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Before Aerys II Targaryen, the man known as the "Mad King" was Britain's King George III, prone to bouts of mania that left him foaming at the mouth and writhing on the floor.

There have been no shortage of theories as to what afflicted George III, from psychotic to physiological disorders. Now, new research shows the king, who ruled Britain and Ireland from 1760 to 1820 (yes, America successfully revolted under his watch), was indeed suffering mental illness in his later years.

Researchers examined hundreds of the king's handwritten letters, Live Science reports, comparing earlier scribblings from when he seemed healthy with those from later in his 60-year reign when his manic episodes flared.

The team programmed a computer to recognize writing characteristics of those with mental disorders using 29 markers, including sentence structure, vocabulary, and word repetition. The loquacious king "wrote very differently when unwell," co-author Peter Garrard of St.

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George's University of London says in a statement. "In the manic periods, we could see that he used less-rich vocabulary and fewer adverbs. He repeated words less often, and there was a lower degree of redundancy, or wordiness," Garrard says.

A 1969 study said the king suffered a metabolic disorder called porphyria that turned his urine blue (a theory borrowed by The Madness of King George), while another said exposure to arsenic aggravated his condition, per Live Science.

Writing in the journal Plos One, the authors lay out their finding: "Acute mania now appears to be the diagnosis that fits best with the available behavioral data." (This American claims he is the rightful next king of England.)

This article originally appeared on Newser: The Real Reason Behind King George III's 'Madness'