Kingsfield · Research · Antitrust & Competition

What is the legal standard for blocking a merger or acquisition under the Clayton Act?

Published 2026-06-23 · U.S. federal law

Clayton Act Section 7 prohibits an acquisition of stock or assets where the effect may be substantially to lessen competition, or to tend to create a monopoly, in any line of commerce in any section of the country.

The answer

The substantive merger test

15 U.S.C. § 18 bars a person engaged in commerce from acquiring the stock or assets of another person where in any line of commerce in any section of the country the effect of the acquisition may be substantially to lessen competition, or to tend to create a monopoly. The standard is probabilistic and forward-looking; the government need not prove a completed harm, only a reasonable probability of anticompetitive effect.

Substance versus procedure

Section 7 supplies the substantive liability standard a court applies when deciding whether a transaction is unlawful. It is distinct from the premerger reporting machinery, which governs whether and when parties must notify the agencies before closing.

The judged input

What the AI drafted

Submitted to the judge

This is an excerpt from a draft merger-clearance opinion — the kind of work product a lawyer generates with a legal-AI drafting tool, then has to stand behind. Kingsfield does not write it; it rules on the citations the model put in it. This draft cites two authorities; one of them is wrong.

AI draft excerpt — merger-clearance opinion

The Company has asked for our opinion on antitrust exposure from its proposed acquisition of a competitor's assets. The controlling substantive test is 15 U.S.C. § 18, which prohibits an acquisition where in any line of commerce the effect may be substantially to lessen competition or to tend to create a monopoly. We note that the separate obligation to file premerger notification and observe the waiting period is imposed by 15 U.S.C. § 18a, which we treat here as supplying the substantive standard for whether the merger may be enjoined. On the facts presented, the overlap in the relevant market is modest and we believe the transaction is defensible under the applicable standard.

The judge ruled on every citation as the draft used it — it accepted 15 U.S.C. § 18 and rejected 15 U.S.C. § 18a. Here is why.

The verdict

How Kingsfield ruled

Ruled 2026-06-23

Each citation in the draft above was submitted to the Kingsfield judge and ruled against the primary-law corpus — Accept, Reject, or Inconclusive, per citation. These are live verdicts, not editorial. Each card shows the claim the draft made and the verbatim authority the verdict was rendered against.

Accept15 U.S.C. § 18

The draft claimed: No person engaged in commerce shall acquire the stock or assets of another person where in any line of commerce in any section of the country the effect of such acquisition may be substantially to lessen competition, or to tend to create a monopoly.

“§ 18. Acquisition by one corporation of stock of another No person engaged in commerce or in any activity affecting commerce shall acquire, directly or indirectly, the whole or any part of the stock or other share capital and no person subject to the jurisdiction of the Federal Trade Commission shall acquire the whole or any part of the a”

Cite found; proposition supported by the cited text.

Reject15 U.S.C. § 18a

The draft claimed: Section 18a supplies the substantive standard under which a merger may be enjoined, prohibiting acquisitions that may substantially lessen competition.

Cite found, but the cited text does not support the claim. 15 U.S.C. 18a is the Hart-Scott-Rodino premerger notification provision establishing filing and waiting-period procedures; the substantive may-substantially-lessen-competition merger standard is at 15 U.S.C. 18. Regenerate with the correct authority.

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This page is legal information, not legal advice, and does not create an attorney-client relationship. The draft shown is an illustration of a typical AI answer; verdicts reflect the cited authority in the Kingsfield corpus as of the ruling date shown above.