Trademark Infringement: Proving Likelihood of Confusion
By Casey Scott McKay ·
Trademark infringement turns on one question - whether an appreciable number of ordinary consumers are likely to be confused about who is behind the goods - and every federal circuit answers it with a slightly different multi-factor test inherited from a 1938 Restatement rather than from Congress. This article explains what a plaintiff must actually prove under Sections 32(1) and 43(a) of the Lanham Act, where the Polaroid, Sleekcraft, and DuPont factor lists came from, and how the thirteen tests differ in ways that matter to forum selection and to what evidence you need to collect. It works through the factors in the order courts actually weight them, showing that similarity of the marks, relatedness of the goods, and hard evidence of real-world confusion do most of the work while the rest break ties. It covers the anti-dissection and no-side-by-side rules, the crowded-field defense, why incontestability does not make a weak mark strong, the hearsay problem that sinks most actual-confusion evidence, and the advice-of-counsel waiver trap on intent. Separate sections map the four theories of confusion - forward, reverse, initial interest, and post-sale - and explain how the same factor list is re-weighted for each. The article closes with the procedural posture questions that decide cases early, the preclusive effect of TTAB findings after B&B Hardware, the defenses that beat or bypass confusion entirely, and the places the doctrine is genuinely unsettled.
IP and Technology > Trademarks | Article | Published 15 January 2026 - Updated 24 February 2026 | Casey Scott McKay - marksy.us
Summary. Courts weigh a set of factors to decide trademark infringement, and this article explains what actually moves the needle. It sets out the three elements of a Lanham Act claim, traces the Polaroid, Sleekcraft, and DuPont factor lists back to a 1938 Restatement, and compares all thirteen circuit tests in a single table. It then works through the factors in order of real-world importance — similarity of the marks, relatedness of the goods, mark strength, actual confusion, intent, sophistication, trade channels — with the evidentiary rules that decide whether your proof on each is worth anything. It maps the four theories of confusion, explains when a TTAB finding binds a district court, identifies the defenses that beat confusion or make it irrelevant, and marks the places the doctrine is genuinely unsettled.
Keywords: likelihood of confusion · trademark infringement · polaroid factors · sleekcraft factors · dupont factors · lanham act section 32 · section 43(a) · reverse confusion · initial interest confusion · post-sale confusion · actual confusion evidence · consumer survey · mark strength · crowded field · anti-dissection rule · consumer sophistication · b&b hardware · trade channels · sponsorship confusion · trademark litigation
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