SECURITIES LITIGATION | 2026-09-30

Halliburton II: How Defendants Rebut the Fraud-on-the-Market Presumption With Price-Impact Evidence

The Supreme Court refused to overrule Basic — but held that a defendant may defeat class certification by showing the alleged misrepresentation had no impact on the stock price.

The fraud-on-the-market presumption from Basic v. Levinson is what makes securities class actions possible — but it is rebuttable. The pivotal modern question is when and how a defendant may rebut it. In Halliburton Co. v. Erica P. John

After Basic, the fight in securities class actions shifted from whether the presumption exists to whether a defendant can knock it out early. Halliburton II defined that fight.

The Setup: A Rebuttable Presumption

Under Basic, plaintiffs who satisfy the predicates (a public, material misrepresentation, an efficient market, and trading before the truth emerged) earn a presumption that they relied on the integrity of the market price. Basic expressly made the presumption rebuttable by evidence severing the link between the misrepresentation and the price paid. Halliburton II addressed how that rebuttal operates.

What the Court Declined to Do

The defendant urged the Court to overrule Basic entirely, arguing that modern economic theory undermines the efficient-market premise. The Court refused. It reaffirmed that the fraud-on-the-market presumption survives, declining to require every plaintiff to prove direct reliance. The presumption remained intact.

What the Court Did Hold: Price Impact at Certification

Crucially, the Court held that a defendant may, at the class-certification stage, present evidence that the alleged misrepresentation had no price impact — no effect on the market price — to rebut the Basic presumption before a class is certified. The logic follows from Basic itself: if the misrepresentation did not affect the price, then investors who bought at that price could not have relied on it through the market, and the presumption’s foundation collapses. Because class certification is often the decisive moment in these cases, allowing the price-impact challenge there is highly consequential.

Price Impact vs. Materiality and Loss Causation

Halliburton II drew careful distinctions that continue to matter:

Materiality is not decided at certification. An earlier decision (Amgen) held that materiality is a merits question common to the class and need not be proven at certification. Halliburton II did not disturb that — price impact is a distinct inquiry.
Price impact differs from loss causation. Loss causation concerns whether the corrective disclosure caused the plaintiff’s loss; price impact concerns whether the misrepresentation affected the price in the first place. A defendant can concede materiality yet still argue no price impact.

The Central Role of Event Studies

Price impact is proven or disproven largely through event studies — statistical analyses that isolate how a security’s price moved in response to specific disclosures, controlling for market- and industry-wide movements. A defendant may point to the absence of a statistically significant price reaction to the alleged misstatement (or to a lack of price decline when the truth emerged) to argue no price impact. Plaintiffs respond with their own event-study evidence, including theories such as price maintenance (where a misstatement keeps an already-inflated price from falling). The battle of experts at certification is now a defining feature of securities litigation.

A Later Refinement

The Supreme Court returned to these issues in a subsequent decision (Goldman Sachs Group v. Arkansas Teacher Retirement System), which addressed how courts should assess price impact when the alleged misstatements are generic, and clarified that the defendant bears the burden of persuasion on the absence of price impact. The through-line remains: price impact is the mechanism by which the Basic presumption is contested at certification.

Why It Matters

For issuers, Halliburton II created a meaningful path to defeat class certification without overturning Basic — by investing early in rigorous price-impact analysis. For investors, it means the certification record must be built with event-study evidence anticipating the price-impact challenge. Either way, the economics of how a disclosure moved the price now sit at the heart of the case.

Frequently Asked Questions

Did Halliburton II overrule Basic?

No — the Court reaffirmed the fraud-on-the-market presumption and declined to require direct proof of reliance from every plaintiff.

What did Halliburton II change?

It held that defendants may rebut the presumption at the class-certification stage with evidence that the misrepresentation had no impact on the stock price.

What is price impact?

Whether the alleged misrepresentation actually affected the market price of the security — the foundation of the fraud-on-the-market presumption.

How is price impact proven?

Largely through event studies that isolate the price reaction to specific disclosures, controlling for market and industry factors.

Is price impact the same as materiality?

No — materiality is a common merits question not decided at certification (per Amgen); price impact is a distinct inquiry that can be raised at certification.

Who bears the burden on price impact?

Later Supreme Court guidance clarified that the defendant bears the burden of persuasion to show the absence of price impact.

Related Securities & Governance Guides

Talk to a Securities Attorney About Price Impact

Price impact can decide whether a securities class is certified. Trembach Law Firm advises California issuers and investors on class-certification strategy. Call (818) 514-7680.

Contact Trembach Law Firm at (818) 514-7680 for a confidential consultation.

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