What Is the g Factor? A Plain-Language Guide to General Intelligence and CHC Theory

The g factor (general intelligence) is the factor that shows up in common across intellectual tasks of different kinds. People who do well on vocabulary problems tend to do well on figure problems too — g was introduced to explain this phenomenon, in which performance on every sort of intellectual task correlates positively, and it is what an IQ score is trying to represent.

What the g factor (general intelligence) is

In the early 20th century, the psychologist Charles Spearman found that grades across school subjects and scores on several perceptual tasks correlated positively with one another despite differing in content. This phenomenon is known as the 'positive manifold.'

Spearman posited a general factor g to account for the shared part, plus specific factors s unique to each task. For more than a century since, the positive correlation itself has been reproduced repeatedly. What g actually is — a single processing resource, or the emergent result of several abilities influencing one another — remains under debate.

The development of CHC theory

The modern standard model is CHC theory, which integrates the Cattell-Horn fluid/crystallized account with Carroll's three-stratum theory. It treats intelligence as a hierarchy with g at the top and several broad abilities beneath it. The main broad abilities are:

• Fluid reasoning (Gf): spotting rules in unfamiliar problems. What the figure problems on IQ tests primarily measure
• Crystallized intelligence (Gc): judgment based on vocabulary, knowledge, and experience
• Processing speed (Gs): how quickly and accurately you handle simple tasks
• Short-term memory (Gsm): holding information while operating on it (see working memory)

Gf and Gc change differently with age

The practical value of CHC theory is that it predicts different behavior for different broad abilities. Age is the clearest example: fluid reasoning (Gf) peaks around age 20 and declines gradually, while crystallized intelligence (Gc) keeps growing or holds steady into the 50s and 60s.

'Minds get rigid with age' and 'judgment comes with age' are both correct — about different abilities. The CHC hierarchy explains that twist cleanly. We cover it in fluid and crystallized intelligence and IQ and age.

NekoIQ's four domains and the g factor

The spatial reasoning, pattern recognition, logical reasoning, and classification sections on this site are all designed to measure fluid reasoning (Gf), based on the rule taxonomy behind Raven's matrix reasoning (Carpenter et al. 1990). The moderate positive correlations among the four domain scores are thought to reflect the shared g factor underneath (see cognitive ability correlation data).

The overall score is the ability parameter theta, estimated jointly from responses across those four domains with an item response theory (Rasch) model and converted to the mean-100, SD-15 scale. Positioned precisely, it is an estimate of the g factor as seen through non-verbal fluid reasoning (see our methodology).

What the g factor does not explain

The g factor captures the shared component of performance on intellectual tasks and nothing beyond it. Creativity, motivation, interpersonal skill, accumulated domain knowledge, and persistence are all separate from g.

The relationship with creativity, for instance, is organized as a correlation up to a point that weakens above it (see IQ and creativity). An IQ score is a strong indicator representing the core of intellectual ability, but the standard understanding in psychology is that it is not a number that determines a person's potential.

See your own estimate of g

NekoIQ's free IQ test implements exactly the design described here, scoring 20 questions across four domains (about 10 minutes) with IRT to produce an estimated IQ, deviation score, and percentile. Domain scores are shown separately, so you can also see the uneven profile of your strengths within Gf.