What is the "g" factor? What is general intelligence?
Robert Neir
Mar 14, 2025TL;DR
➤ The g-factor (general intelligence) represents a single underlying cognitive capability that influences performance across diverse intellectual domains. This concept was initially identified by psychologist Charles Spearman in 1904 through statistical analysis of test score correlations.
➤ General intelligence comprises multiple cognitive components: fluid reasoning, working memory, verbal reasoning, processing speed, and spatial ability.
➤ The g-factor is quantified through standardized psychometric instruments and is typically expressed as Intelligence Quotient (IQ) scores.
➤ Research demonstrates substantial predictive validity of the g-factor for academic achievement and occupational performance.
➤ The g-factor accounts for 40 to 50 percent of the variance in performance differences across cognitive tests, making it a robust predictor of individual cognitive ability [Kamphaus et al., 2005].
g-factor: The Hidden Force Behind Intelligence
Ever wondered why some people seem to excel across diverse intellectual challenges? Why do student who aces mathematics often perform impressively in verbal reasoning too? The answer might lie in one of psychology's most powerful yet controversial concepts: the g-factor.
Spearman's Discovery of g
In 1904, British psychologist Charles Spearman made a discovery that would forever change how we understand human cognition. While analyzing test scores, he noticed something remarkable – people who performed well on one cognitive test typically performed well on others, regardless of the tests' content. This wasn't just a coincidence. Spearman had uncovered the fingerprint of what he called the "general intelligence factor," or g-factor – a single underlying mental capability that influences performance across all intellectual tasks. He observed that various measures of different mental abilities consistently show correlations, leading him to conclude that these positive correlations arise from a general factor, g. This phenomenon, known as the "positive manifold," has been described as "arguably the most replicated result in all psychology" due to its consistent replication across diverse cognitive tests [Deary, 2000].
Spearman’s work laid the foundation for psychometric research on intelligence, using factor analysis to identify g as a common factor explaining the variance shared among cognitive tasks. However, his findings were challenged by Godfrey Thomson, who argued that such intercorrelations could arise without a single elderly without a unitary g-factor [Thomson, 1916].
Spearman's Theory
Spearman's theory of general intelligence is originally known as the Two-Factor theory of Intelligence, which boils down to two core ideas: “g,” your general intelligence that powers everything you do, and “s,” the specific skills that shine in particular tasks. According to Spearman, every task on an intelligence test taps into this g factor to some extent, no matter the challenge, whether it’s cracking a puzzle, mastering words, or tackling numbers, your general intelligence, or g, plays a key role in how well you do. The more varied the mental tasks you face, the more g steps up, making it a powerful predictor of your overall performance. Spearman proposed the principle of the "indifference of the indicator," suggesting that the specific content of intelligence tests is less important than their ability to measure g, as g influences performance across all cognitive tasks [Spearman, 1927]. This theory helps explain why people can be strong in many areas while still having unique talents.
Mental Toolbox: Components of General Intelligence
Picture general intelligence, or g, as the master key to your mental toolbox—a single, adaptable tool that unlocks your ability to tackle life’s challenges, whether you’re piecing together a tricky puzzle, grasping big ideas, or navigating new situations. It’s the spark that fuels your capacity to learn and think logically.
The thing is, general intelligence isn't a one-dimensional tool; it's a mixture of different cognitive abilities collaborating together. It is made up of several key components, including, but not limited to:
Verbal Reasoning. The depth and breadth of acquired knowledge, vocabulary, and verbal concept formation that accumulates throughout life.
Visuospatial Ability. The ability to think logically and solve problems in novel situations, independent of acquired knowledge, and to understand and mentally manipulate visual patterns, spatial relationships, and three-dimensional structures.
Working Memory. The capacity to hold information in mind while simultaneously manipulating it for complex cognitive tasks.
Processing Speed. The efficiency and rapidity with which simple cognitive operations can be executed, affecting performance across all domains
Reaction Time. The ability to quickly respond or react to a stimuli in the environment.
These components align with contemporary hierarchical models of intelligence, such as the Cattell-Horn-Carroll theory, which places g at the apex of a three-level structure, above broad abilities like fluid intelligence (Gf) and crystallized intelligence (Gc) [McGrew, 2005]. For a more detailed discussion about the mentioned cognitive abilities, read the article: 5 Cognitive Abilities of the RIOT.
Measuring the Invisible: How General Intelligence is Quantified
Now, is there a way to measure intelligence? How do we measure something as abstract as general intelligence? The answer lies in sophisticated psychometric instruments designed to capture g through diverse cognitive tasks. The g-factor is typically extracted using factor analysis, a statistical method that identifies a common factor accounting for the correlations among test scores. Full-scale IQ scores from test batteries, such as the Wechsler Adult Intelligence Scale, are highly correlated with g, often exceeding .95, making them reliable estimates of an individual’s standing on g [Jensen, 1998].
Modern intelligence tests like the Wechsler Adult Intelligence Scale (WAIS), Stanford-Binet, and Raven's Progressive Matrices assess multiple cognitive abilities and combine these scores to estimate g. Raven’s Progressive Matrices, for instance, is noted for its high g-loading (around .80), as it heavily taps into abstract reasoning [Jensen, 1998]. Read the article 5 Best IQ Tests in 2025 for more information about different intelligence tests. The resulting Intelligence Quotient (IQ) score, typically with a mean of 100 and a standard deviation of 15, serves as our best proxy for general intelligence. To know more about calculating IQ, we recommend reading this article: How to Calculate IQ.
In an engaging conversation between neuroscientist Richard Haier and AI researcher Lex Fridman, they discussed general intelligence in-depth and how it can possibly be measured:
Impact of General Intelligence
What's particularly amazing is that g isn't just academic curiosity – it has real-world implications. Studies show that g predicts life outcomes such as:
Academic achievement
Studies indicate a strong link between mental ability and academic success, yet its influence is debated as intelligence alone doesn't fully explain outcomes. For instance, the correlation between IQ and grades in elementary school ranges from .60 to .70, though this decreases at higher educational levels due to range restriction [Jensen, 1998]. The SAT, widely used in college admissions, correlates with g at .82, underscoring its role as a measure of general intelligence [Frey & Detterman, 2004].
Work Performance
Intelligence is often linked to career success, driving the use of psychological testing in hiring and career placement. However, debate persists over whether general intelligence outweighs specific mental abilities. The role of general intelligence grows more critical as job complexity increases, making it a key asset in demanding roles. Meta-analyses show that g has an average validity coefficient of .55 for job performance and .63 for job training performance, with higher validity in complex roles like professional and scientific positions [Schmidt & Hunter, 2004].
Other Outcomes
Beyond academics and work, g is negatively correlated with social issues like chronic welfare dependency, accident proneness, and crime, independent of social class [Gottfredson, 2007]. Higher g scores in childhood also predict better health and mortality outcomes in adulthood, highlighting its broad societal impact [Gottfredson, 2007].
What's Next for g
Researchers are probing deeper into g using advanced brain imaging, genetic analysis, and computational models. They're discovering intriguing connections between g and neural efficiency, brain structure, and information processing networks. For example, MRI studies show moderate correlations (r~.3–.4) between g and total brain volume, with specific brain regions like the frontal and parietal cortices also linked to g [Deary et al., 2010]. One of the most recent developments is a study examining how well-connected certain brain areas were while people solved a common intelligence test called Raven's Progressive Matrices. Some of the most exciting questions remain unanswered: Can we enhance g through training? How does it develop throughout childhood? What exact biological mechanisms create the pattern we identify as general intelligence?
Research also highlights the high heritability of g, estimated at 50–80% in adults, increasing with age [Plomin & Spinath, 2004]. However, environmental factors play a significant role, particularly in childhood, though their impact diminishes in adulthood [Deary et al., 2006].
Criticisms and Controversies
While the g-factor is widely accepted in psychometrics, it has faced criticism. Some, like Stephen J. Gould, argue that g is a reified construct—a statistical artifact rather than a tangible entity [Gould, 1996]. Critics contend that emphasizing g devalues other abilities, such as creativity or emotional intelligence, and point to its historical ties to eugenics as a concern [Graves & Johnson, 1995]. Alternative theories, like Howard Gardner’s multiple intelligences or Robert Sternberg’s triarchic theory, propose that intelligence is multifaceted and not fully captured by g [Mackintosh, 2011]. Despite these critiques, the practical validity of g, its invariance across diverse test batteries, and its biological correlates continue to support its significance [Jensen, 1998].
Bottom Line
Whether viewed as the central pillar of human cognition or a useful statistical framework, the g-factor continues to drive our understanding of intelligence forward. It reminds us that while human abilities are diverse and multifaceted, they're also connected in profound ways we're still working to fully comprehend.
The next time you tackle a challenging problem or learn a new skill, remember that behind your specific abilities lies g, the invisible thread connecting your mental capabilities into what we recognize as intelligence.
Key Citations
🔗 Spearman, C. (1904). 'General intelligence,' objectively determined and measured. American Journal of Psychology, 15(2), 201–293. https://doi.org/10.2307/1412107
🔗 Thomson, G. H. (1916). A Hierarchy Without a General Factor. British Journal of Psychology, 8(3), 271–281. https://doi.org/10.1111/j.2044-8295.1916.tb00133.x
🔗 Kamphaus et al. (2005). A history of intelligence test interpretation. In Contemporary intellectual assessment. https://psycnet.apa.org/record/2005-09732-002
🔗 Deary, I. J. (2000). Looking Down on Human Intelligence: From Psychometrics to the Brain. https://doi.org/10.1093/acprof:oso/9780198524175.001.0001
🔗 Jensen, A. R. (1998). The G Factor: The Science of Mental Ability. https://www.jstor.org/stable/4236447
🔗 Frey, M. C., & Detterman, D. K. (2004). Scholastic Assessment or g? Psychological Science, 15(6), 373–378.
🔗 Schmidt, F. L., & Hunter, J. (2004). General Mental Ability in the World of Work. Journal of Personality and Social Psychology, 86(1), 162–173. https://doi.org/10.1037/0022-3514.86.1.162
🔗 Gottfredson, L. S. (2007). Innovation, fatal accidents, and the evolution of general intelligence. In Integrating the mind. https://psycnet.apa.org/record/2007-02338-017
🔗 Deary, I. J., Penke, L., & Johnson, W. (2010). The neuroscience of human intelligence differences. Nature Reviews Neuroscience, 11(3), 201–211. https://doi.org/10.1038/nrn2793
🔗 Plomin, R., & Spinath, F. M. (2004). Intelligence: genetics, genes, and genomics. Journal of Personality and Social Psychology, 86(1), 112–129. https://doi.org/10.1037/0022-3514.86.1.112
🔗 Graves, J. L., & Johnson, A. (1995). The Pseudoscience of Psychometry and The Bell Curve. The Journal of Negro Education, 64(3), 277–294. https://doi.org/10.2307/2967209
🔗 McGrew, K. S. (2005). The Cattell-Horn-Carroll Theory of Cognitive Abilities. In Contemporary Intellectual Assessment. https://doi.org/10.1002/9781118660584.ese0431
🔗 'General intelligence,' objectively determined and measured
🔗 Theory of fluid and crystallized intelligence: A critical experiment
🔗 Refinement and test of the theory of fluid and crystallized general intelligences
🔗 Spearman’s Two-Factor Theory: General Intelligence (g) and Specific Abilities (s)
🔗 General Intelligence in Psychology: Exploring Spearman’s g factor and Its Impact
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