King George III probably suffered from bouts of mania symptomatic of bipolar disorder, new research suggests.
A computer analysis of hundreds of the King's letters challenges the theory that the monarch's strange behaviour was due to an inherited blood condition, porphyria.
Scientists found evidence of "acute mania", a term used to describe the kind of excitable, hyperactive state associated with bipolar disorder.
Once described as manic depression, bipolar disorder is characterised by extreme mood swings that include episodes of high excitement.
Lead researcher Peter Garrard, Professor of Neurology at St George's, University of London, said: "King George wrote very differently when unwell, compared to when he was healthy.
"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.
"It would be fascinating to look at how modern patients write during the manic phase of bipolar disorder, as this could create a definite link to King George and possibly other historical cases of the illness."
The monarch's problems were portrayed in the 1994 film The Madness of King George, starring Nigel Hawthorne in the title role.
The findings appear in the journal Public Library of Science ONE.