Theories of Intelligence

Dr. Russell T. Warne

Sep 8, 2023

Ever since the scientific study of intelligence began over 100 years ago, there have been many theories of intelligence. Both scientists and non-experts have asked about the nature of intelligence, whether there is just one intelligence or more, and the practical implications of intelligence. It has been a long, winding road, but today, most psychologists have reached a general consensus on these issues, though details remain to be worked out.

One Intelligence

The earliest intelligence researchers in the late 19th and early 20th century believed that there was just one general intelligence. This started as a folk belief, but it received a lot of support in 1904 when Charles Spearman created the statistical procedure of factor analysis, which is a procedure that mathematically identifies variables that are more strongly related to each other. These groups of variables are called “factors.” Spearman discovered that the similarities in performance across tests could be explained with a single factor, which he called a general factor, or g. Spearman believed that g was equivalent to general intelligence, a belief that many scientists hold today.

Is There More Than One Intelligence?

Spearman’s g theory could not account for all of the similarities--and none of the differences--in people’s performance across tests. Louis L. Thurstone used a more advanced method of factor analysis, and by the 1930s, he had identified seven factors: number facility, word fluency, visualizing, memory, perceptual speed, induction, and verbal reasoning. Thurstone’s theory was an important challenge to Spearman’s theory. After all, if intelligence is not one ability, then an overall IQ score may not make sense.

In time, Spearman and Thurstone came to acknowledge the strengths of one another’s positions. Spearman did recognize that there were more factors than just g, and Thurstone admitted that his seven factors were all correlated and could produce a g factor. Still, the question of whether intelligence was one ability or more than one remained unresolved through most of the 20th century and persisted long after Spearman and Thurstone had died.

Crystallized and Fluid Intelligences

In the 1960s, John L. Horn and Raymond B. Cattell proposed a theory that had two intelligences: “crystallized intelligence” and “fluid intelligence.” In their theory, crystallized intelligence was learned knowledge, and fluid intelligence was the ability to solve problems with little or no context. Their theory was supported by research on the aging process, which showed that individual IQ test subscores followed one of two distinct trajectories: either increasing slowly and declining late in life (crystallized intelligence) or peaking early and decaying more quickly (fluid intelligence). This theory was more elegant than Thurstone’s, and it had the advantage of relying on studies outside of factor analysis. But crystallized and fluid intelligences were correlated and often formed a g factor. So, this theory did not resolve the question of whether intelligence was one ability or multiple abilities.

Gardner’s Theory of Multiple Intelligences

In the 1980s, it seemed that the disagreement on the number of factors/mental abilities would continue with no end in sight. At this time, Harvard education professor Howard Gardner proposed his theory of multiple intelligences. Gardner proposed seven intelligences, which were later expanded to eight: linguistic, musical, logical-mathematical, spatial, bodily-kinesthetic, interpersonal, intrapersonal, and naturalistic. Gardner has also suggested that an existential intelligence may exist. The theory denies any existence of g or a general intelligence and emphasizes that the different intelligences develop in the brain separately.

Gardner’s theory of multiple intelligences is wildly popular in educational circles, but the evidence behind it is very slim. The biggest problem with Gardner’s theory is that he states that the intelligences are independent of one another, but all of the data going back to the Spearman shows that scores on educational, cognitive, and aptitude tests show that performance is positively correlated (meaning, not independent). This is one of the most consistent findings in all of the social sciences, and it contradicts the fundamental claim of Gardner’s theory.

Sternberg’s Triarchic Theory of Intelligence

The 1980s also gave birth to another theory of intelligence that challenged general intelligence theory. Robert Sternberg’s triarchic theory of intelligence proposes that there are three important mental abilities: analytical intelligence (which is the “intelligence of the schoolhouse” and measured by traditional IQ tests and equivalent to g), creative intelligence, and practical intelligence (which shows itself in everyday life). Sternberg’s theory has the strength of incorporating creativity into intelligence theory, something that all other theories have struggled to do. It also has the innovation of practical intelligence, which Sternberg uses to explain how people learn the tacit, unspoken rules of functioning in their daily environment.

But, like Gardner’s theory, it is at odds with the data. Sternberg has never shown that the “practical intelligence” that one person uses to function to navigate a complex workplace is the same practical intelligence that a parent uses to deal with their child’s behavioral challenges. Sternberg’s theory also has the problem that traditional IQ -- which he says only measures intelligence in academic settings -- is an excellent predictor of a lot of life outcomes outside of education settings.

Modern Theories of Intelligence

At the start of the 1990s, there seemed to be little hope for intelligence theory. The Spearman-Thurstone argument had lasted for generations, and more theories were popping up proposing different numbers of intelligences. This situation changed remarkably quickly with the publication of John B. Carroll’s book Human Cognitive Abilities: A Survey of Factor-Analytic Studies in 1993. Using data from over 400 studies, Carroll found that there was indeed a g factor, but that this was not the entire story. Carroll placed g at the top of a hierarchy of cognitive abilities, and the narrow abilities -- such as Thurstone’s primary factors and Horn and Cattell’s crystallized and fluid intelligences -- were incorporated into the theory. In this theory, general intelligence and other abilities are parts of a network of abilities, and the either/or false dichotomy of the Spearman and the Thurstone positions vanished.

The hierarchical theory of intelligence is often called the Cattell-Horn-Carroll theory (or CHC theory) to honor the psychologists who made the most important contributions to it. Psychologists are still figuring out the details, such as the number of cognitive abilities in the hierarchy, but today the CHC theory is the mainstream position in intelligence research.

Most modern intelligence tests are based on the CHC theory, including the Reasoning and Intelligence Online Test (RIOT) is based on the theory. The CHC theory is why, for example, the RIOT and other tests report a global IQ and also more granular scores, such as the RIOT’s index scores. The history of intelligence theories shows how IQ tests can inform theory -- and then be shaped in turn by changes in those theories. This has been a long process, but the cycle of mutual feedback between theorists and test creators is one of the reasons why intelligence research is one of the strongest areas in psychology.

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