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Mono-savants like Christopher, (THES, March 29) show very specialised, narrow abilities, low on abstract reasoning and linked to intense practice. Christopher may shed light on language learning, but he does not show "that assigning someone, anyone, a single IQ is silly".

IQ is an index of general intelligence, that is intellectual capacity underlying a range of cognitive tasks. To argue as Neil Smith and Ianthi-Maria Tsimpli do is like saying: "We have discovered an individual who with years of exercise has developed an exceptionally strong little finger on his left hand - but nothing else whatever. This proves that Daley Thompson never won gold medals for the decathlon. He could not have done, because there is no such thing as all-round athletic ability."

John Radford Emeritus Professor of Psychology University of East London