Did the inability to complete IQ assessments in adult life reflect a true reduction in intellectual capacity?: Previous cognitive data (many individuals had been assessed several times as children/ adolescents) and clinical case records of participants who could no longer be directly assessed were examined in order to explore possible reasons for the inability to complete formal IQ tests in adulthood. In the case of one man, his current level of impairment was due to a severe stroke in his mid-50s. Of the remaining 14, five individuals had shown no evidence of IQ decline in childhood, but in their teens they had developed severe and persisting behaviour problems that subsequently affected their ability to complete IQ assessments as adults. In four individuals, early IQ results had been inconsistent, so it was not possible to determine if there had been cognitive decline during childhood. However, five individuals had shown clear evidence of a decline in IQ scores during the childhood years, and all but one of these had developed epilepsy. Decline in IQ scores in childhood was also associated with the presence of severe behavioural problems and significant language impairment that made cognitive testing impractical (see Table S4 for characteristics of these individuals).