The Causation Standard That Decides Most Retaliation Cases

A manager writes someone up for performance problems six weeks after that employee filed an internal harassment complaint. The write-up may be entirely justified on the merits. It may also be exhibit one in a retaliation lawsuit. The legal question that decides which one it is — causation — is where most retaliation claims actually live or die, and the standard governing it is more fact-dependent, and more contested across circuits, than the clean rule stated in most training materials.

The Legal Standard: But-For Causation After Nassar

Title VII retaliation claims are governed by a causation standard the Supreme Court set out in University of Texas Southwestern Medical Center v. Nassar, 570 U.S. 338 (2013). The Court held that retaliation claims require proof that the adverse action would not have occurred but for the protected activity — a stricter standard than the "motivating factor" test that governs status-based discrimination claims under the 1991 Civil Rights Act amendments. The distinction matters practically: an employer defending a discrimination claim can lose even if bias was only one of several reasons for a decision. An employer defending a retaliation claim wins if it can show the same decision would have been made regardless of the protected activity.

But-for causation does not require the protected activity to be the sole cause of the adverse action — only a necessary one. The Supreme Court's later discussion of but-for causation in Bostock v. Clayton County, 590 U.S. 644 (2020), reinforced that point in the discrimination context: a factor can be a but-for cause even when other factors also contributed, as long as the outcome would have been different without it. Investigators and ER teams sometimes read Nassar as requiring retaliation to be the only motive. It doesn't. It requires that removing the protected activity from the equation would have changed the outcome.

What Courts Do With Temporal Proximity

Because direct evidence of retaliatory intent is rare, plaintiffs frequently rely on timing — the adverse action followed the protected activity closely enough to suggest a connection. The Supreme Court addressed the limits of this inference in Clark County School District v. Breeden, 532 U.S. 268 (2001) (per curiam), holding that a twenty-month gap between protected activity and an adverse action was, standing alone, too attenuated to establish causation. The opinion's language — that temporal proximity must be "very close" to carry an inference of causation on its own — has become the baseline test cited in retaliation opinions across circuits.

What counts as "very close" is where the law gets genuinely inconsistent. The Sixth Circuit has treated proximity of a few weeks, and in some cases days, as sufficient by itself to establish a prima facie case — see Mickey v. Zeidler Tool & Die Co., 516 F.3d 516 (6th Cir. 2008), where three days between complaint and termination supported an inference of retaliation without additional evidence. Other circuits are more skeptical of proximity alone, treating it as a factor that raises suspicion but requires corroboration — comparative evidence, inconsistent explanations, or deviation from stated policy — before it can carry a case past summary judgment. Practitioners defending a personnel decision made shortly after a complaint should not assume that a plausible business justification automatically neutralizes the timing; in several circuits, proximity plus a thin or shifting explanation is enough for a retaliation claim to survive dismissal.

Where the Circuits Split, and Why It Matters for Documentation

The deeper disagreement is less about the but-for standard itself, which Nassar settled, and more about how much evidence is needed to meet it at the summary judgment stage — and how courts should weigh an employer's contemporaneous documentation against post-hoc justification. This is where investigative recordkeeping does real legal work. A performance concern documented before a complaint was filed is powerful evidence that the same decision was already in motion. A performance concern that surfaces for the first time in a termination memo written after the complaint invites exactly the inference Nassar's but-for standard is designed to test. Courts evaluating a retaliation defense are, in practice, asking whether the paper trail predates or postdates the protected activity — a question closely related to how courts evaluate the quality of the underlying investigation record more broadly.

What Good Practice Looks Like

Investigators and HR partners handling personnel actions that follow protected activity — even by weeks — should treat the sequence itself as a documentation problem, not just a legal one. That means dating performance concerns as they arise rather than reconstructing them later, separating the decision-maker's knowledge of the protected activity from the decision timeline where the facts allow it, and being explicit in the record about why an action's timing coincided with, rather than resulted from, the protected conduct. None of this changes the underlying but-for standard. It changes whether an employer can actually prove it met that standard when a court asks.

Marshal's audit chain logs every investigator action with a tamper-evident hash — the kind of documented diligence that makes retaliation defenses possible.