Alfred Binet was born in Nice, France in 1857. His family was interested in his pursuing Law, he preferred Psychology. He became interested in intelligence and the teaching of children and began to develop a testing system to evaluate school children and their requirements for teaching. He tested different ages and compared the results. His purpose was to identify students who needed more attention.
In 1905, this vision of identifying at-risk students became reality as Binet developed the first test comprised of 30 different types of questions relating to everyday life. Participants were asked questions ranging from sensory tasks to verbal abstractions. In 1911, revisions to the original intelligence test allowed mental ages to be assigned to test participants based on their performances. The term mental age refers to the mental sophistication of an individual, as opposed to chronological age, which refers to the actual physical age. An eight-year-old who performed at an twelve-year-old's standards would have a mental age of twelve. 12/8. 150. However, an eight-year-old who performed at a four-year-old's level would acquire a mental age of four. 4/8. 50. It is from these first few tests that the entire field of modern intelligence testing bases its foundations upon. A really amazing leap.
It is important that Binet be understood. He was making no generalizations about intelligence; he was trying to identify vulnerable students whose learning needed support. When Stanford psychologist Lewis M. Terman imported Alfred Binet's original test from France and developed the Stanford–Binet IQ test, he gave a hereditarian interpretation to the results. Binet had vigorously rejected developing this style of test.
In the Bell Curve, Herrnstein and Murray--because of their racial generalities--actually made people care about the arcane IQ measurements. They argued that IQ was predicable across groups. But they also said this: "...you cannot predict what a given person will do from his IQ score.... On the other hand, despite the low association at the individual level, large differences in social behavior separate groups of people when the groups differ intellectually on the average."
So IQ testing has no predictive value for individuals. (Put aside the group question.) There are many reasons for this but the basic problem is that no one, no one, can define intelligence. No one knows what they are measuring. Binet in his original work was making no effort to identify smart or dumb people; he was trying to measure which kids needed more attention than others in their peer group. That's it. We now know how wise Binet was. IQs change. IQ are not consistent among people. Why? Because we do not know what we are measuring. For example, how would one distinguish intellect from wisdom? From cleverness? From profound?
As an aside, one need only mention IQ testing among sensitive people to initiate a hurricane. (Look, for example, on Quora.) There is good reason. Testing people to grade current or future potential has a huge danger: The risk of the grade being expanded into "quality," into who is more valuable. You will never hear this directly, only through inference, but that fear is there and it is valid. This kind of thing can create hierarchies. And selection. The outrage at Murray after The Bell Curve was fierce and heartfelt.
Giving people a number, like declaring beef "choice" or "prime," is just criminal.