Functional Neurological Disorder (FND) is caused by a problem in how the brain and nervous system send and receive signals, leading to disrupted functioning. The exact cause is unknown, but it is considered multifactorial, involving a combination of biological, psychological, and social risk factors. Stress, physical or mental trauma, and adverse life events can trigger or exacerbate symptoms, but FND can also occur with no known trigger. Some patients show changes in brain connectivity affecting voluntary movement and self-agency, indicating a neurological basis rather than purely psychological.

Predisposing factors may include other neurological or mental health conditions, chronic pain, ongoing life stress, childhood trauma, or neglect, though not all patients have such histories. FND is sometimes described as a "software issue" where the brain's hardware is intact, but the way it communicates and controls body functions is disrupted.

In summary, FND results from complex interactions between brain function abnormalities and psychological or social stressors, and its symptoms are real and disabling though the underlying cause remains incompletely understood.