Is Intelligence Too Complex to Summarize with One Number?

Since Spearman identified g in 1904, critics have questioned whether intelligence could be captured by a single metric. Throughout the twentieth century, psychologists debated this issue using factor analysis—Spearman advocating for one unified ability, while Thurstone argued for multiple separate abilities. After decades of data collection and analysis, a consensus emerged: intelligence operates as a general ability (similar to Spearman's g) that relates to other mental capacities. Current leading theories, including the Cattell-Horn-Carroll model and the bifactor model, represent this middle-ground position. In essence, both theoretical camps were partially right, though neither fully captured how human cognitive abilities interconnect.

This scientific progress was driven by empirical evidence. Researchers constructed, evaluated, and refined their theories based on collected data, demonstrating how the scientific method moves us toward truth. In contrast, some critics simply assert—typically without empirical testing—that intelligence is too multifaceted to reduce to a single number like IQ. However, intelligence not only can be represented as one entity, but it's actually impossible for it to be as multidimensional as critics suggest.

The general factor isn't merely theoretical speculation—it's an empirical reality emerging from the positive manifold, which appears when virtually all cognitive task scores correlate positively. Spearman observed this phenomenon in 1904, finding that school subject grades and discrimination tests all showed positive correlations. Contemporary assessments confirm this pattern. The Stanford-Binet Intelligence Scales and various Wechsler tests all display consistent positive intercorrelations among their subtests.

The positive manifold matters because g derives directly from it. Spearman's key insight was that test scores correlated because one underlying ability—g—caused them all. He developed factor analysis to prove this single ability could explain these correlations. Without positive intercorrelations, extracting a g factor becomes impossible.

To disprove g's existence, one would only need to identify a cognitive variable uncorrelated with others, or find independent ability clusters producing unrelated factors. Despite over a century of searching, neither has been discovered. This provides compelling evidence that intelligence functions as a unified entity.

Once g's existence is established, creating a single score becomes straightforward. Since g represents general problem-solving capacity, scoring involves ranking individuals by their problem-solving success. Quality intelligence tests assess problem-solving across various tasks, and tallying performance on problems of different difficulties reveals who performs best and worst, approximating relative g levels.

Nevertheless, critics are partly correct about cognitive complexity. Early theorists like Spearman underestimated human cognitive breadth, and no recent expert claims g is the sole important ability. Attacking intelligence research by arguing "IQ isn't everything" targets a nonexistent position. Modern perspectives acknowledge cognitive complexity while recognizing g's role. Both major theories accept that g doesn't encompass all mental abilities—other important capacities exist in their hierarchical structures.

Consequently, well-designed tests generate more than global IQ scores. The WISC-V, for instance, provides scores for verbal comprehension, visual-spatial ability, fluid reasoning, working memory, and processing speed alongside full-scale IQ. Two people with identical IQ scores may show very different ability profiles. These profiles reveal important strengths and weaknesses, particularly for career and educational decisions. Research indicates that in societies offering occupational choice, people gravitate toward fields utilizing their strengths.

While IQ usefully summarizes general problem-solving ability, it's not the only important score for understanding someone's cognitive profile.



From Chapter 2 of "In the Know: Debunking 35 Myths About Human Intelligence" by Dr. Russell Warne (2020)