What is "IQ"

It was also observed that the gaps between children's mental ages and their chronological ages widened as the children got older.

The 6-year-old with the mental age of 8 had a mental age of 12 by the time he was 9 and a mental age of 16 by the time he was12. Similarly, the 6-year-old with a mental age of 4 had a mental age of 6 when he was 9 and a mental age of 8 when he was 12. In 1912, the German psychologist, William Stern, noticed that even though the gap between mental age and chronological age widens as a child matures, the ratio of mental age to chronological age remains constant (and, as we will see, remains essentially constant throughout life).

This constant ratio of mental age divided by chronological age was given the name "Intelligence Quotient". Actually, the intelligence quotient is defined as 100 times the Mental Age (MA) divided by the Chronological Age (CA).

IQ = 100 MA/CA.