What is IQ? What is Intelligence?

According to a 1997 consensus statement signed by over 50 leading intelligence researchers, intelligence is “a very general mental capability that, among other things, involves the ability to reason, plan, solve problems, think abstractly, comprehend complex ideas, learn quickly and learn from experience. It is not merely book learning, a narrow academic skill, or test-taking smarts. Rather, it reflects a broader and deeper capability for comprehending our surroundings--‘catching on,’ ‘making sense’ of things, or ‘figuring out’ what to do.”

This definition aligns very well with most non-scientists’ understandings of intelligence. More intelligent people tend to be good at learning, problem solving, and applying the knowledge that they gain. This makes people who score high on IQ tests perform better in school and the workplace, though there are exceptions.

Intelligence is measured with IQ tests. Many different tasks appear on different tests, but they all measure the same ability. There is no one task that appears on every intelligence test. That is not a problem. What matters is what the tasks requires the test taker to do: As long as the task requires people to think, solve problems, reason, or make judgements, it will measure intelligence to some extent. Some tests are administered individually in a face-to-face setting, while others can be administered to large groups of people at once. The scores from these IQ tests are usually “IQ,” though there are some tests that measure intelligence that are not called “IQ tests” and that produce different scores. These can be converted to IQs anyway.

The first intelligence test was created by Alfred Binet and Theodore Simon, who published a 30-question test in 1905 that was designed to identify children who might struggle to learn in regular classrooms. (Today, we would say that these children qualify for special education services.) Their test was successful, and revisions were published in 1908 and 1911 (the year of Binet’s death).

The Binet tests, as they came to be called, were very popular and quickly translated and adapted into several languages all over the world. Most notably, Stanford University professor Lewis Terman revised and expanded the Binet test to create the Stanford-Binet Intelligence Test, which was designed as an IQ test for American children. The Stanford-Binet was wildly popular, and a modern revision still exists today.

Terman also was part of a committee of psychologists that worked with the U.S. Army to create two intelligence tests that were the first that could be administered to groups of people: the Army Alpha (for literature soldiers) and Army Beta (for illiterate soldiers). Today group testing is widespread in education and sometimes employment settings.

David Wechsler was another important early IQ test creator. He published the Wechsler-Bellevue test for adults (later revised to the Wechsler Adult Intelligence Scale, or WAIS) and followed up with the Wechsler Intelligence Scale for Children (WISC), and the Wechsler Preschool and Primary Scale of Intelligence (WPPSI). Today, modern versions of the WAIS, WISC, and WPPSI are available for trained psychologists to administer.

Simply put, an “IQ” is the score that someone obtains from an IQ test (also called an “intelligence test”). When IQ tests were first created, “IQ” stood for “intelligence quotient.” To score these tests, a psychologist would find the age of the examinees that performed (on average) as well as the examinee, called the “mental age” and divided it by the examinee’s chronological age. For example, if an 8-year-old child performed as well as the average 10-year-old on an IQ test, then, the child’s IQ would be 10 / 8 = 1.25. To eliminate the decimal, the number would then be multiplied by 100 to produce an IQ of 125.

This formula has the advantage of being easy to calculate and interpret. An IQ of 100 is always average. An IQ greater than 100 indicates that the person is smarter than average, and an IQ less than 100 indicates that the person is less intelligent than average.

On the other hand, the formula breaks down on closer inspection. One obvious problem is that it does not make sense for adults. Because children, presumably, get smarter as they age, it is meaningful to say that a child has a certain “mental age.” But because cognitive growth levels off in adulthood, it does not make much sense to say that (for example), a 40 year old has the “mental age” of a 50-year-old when they take an IQ test.

Another important limitation to the quotient formula was that the IQs are not comparable across ages. If a child has a mental age that is two years ahead of their chronological age, then at age 4, their IQ would be 150. At age 6, their IQ would drop to 133, and at age 8, it would only be 120. Their IQ would decrease even though they are always 2 years more advanced than an average child their age.

To fix these problems, psychologists who design IQ tests now use a formula called a deviation score IQ. It still has an average of 100, but it can be used for children and adults, and scores have the same meaning across the age range. You can learn more about calculating deviation score IQs here.