Peter, thank you for posing this question, because it will help clarify the meaning of IQ for many of our visitors. We’ll start first with the concept of intelligence and then address the concept of IQ. 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 n different cognitive skills.) By measuring an individual’s performance on n different cognitive skills, Spearman sought to quantify this enigmatic “general intelligence,” g.
However, some psychologists charged that g is a mathematical fiction and has no real existence, while others celebrated g as a long sought-after metric for comparing humans by intelligence. Gradually, IQ became widely accepted as a proxy for intelligence. In 1955, American psychologist David Wechsler published a new intelligence test for adults that became known as the Wechsler Adult Intelligence Scale (WAIS). Defining intelligence as “the global capacity of a person to act purposefully, to think rationally, and to deal effectively with his environment,” he created two sub-tests–one measuring “verbal intelligence” and the other, “non-verbal (performance) intelligence”. Wechsler made the assumption that intelligence is normally distributed, i.e., has a bell-curve distribution throughout the population, and mapped his test scale onto a normal distribution with mean 100 and standard deviation 15. By standardizing his tests in this way, he linked his scale directly to percentiles, allowing for immediate comparisons to average intelligence. A person with an IQ of 100 would be in the 50th percentile, meaning that she scored higher than 50% of adults in her age range who took the test. This definition of IQ was age independent since all adults were believed to have reached full brain development. The correspondence between IQ and percentile ranking is displayed in the figure below.
For example, the area of the shaded region left of IQ 115 is 84% (i.e., 50% + 34%) of the total area under the curve. So, someone with an IQ of 115 scored higher than 84% of the population and ranks in the 84th percentile. This is called the percentile ranking. The table below gives the percentiles that match some familiar IQs.
Percentiles corresponding to Some IQs
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 short answer to your question. Any statement such as, “IQ is highly correlated to intelligence,” is misleading because we can only correlate two quantifiable variables. We can state that there is a high correlation between IQ and performance on SATs, because scores are variables. Since IQ is an approximate measure of intelligence, it would be circular reasoning to say that IQ correlates with intelligence. IQ can only correlate with another measure of intelligence, but not with intelligence itself.
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. A free chapter titled “The Many Faces of Intelligence,” is available for downloading at: Free Downloads – Intelligence and IQ. I hope you enjoy the read.
