Mentally dull children are those who, at about the middle of their school career and without being mentally deficient, are behind the normal average of their age to the extent of two years or two classes. This corresponds to a retardation of between 15 and 20% of their age or to about twice the 'standard deviation', and roughly to an IQ of between 80 and 85.
In the 1980's, a study showed that up to 25% of the Chinese children in areas low in iodine were mildly retarded, with IQ's of 50 to 69. In 1997, an iodine deficiency in Georgia is thought to have reduced the average IQ of 50,000 babies by 10 points.