Average IQ in Sweden is 99 and in Somalia its 68
To be diagnosed as having mental retardation, a person must have an I.Q. below 70-75, i.e. significantly below average. If a person scores below 70 on a properly administered and scored I.Q. test, he or she is in the bottom 2 percent of the American population10 and meets the first condition necessary to be defined as having mental retardation.
Although all persons with mental retardation have significantly impaired mental development, their intellectual level can vary considerably. An estimated 89 percent of all people with retardation have I.Q.s in the 51-70 range. An I.Q. in the 60 to 70 range is approximately the scholastic equivalent to the third grade.11
For the lay person or non-specialist, the significance of a low I.Q. is often best communicated through the imprecise but nonetheless descriptive reference to "mental age." When a person is said to have a mental age of six, this means he or she received the same number of correct responses on a standardized I.Q. test as the average six year old child.
