IQ stands for Intelligence Quotient, although it originally referred to the first intelligence scale, developed by French psychologist, Alfred Binet, to measure mental ability in children, it is now used in conjunction with several intelligence scales and is a considered a general indicator of intelligence.
IQ and Intelligence Scales
The term 'intelligence quotient' was first coined in 1912 by German psychologist, William Stern in reference to the intelligence tests developed by psychologists Alfred Binet and Theodore Simon, who wanted to identify students that needed special help with the school curriculum. The original formula for calculating IQ was:
(mental age divided by chronological age) x 100
In other words, a person's intelligence level was measured as a quotient of their estimated mental age and chronological age. A 10-year-old child that had the mental age of a 12-year-old was considered to have an above average IQ of 120 [(12/10) x 100]
In the 1930's an intelligence test explicitly for adults was designed by American psychologist, David Wechsler, called the Wechsler Adult Intelligence Scale (WAIS). This was followed by the Wechsler Intelligence Scale for Children (WISC). These scales differed from the earlier versions by basing the scores on a standardised normal distribution rather than on an age-based quotient, because the latter only worked for children. Using the newer method, an average IQ of 100 was set as the centre value on a 'bell curve', with a standard deviation of 15 points and each score was "ranked" by a projection on the normal distribution.