<p>Alzheimer's disease clinical trials have failed at a rate exceeding 99% over two decades, and neuroimaging biomarkers developed in discovery cohorts have repeatedly disappointed in validation and clinical application. Standard explanations address symptoms rather than structural causes. I argue that Alzheimer's disease research is caught in a Dual Diversity Crisis: the simultaneous neglect of population diversity and individual neurobiological heterogeneity as compounding validity threats. The first dimension concerns the systematic over-reliance on Western, young, healthy, and university-affiliated (WYHU) research samples, from which biomarker normative standards are derived and then applied universally to a demographically distinct clinical population. The second concerns the erasure, through group-level analysis, of the substantial neurobiological heterogeneity that exists within any sample regardless of its demographic composition. The critical structural insight is that these two problems do not add; they multiply. A WYHU-derived group average is doubly unrepresentative: it misrepresents the target population demographically and conceals the individual variance that exists even within that already-unrepresentative sample. FDG-PET evidence demonstrates that biological sex accounts for approximately 30 times more metabolic variance than diagnostic category in patients with equivalent symptom profiles, directly challenging the construct validity of threshold-based trial inclusion. Correcting the Dual Diversity Crisis requires treating demographic diversity and individual neurobiological characterization as primary design parameters, not post-hoc corrections. Until this reconceptualization occurs, translational failure in Alzheimer's disease research remains the structurally expected outcome.</p>