The concept of Intelligence Quotient or IQ was developed by—

The concept of Intelligence Quotient (IQ) was developed by German psychologist William Stern in 1912. He proposed the formula IQ = (Mental Age / Chronological Age) × 100. While Alfred Binet developed the first practical intelligence test, it was Stern who introduced the IQ ratio concept. Lewis Terman later adapted Binet's test and Stern's IQ formula to create the Stanford-Binet Intelligence Scale, which became widely used in the United States.