But-For Causation in Civil Litigation: How Expert Analysis Tests the Causal Chain

But-for causation asks whether an injury or loss would have occurred without the conduct alleged to be wrongful. When the answer requires specialized knowledge, a qualified expert may help analyze the factual causal chain.

This guide explains how expert analysis may help counsel and the factfinder evaluate a proposed causal explanation, consider alternative causes, and identify areas of uncertainty. An expert does not decide the governing law or determine liability.

What But-For Causation Means

But-for causation generally asks whether the outcome would have occurred if the identified factor were absent. In Comcast Corp. v. National Association of African American-Owned Media, the Supreme Court of the United States held that a plaintiff bringing a claim under 42 U.S.C. § 1981 must show that race was a but-for cause of the plaintiff’s injury.

The conduct need not be the only cause. In Bostock v. Clayton County, a Title VII case, the Court explained that an event may have multiple but-for causes.

The governing causation standard depends on the claim and applicable law. Counsel should also determine how that law treats concurrent and independently sufficient causes.

But-For Cause and Proximate Cause Are Different

But-for cause usually addresses whether the conduct was factually necessary to the outcome. Proximate-cause or scope-of-liability rules address whether the harm is sufficiently connected to the conduct for legal responsibility. Establishing factual cause does not automatically establish proximate cause.

In Bank of America Corp. v. City of Miami, the Supreme Court held that foreseeability alone was insufficient to establish proximate cause under the Fair Housing Act. The required causal connection depended on the specific statutory claim.

Counsel evaluates the expert’s factual analysis within the governing legal framework. The court decides questions of law and admissibility, including preliminary questions governed by the applicable evidentiary rules. The appropriate factfinder evaluates disputed facts concerning the merits and the weight of admitted testimony.

When Expert Causation Analysis Helps

Expert analysis may help when the causal issue requires knowledge outside ordinary experience, with the appropriate discipline depending on the specific causal link in dispute.

For example:

• A forensic engineer or accident reconstruction professional may examine physical evidence, scene data, maintenance and design records, testing, and calculations to evaluate an event sequence or failure mechanism.

• A medical professional, toxicologist, epidemiologist, or exposure scientist may analyze clinical findings, timing, exposure information, scientific literature, and competing explanations.

• An economist, accountant, or valuation professional may compare actual financial results with a supported counterfactual using historical performance, market information, forecasts, and sensitivity testing.

• A statistician may assess patterns, comparison groups, uncertainty, and potential confounding variables.

The Reference Manual on Scientific Evidence: Fourth Edition, published by the National Academies of Sciences, Engineering, and Medicine and the Federal Judicial Center, includes reference guides relevant to many of these disciplines.

How Experts Test the Causal Chain

A well-supported but-for analysis generally identifies the outcome and factor at issue and evaluates the available evidence. Where appropriate, the expert may compare the supported factual scenario with a counterfactual, meaning an evidence-based scenario in which the disputed factor is changed or removed.

Depending on the field, that process may include:

• Organizing records, measurements, physical evidence, testimony, or electronic data into a timeline

• Identifying disputed facts and assumptions provided by counsel

• Applying a method appropriate to the expert’s discipline

• Evaluating plausible alternative explanations

• Testing whether reasonable changes to important inputs alter the conclusion

• Explaining missing information, uncertainty, and methodological limits

Timing or sequence alone does not necessarily establish causation. The conclusion should explain how the method applies to the case-specific facts rather than rely on a generic theory or assume the desired counterfactual outcome.

Expert Admissibility and Disclosure in Federal Civil Litigation

In federal proceedings governed by the Federal Rules of Evidence, Rule 702 governs the admissibility of expert testimony. A witness must be qualified as an expert by knowledge, skill, experience, training, or education. The proponent must also demonstrate to the court that it is more likely than not that the expert’s specialized knowledge will help the trier of fact, the testimony is based on sufficient facts or data and is the product of reliable principles and methods, and the expert’s opinion reflects a reliable application of those principles and methods to the facts of the case. Other evidentiary rules may also apply.

In civil actions and proceedings governed by the Federal Rules of Civil Procedure, Rule 26(a)(2) separately governs expert disclosures. Unless the parties stipulate otherwise or the court orders otherwise, Rule 26(a)(2)(B) requires the disclosure to include a written report prepared and signed by a witness retained or specially employed to provide expert testimony. It also applies to a party’s employee whose duties regularly involve giving expert testimony.

The report must include a complete statement of the expert’s opinions and their bases and reasons, along with the facts or data considered. It must also identify any exhibits that will be used to summarize or support the opinions and provide the required information about qualifications, prior testimony, and compensation.

For state-court proceedings, counsel should review the applicable substantive law, evidentiary and procedural rules, jury instructions, and scheduling orders.