Sir Keith Joseph, the father of Thatcherism whose free market principles are still followed to some extent by Tony Blair, had a form of autism that is reflected in his political philosophy, a psychiatrist believes.

The former Conservative education secretary, who was Mrs Thatcher's mentor in the 1970s and 1980s, had Asperger's syndrome, a condition that renders sufferers unable to interpret social situations or to empathise with other people, according to Michael Fitzgerald, professor of child and adolescent psychiatry at Trinity College Dublin.

Professor Fitzgerald conducted armchair diagnoses on prominent politicians, artists and scientists using contemporaneous records. He presented the results to the conference of the Royal College of Psychiatrists in Glasgow yesterday.

Joseph - who died in 1994 - was a brilliant lawyer who served in Harold Macmillan's government in the 1960s but was prone to eccentric behaviour and errors of judgement that can be attributed to his mental condition, Professor Fitzgerald said. In the early 1970s, he was urged by friends to challenge Edward Heath for the Tory leadership but he lost any chance of winning after making a speech in Birmingham in 1974 in which he implied that the lower classes should be deterred from having children.

Professsor Fitzgerald said: "That is the kind of comment he would make and mean it from the depths of his heart but it was absolutely strange. He had a lack of empathy and he was naïve in social situations. Once attending a camping exhibition, he surprised visitors by giving a lecture on Communism. He was regarded as so eccentric that the other members of the Cabinet suspended normal rules of behaviour for him."

After accepting he had no chance of winning the leadership of the party, Joseph urged Mrs Thatcher to stand and they became close political friends. In the mid-1970s, Joseph set up the Centre for Policy Studies which developed the free market ideas of the US economist Milton Friedman and impressed them on Mrs Thatcher.

"His ideas have influenced politics for 20 years. Monetarism has some of the characteristics of Asperger's in its insensitivity and its harshness - that is my point, the man and what he does in life are one. It is important to know this because these people control the destiny of the nation," Professor Fitzgerald said.

According to Professor Fitzgerald, other politicians who showed symptoms of Asperger's include Enoch Powell, the Tory MP and former health secretary whose "Rivers of Blood" speech in 1968 provoked widespread protests and blighted his career.

Eamon de Valera, one of the dominant political figures in 20th-century Ireland and a leader of Ireland's struggle for independence, was also afflicted, according to Professor Fitzgerald.

He also claimed that creative geniuses including W B Yeats, George Orwell and Sir Isaac Newton had Asperger's. Professor Fitzgerald said: "I take all the biographical data that was collected contemporaneously. It is unbiased information and that gives it increased validity.

"Such people are not distracted by human things like wanting to chat or have coffee."

The 'maleness condition'

Asperger's syndrome is rare but increasing. Two to three children in 10,000 are affected and the condition is four times commoner in boys. People with Asperger's are mostly of normal intelligence but have narrow, intense or obsessive interests. They suffer social impairment and have difficulty making friends and forming and keeping relationships.

They lack the ability to understand the subtext of social situations, and make remarks that cause offence or behave in ways that breach the rules of acceptability. They have to learn social skills intellectually rather than intuitively. Like autism, of which it is a mild form, Asperger's runs in families. The university professor of computer science with the brilliant mind, few friends and an obssessive interest in model trains is the classic sufferer. Michael Fitzgerald, professor of psychiatry at Trinity College Dublin, claims his research has proved that the former Irish politician Eamon de Valera, right and W B Yeats, left, showed symptoms of the condition.