Intellectual ability and social competence are continuously distributed variables, so there is no clear cut-off between normality and disability, or between the various degrees of disability. It became possible to examine the distribution of intelligence in a population when psychologists developed methods for measuring cognitive performance in the ®rst half of the twentieth century. Performance on intelligence tests approximates to a Gaussian (`normal') distribution. The frequency distribution of a Gaussian curve may be described completely by the mean and standard deviation: about 95 per cent of the population falls within two standard deviations either side of the mean. IQ (`intelligence quotient') scores are adjusted so that the population mean is 100 and the standard deviation is 15. An IQ of less than 70 (i.e. two standard deviations below the mean) is generally considered subnormal. About 2.5 per cent of the population might be expected to have an IQ below 70, but in practice the ®gure depends on factors such as education and welfare provision.