The mental health disorders of George III, their causes and consequences are important issues for historians. If accurate and correctly interpreted, they will provide valuable insights into his behaviour and decisions during his 50-year reign.

George was born 2 months premature and unlikely to survive. After a somewhat dysfunctional childhood and adolescence, he succeeded to the throne aged 22. Initially, his reign was turbulent, with frequent changes of ministers, and it was only in 1783 with the appointment of William Pitt as First Minister that he had a more stable government.

George had four, possibly five, episodes of mental ill health. In 1765 during this turbulent period, he had recurrent chest infections with some features suggestive of depression but no medical notes are extant. In 1788-1789 he had his first episode of serious mental ill health following probable cholelithiasis for which he spent 5 weeks taking the waters at Cheltenham. His behaviour during this period is suggestive of hypomania and was followed in October 1788 by an episode of acute mania meeting the DSM-IV and ICD-10 criteria. Recent studies using the OPCRIT programme support this diagnosis: the Young Mania Scale indicates a severe episode of mania (grade 4/4).

There are more than 100 volumes of medical notes together with many primary sources describing his behaviour during this and subsequent periods of mental ill health. By March 1789 he was in remission for 12 years before relapses in 1801, 1804 and 1810. During the intercurrent periods, there is evidence from his writings and behaviour of possible dysthymia and after his 1810 episode at the age of 70 he had a decade of chronic mental ill health, the subject of current research by my colleagues and I.

Variegate porphyria is one of the rarer forms of acute porphyria with a patient prevalence of 3 per million. Attacks are usually precipitated by exposure to medicinal agents unavailable to George III and characterised by severe abdominal pain often extending to the lower back and thighs. It is persistent, unrelenting and certainly not colicky or cramping, a feature of George's episodes. There are characteristic photosensitive skin lesions also not seen in George. Psychiatric symptomatology, usually a transient confusional state, occurs in less than 1% of acute attacks. Untreated severe attacks are often fatal or recur with increasing frequency and severity. From the prevalence data it would be predicted that some 180 living descendants would have clinically manifest porphyria; none have been reported.

In 1964, Ida Macalpine and Richard Hunter, mother and son psychiatrists, categorically stated that George was not ‘psychiatrically ill’ but suffered from acute intermittent porphyria later changed to the rarer and milder variegate porphyria; they rejected three detailed papers by experienced American psychiatrists reporting manic-depressive psychosis. In spite of detailed objections by porphyria experts, Macalpine and Hunter were able to garner support from historians, some psychiatrists and, surprisingly, The Royal Society. With the support of the play and film The Madness of King George by the former historian Alan Bennett and the composition Eight Songs for a Mad King by Peter Maxwell Davies, the diagnosis has gained general acceptance.

It might be argued that their claims, now shown to be unfounded, are of little current concern. However, their diagnosis has had untoward consequences. The diagnosis of acute porphyria rather than bipolar disorder has inhibited historians from providing explanations of George's final decade of cognitive impairment, his offers of abdication at times of stress, his persistence with policies well after their failure and the damaging relationships with his children. Finally, the portrayal of the incorrect diagnosis and by insinuation his apparent mistreatment has distorted the contributions of the King's specialist ‘mad doctors’, Dr Francis Willis and his colleagues, to the development of effective psychiatric practice.