Image caption Nigel Hawthorne in the National Theatre production of The Madness of George III. Modern medicine may help us to discover the real reasons behind King George III's erratic behaviour, writes historian Lucy Worsley. George III is well known in children's history books for being the mad king who lost America. In recent years, though, it has become fashionable among historians to put his madness down to the physical, genetic blood disorder called porphyria. Its symptoms include aches and pains, as well as blue urine.