From Spearman’s g to the CHC Model: Theoretical Evolution of Intelligence and Contemporary Developments

Article written in collaboration with @mensana.psy

Introduction

The history of intelligence theories represents a trajectory shaped by theoretical shifts, methodological debates, and continuous attempts at integration. From Charles Spearman’s early factorial models to today’s multilayered frameworks, the concept of intelligence has evolved from a monolithic construct to a dynamic and stratified system. Psychometric and neuroscientific research has progressively shown that cognitive abilities can be understood both as manifestations of a unitary principle and as differentiated competencies shaped by development, experience, and culture (Deary et al., 2010).

This article retraces this evolution, integrating the original content with updated references and additional theoretical contributions essential for understanding the contemporary Cattell–Horn–Carroll (CHC) model.

Spearman and the Original Insight of General Intelligence (1904)

Charles Spearman inaugurated modern psychometrics by observing that cognitive performances tended to covary: individuals who excelled in one task often performed well in others. This regularity led to the formulation of the g factor, described as the mental energy common to all cognitive processes (Spearman, 1904).

Alongside g, Spearman proposed specific abilities (s-factors) that explained residual variance in individual tasks. His approach was revolutionary because it mathematically modeled the structure of intelligence, anticipating the development of modern factorial techniques.

Contemporary neuroscience has largely confirmed Spearman’s intuition: the efficiency of fronto-parietal networks is strongly associated with general intelligence (Duncan, 2010; Jung & Haier, 2007). Recent studies suggest that g reflects the brain’s capacity to efficiently coordinate distributed systems, reducing metabolic cost during complex cognitive processes (Colom et al., 2006; Dubois et al., 2018).

Thurstone and the Plurality of Abilities

Louis Thurstone challenged the centrality of g, proposing a model based on Primary Mental Abilities that were relatively independent: verbal comprehension, reasoning, memory, number facility, perceptual speed, and spatial visualization (Thurstone, 1938).

Thurstone shifted focus toward the variety of cognitive functioning, promoting a conception of intelligence as an articulated profile rather than a single measure. His approach strongly influenced assessment practices, emphasizing intraindividual differences and functional specificity.

Later studies showed that these abilities are not incompatible with g; instead, they represent intermediate manifestations within a hierarchically organized cognitive structure (Carroll, 1993).

Cattell and Horn: From Unity to Dynamics

Raymond Cattell introduced a developmental and structural distinction between fluid intelligence (Gf)—the capacity to solve novel problems—and crystallized intelligence (Gc)—knowledge derived from experience and learning (Cattell, 1963).

John Horn expanded the theory, adding further broad abilities (such as processing speed, long-term storage and retrieval, visual-spatial ability) and supporting the idea of multiple interconnected but not fully g-dependent domains (Horn, 1965).

The Cattell-Horn framework added an essential developmental dimension: Gf tends to decline with age, whereas Gc is maintained or increases through cultural experience (Salthouse, 2004).

Carroll and the Unified Hierarchy

John B. Carroll conducted the most extensive factorial analysis in psychometric literature, proposing a three-stratum hierarchical model (Carroll, 1993):

• Stratum I: Narrow abilities

• Stratum II: Broad abilities (fluid reasoning, comprehension-knowledge, memory, visual-spatial skills, processing speed)

• Stratum III: The general intelligence factor g

Carroll’s work provided a rigorous architecture that reconciled Spearman’s unity with Thurstone’s plurality, forming the structural basis for later theoretical integration.

The CHC Model: The Final Integration

The CHC model, resulting from the convergence of Cattell-Horn’s theory and Carroll’s hierarchy, is now considered the standard in cognitive assessment (McGrew, 2009; Flanagan & Dixon, 2014). The model integrates:

• Narrow abilities,

• Broad abilities,

• The general factor g.

This layered structure allows for a refined interpretation of individual cognitive profiles and has significant clinical applications in developmental disorders, giftedness, and neurocognitive decline.

Recent studies show that the CHC model aligns with both psychometric and neural evidence, acting as a bridge between cognitive psychology and neuroscience (Kievit et al., 2016).

Conclusions

The evolution from Spearman’s g to the CHC model is not a simple linear progression but a complex synthesis of unity and multiplicity.

• Spearman provided the unifying root.

• Thurstone highlighted functional diversity.

• Cattell and Horn introduced developmental and dynamic aspects.

• Carroll mapped the hierarchical structure.

• The CHC model offered the most empirically grounded synthesis.

Future research aims not to replace these contributions but to integrate them further with network models, developmental perspectives, and advanced neuroscientific analyses to address the fundamental question: what does it mean to be intelligent?

Bibliographic References

Carroll, J. B. (1993). Human cognitive abilities: A survey of factor-analytic studies. Cambridge University Press.

Cattell, R. B. (1963). Theory of fluid and crystallized intelligence: A critical experiment. Journal of Educational Psychology, 54(1), 1–22. 

Colom, R., Jung, R. E., & Haier, R. J. (2006). Distributed brain sites for the g‐factor of intelligence. NeuroImage, 31(3), 1359–1365. 

Deary, I. J., Penke, L., & Johnson, W. (2010). The neuroscience of human intelligence differences. Nature Reviews Neuroscience, 11(3), 201–211. 

Dubois, J., Galdi, P., Paul, L. K., & Adolphs, R. (2018). A distributed brain network predicts general intelligence from resting-state human neuroimaging data. Philosophical Transactions of the Royal Society B, 373(1756), 20170284. 

Duncan, J. (2010). The multiple-demand (MD) system of the primate brain. Trends in Cognitive Sciences, 14(4), 172–179.

Flanagan, D. P., & Dixon, S. G. (2014). The Cattell–Horn–Carroll theory of cognitive abilities. In D. P. Flanagan & P. Harrison (Eds.), Contemporary intellectual assessment (pp. 249–402). Guilford Press.

Horn, J. L. (1965). Fluid and crystallized intelligence. Psychometrika, 30(2), 179–185.

Jung, R. E., & Haier, R. J. (2007). The parieto-frontal integration theory of intelligence. Behavioral and Brain Sciences, 30(2), 135–187. 

Kievit, R. A., et al. (2016). Integrated cognitive neuroscience approaches to individual differences in intelligence. Current Opinion in Behavioral Sciences, 11, 74–82. 

McGrew, K. S. (2009). CHC theory and the human cognitive abilities project. Intelligence, 37(1), 1–10. 

Salthouse, T. A. (2004). What and when of cognitive aging. Current Directions in Psychological Science, 13(4), 140–144. 

Spearman, C. (1904). “General intelligence,” objectively determined and measured. The American Journal of Psychology, 15(2), 201–292.

Thurstone, L. L. (1938). Primary mental abilities. University of Chicago Press.