No one knows exactly what we mean by intelligence, but as Justice Potter Stewart of the US Supreme Court said in 1964, “Intelligence is like porn, difficult to define, but you know it when you see it.” When we observe members of our species we see some who learn quickly, can solve problems easily, and can articulate ideas clearly. Others struggle with abstract concepts, learn more slowly, and have difficulty expressing themselves. To describe this variation in human capability, we use the term, intelligence. Unlike concepts in physics where qualities tend to have a more precise mathematical definition, the term intelligence is somewhat enigmatic. Perhaps the most widely accepted definition is the one articulated by psychologist Linda Gottfredson:
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.
The big question that has engaged psychologists since the time of Sir Francis Galton (1822–1911) is: How do we measure this elusive human characteristic? At the beginning of the 20th Century, Charles Spearman discovered that a student who performed well in one academic subject, seemed to perform well in the others, suggested that intelligence is not domain specific and is therefore, not a result of prior learning. The fact that significant individual differences were observed in very young children in their ability to learn concepts, to draw inferences, and to solve problems suggested that these differences are innate, i.e., biological, or acquired at a very early age. Using a sophisticated statistical technique, called factor analysis, he proposed the existence of a general intelligence, denoted g, that could quantify an individual’s ability to learn, problem solve, etc., relative to others of the same age. (If you’re familiar with vectors, you can think of g as an n-dimensional vector whose components in an n-dimensional space are measures of ndifferent cognitive skills.) By measuring an individual’s performance on n different cognitive skills, Spearman sought to quantify this enigmatic “general intelligence,” g. In the years that followed, John Raven and others developed tests to measure g.
In 1955, American psychologist David Wechsler published a new intelligence test for adults that became known as the Wechsler Adult Intelligence Scale (WAIS). Wechsler’s recognition that intelligence may have more than one dimension had prompted him to depart from a single measure of intelligence offered by the original Binet tests and the Stanford-Binet test. Subsequent revisions of the Wechsler tests included measures of verbal comprehension, perceptional reasoning, working memory, and processing speed. WAIS-IV now has 10 subtests and 5 supplemental tests that summarize intelligence with two measures–a final IQ score and a General Ability Index.
This brings us to the original question, “How do we know that IQ tests measure the cognitive abilities?” The short answer is that there is a close connection between IQ and job performance that Linda Gottfredson compiled from the Wonderlic Personnel Test and Scholastic Level Exam yielding the figure below.
The row labeled career potential shows the spectrum of employment ranging from routine, relatively unskilled occupations at the low IQ levels to the occupations demanding complex cognitive skills at the high IQ levels. As we move from left to right along this employment spectrum, we see that the level of job complexity increases in parallel with the corresponding IQ scores. It is not surprising that the increasing levels of complexity in the jobs match the increasing levels of complexity in the items on the IQ tests.
It is generally accepted that IQ is the best measure of intelligence that we have, because people who perform at the highest levels in life are usually found to be of significantly higher IQ than average, and those of below average IQ are rarely found in intellectually demanding professions. Yet, we all know that intelligence has many different faces and is sometimes manifest in ways that don’t register on IQ tests. Qualities like creativity, curiosity and inventiveness may not show up on the IQ radar, but often manifest as genius as life unfolds. Charles Darwin is an example of someone who admitted to personal limits in his capacity for abstraction, yet he changed our understanding of who we are more than anyone before or since. See also the correlation between IQ and college majors at: https://www.intelligence-and-iq.com/how-does-iq-affect-everyday-life-career-opportunities/
