it serves. No intent to discriminate needs to be proved; the effect and the justification are what is litigated.
The claim proceeds in stages. The person challenging the practice must identify a specific practice and show, usually with statistics, that it causes a significantly disproportionate effect on a protected group. The defendant then must show that the practice is job-related and consistent with business necessity, or serves a substantial, legitimate, non-discriminatory interest. If it does, the challenger may still prevail by showing that a less discriminatory alternative would serve the same interest and the defendant refused to adopt it. Statistical evidence is central, and the choice of comparison population is frequently the whole argument.
The theory is not available under every civil-rights law. It is written into Title VII for employment, recognised by the Supreme Court under the Fair Housing Act, applied to age claims in a narrower form, and available under the Equal Credit Opportunity Act and Title VI regulations in various ways - but it is not available under the Constitution's equal-protection guarantee, which requires proof of discriminatory purpose, and its scope under particular statutes has been narrowed and broadened by regulation and litigation over the years. Whether disparate impact can be pleaded at all is therefore the first question in any case built on it.
This is not a claim an individual assembles alone: it depends on data about how a practice affected a whole group, which the defendant holds and will produce only in litigation or an agency investigation. A lawyer or a fair-employment or fair-housing agency can tell you whether the practice you are describing is one the theory reaches and whether there is enough of a pattern to investigate. An individual who was turned down under a neutral rule usually has a stronger and faster claim if there is any evidence of intent - and a lawyer will look for that first.
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