The g Factor: General Intelligence

The g factor (general intelligence) is considered by many psychometricians to be one of the most important discoveries in the field. It represents the shared variance among all cognitive abilities—the statistical finding that people who perform well on one type of cognitive task tend to perform well on others. However, others remain skeptical of most reported g-loadings and question the commercial and methodological practices behind many g-factor claims. For a comprehensive discussion of these critical perspectives, see Views on g-loading in our adaptive test documentation.

Discovery of g

The g factor was discovered by Charles Spearman in 1904 through factor analysis of children's academic performance. Spearman noticed something remarkable: students' grades in seemingly unrelated subjects showed positive correlations.

1904

Charles Spearman publishes "General Intelligence, Objectively Determined and Measured," introducing factor analysis and discovering g.

1927

Spearman refines his two-factor theory, proposing g (general) and s (specific) factors.

1940s

Thurstone challenges g with his theory of primary mental abilities, but later acknowledges a higher-order g factor.

1993

Carroll publishes his three-stratum theory, placing g at the apex of cognitive abilities.

What Exactly is g?

Understanding g requires grasping what it is and isn't:

What g IS:

  • A statistical abstraction derived from the positive correlations among cognitive tasks
  • The first principal component in factor analysis of cognitive abilities
  • The best single predictor of performance across diverse cognitive tasks
  • A latent variable that influences all cognitive abilities to varying degrees

What g IS NOT:

  • A physical thing or location in the brain
  • The only aspect of cognitive ability that matters
  • A complete description of intelligence
  • Fixed or entirely genetic

The Positive Manifold

The foundation of g is the "positive manifold"—the finding that all cognitive abilities correlate positively with each other. This means:

Simplified representation:

Performance on any cognitive task = g × loading + specific ability + error

Evidence for g

1. Statistical Evidence

2. Predictive Validity

g predicts a remarkable range of life outcomes:

3. Biological Correlates

Biological Basis of g

Research has identified several biological factors associated with g:

Neurological Findings

Heritability

Twin and adoption studies consistently show that g is substantially heritable:

  • Heritability increases with age: ~20% in infancy to ~80% in adulthood
  • Shared environment effects decrease with age
  • Molecular genetics has identified numerous small-effect variants

Measuring g

g-Loading

Different cognitive tasks vary in their g-loading (correlation with g):

See Views on g-loading for a critical perspective on g-loading claims in psychometrics.

Extraction Methods

  1. Principal Component Analysis: First unrotated principal component
  2. Factor Analysis: Common factor underlying cognitive tasks
  3. Bifactor Models: Separate g from group factors
  4. Item Response Theory: Modern psychometric approaches

Theoretical and Practical Implications

Theoretical Implications

Practical Applications

Important Caveats

While g is a powerful construct, it's important to remember:

  • g doesn't capture all important aspects of human capability
  • Specific abilities matter, especially for specialized tasks
  • Non-cognitive factors (personality, motivation) also predict success
  • g should never be used to judge human worth or limit opportunities

Future Directions

Current research on g focuses on:

The g factor remains one of the most robust findings in psychology, providing a scientific foundation for understanding human cognitive abilities while acknowledging the full complexity of human intelligence.