A system for diagnosing a neurological disorder, Parkinson's Disease, and a comorbid mental health condition, major depressive disorder is provided. The system comprises a smart device and a device including a memory and a processor. The smart device allows a participant to perform a cognitive task and the device receives data collected from the smart device in connection with the cognitive task performed by the participant. The device determines whether the participant has Parkinson's Disease based on the data collected and via a classification algorithm. If the participant has Parkinson's Disease, the device determines whether the participant has comorbid major depressive disorder.