["Journal of the History of the Behavioral Sciences, Volume 62, Issue 3, Summer 2026. ", "\nABSTRACT\nThe existence of a “severe discrepancy” between measured ability and achievement has been the dominant criterion for identifying learning disabilities (“LD”) in American education since its codification into federal law in 1977. In this article, I argue that the history of LD is best understood not through the emergence of a diagnostic label, but through the evolution of its methods of identification. By tracing the rise and decline of the IQ–achievement discrepancy model amid shifting legal, psychometric, and policy contexts, I aim to show how declining faith in IQ tests as measures of potential threatened the credibility of LD as a viable construct for identifyin and treating “unexpected” underachievement. Abandoning discrepancy and embracing more pragmatic approaches like response to intervention (RTI) reflected political compromises as much as discrepancy had represented decades before. But it also represented an effort by experts in the LD field to protect the scientific integrity of LD by more effectively targeting its most recognizable characteristic—the failure to learn in comparison to age‐level peers. In the era of standards and accountability, this shift reflected the priorities of education reformers and the legislative impetus of No Child Left Behind. But it also represented a profound reorientation of the measurement of unexpected underachievement. By supplanting an increasingly discredited metric of intellectual potential (IQ), RTI and similar methods helped reinterpret LD as a specifically skills‐based construct unrelated to perennially ambiguous notions of intelligence.\n"]