(Revised May 2025)

The mechanisms driving disparities in mental healthcare are not well understood. This paper develops a structural model of diagnosis for Attention Deficit Hyperactivity Disorder (ADHD), highlighting how patient and physician factors can result in disparities. Using electronic health record data, I estimate model parameters and decompose the observed male:female ADHD diagnostic difference of 2.3:1. Simulations show that only 46-55% of this diagnostic difference can be explained by underlying symptom prevalence, with the remainder driven by differences in diagnostic thresholds. I find that physicians view missed diagnosis to be costlier for their male patients, which I argue may have economic justifications.