Why are long term disability claims related to mental health sometimes denied?

As an "invisible" disability, diagnosing and assessing the severity mental illnesses often requires medical providers to consider an individual's self-reported symptoms. Insurers seeking to validate disability claims may their own doctors to review an individual's medical history, require new testing or assessments, and may conduct surveillance to determine if an applicant is as disabled as they claim. Sometimes insurers will use information or alternative opinions in these new findings as a basis to deny a claim.