I know IQ testing has been around for over a century but I have never really understood the history behind it. Who came up with the idea, what were they originally trying to measure, and how different is a modern IQ test from whatever was being used 100 years ago? I am curious whether the original purpose of IQ testing matches how we use it today.
The origin story is more interesting than most people realize and the original purpose was actually pretty practical rather than theoretical. Alfred Binet and Theodore Simon developed the first practical intelligence test in France in 1905 at the request of the French government, which wanted a tool to identify children who needed additional educational support. Binet was actually skeptical of the idea that intelligence was a single fixed inherited trait and designed the test as a practical screening tool rather than a measure of innate ability. The concept of the intelligence quotient itself came later from German psychologist William Stern who proposed dividing mental age by chronological age and multiplying by 100. Lewis Terman at Stanford then adapted and expanded Binets work into the Stanford Binet, which became the dominant American intelligence test and introduced many of the conventions still used today. The modern deviation IQ replacing the original ratio formula came later still through David Wechsler whose test battery eventually became the most widely used clinical assessment in the world. So the tool went through multiple major revisions across several countries before becoming what we recognize today 
The irony is that Binet himself would probably be uncomfortable with how his test got used 
He explicitly warned against treating intelligence scores as fixed measures of innate ability and here we are a century later having exactly the arguments he was trying to prevent 