We can thank the French government for IQ tests! Early in the 1900’s, when universal education was mandated in France, Alfred Binet was commissioned to develop a test to identify children in need of special assistance in school. He came up with the concept of “mental age”: measuring intelligence by comparing individual children to those with average abilities in their age group. Binet defined intelligence as “judgement, otherwise called good sense, practical sense, initiative, the faculty of adapting one’s self to circumstances.”
The first IQ test, the Binet-Simon Scale, was introduced in 1905. The first American IQ test, a revision of the original Binet-Simon, was introduced in 1916. It was called the Stanford-Binet Intelligence Scale and, in its latest revision, is still in use today.
In 1955, David Wechsler, an American psychologist, developed the Wechsler Adult Intelligence Scale (WAIS), followed by the Wechsler Intelligence Scale for Children (WISC) and the Wechsler Preschool and Primary Scale of Intelligence (WPPSI). He defined intelligence as “the global capacity of a person to act purposefully, think rationally, and deal effectively with his environment”. The Wechsler tests are the most commonly used in the United States today.
But what does IQ mean? IQ is an ill-defined human construct: something we have made up to help understand the world and how people fit into it. Really, IQ is best defined as the score someone gets on an IQ test! As one would guess given the origin of IQ testing, IQ tests generally measure those skills that are most closely allied with success in academic learning: logical, sequential reasoning skills and a good memory.
IQ scores are designed to mirror the normal curve – also called the “bell curve”. The normal curve illustrates how, theoretically, different traits and abilities are distributed through the human population. Here is what it looks like:
The big middle part of the curve, which covers 68% of the population, shows what we can call “within normal limits” – the range that would describe most people for that characteristic. For example, if you took everyone in your town and stacked them up by height, the stack would look like the bell curve – 68% of people are of average height, 14% of people are tall, 2% of people are very tall, 14% of people are short, and 2% of people are very short. IQ scores work the same way. If you took everyone in your town and stacked them up by their IQ score, 68% of the people would have an average IQ, 14% would be smart, 14% would be slow learners, 2% would be intellectually gifted, and 2% would be intellectually disabled.
In the next article, we will look at what IQ scores (and all standardized test scores) mean.