What Does ‘IQ’ Actually Stand For?

In the simplest terms, IQ is a number that scores a person’s intelligence in relation to that of their peers. The average IQ in any given peer group is 100, which means anyone who has an IQ above 100 is considered smarter than average, at least by those metrics. While it’s common knowledge that IQ is shorthand for intelligence, how the number is calculated is less understood — and for that matter, so is what it stands for.

The modern intelligence test was created in 1904 when French psychologist Alfred Binet wrote one to help the French government determine which children might need additional help at school. Stanford University psychologist Lewis Terman adapted Binet’s test in 1916 into the Stanford-Binet IQ test, which determined someone’s IQ score. The Stanford-Binet IQ test is now in its fifth iteration and still being used in modern cognitive testing.

But Terman didn’t come up with the initialism “IQ.” In 1912, psychologist William Stern coined the German-inspired term intelligenzquotient, or “intelligence quotient” (IQ). If you think back to math class, “quotient” means “the number resulting from the division of one number by another.” Terman borrowed this term for his test. The first IQ scores were determined by dividing two relevant numbers — the person’s mental age divided by their chronological age — and then multiplying that quotient by 100. The concept of mental age was introduced by a test created by Binet and Théodore Simon in 1908 and adopted by Stern for the IQ score determined by ratio. To give an example, a 25-year-old who had the intelligence of a 35-year-old would have an IQ of 140 (35 divided by 25, times 100). 

Today, IQ still ranges from 40 to 160, but the formula of dividing mental age by chronological age was thrown out as of 1960. In the newer model, a statistically significant number of IQ tests are compared, and the mean (average) is set at 100. Then the standard deviation is calculated to place the rest of the scores in a bell curve. The 25-year-old who previously had an IQ of 140 will now have a different IQ because it’s not based on their mental age. Instead the Stanford-Binet test scores are plotted in a bell curve to determine the IQ score of each individual. Approximately 68% of IQs fall within the range of 85 to 115, or within one standard deviation of the mean. Scores below 70 may indicate some type of developmental disability, and scores above 130 indicate gifted intelligence. 

While it’s fun to talk about IQ, this number shouldn’t serve as the be-all and end-all determiner of intelligence. If anything, IQ is an indication of one’s reasoning ability through problem-solving, compared with others who may be considered their peers. If someone took an IQ test before and after 1960, they could get a very different score, based on how the numbers are calculated. Furthermore, research shows that IQ scores have been rising steadily in recent decades, likely due to factors such as better education, improved nutrition, and smaller family sizes.