The g Factor

Dr. Russell T. Warne

Sep 8, 2023

Many psychologists who study human intelligence discuss the “g factor” or simply “g”. For outsiders, this terminology raises some questions: What is the g factor? What does g stand for? How does it relate to intelligence? Are IQ and g the same thing?

The g Factor and General Intelligence

The g factor originates in an article published by Charles Spearman in 1904. In this article, Spearman discovered that performance on a series of educational and psychological tests was positively correlated for every test. In other words, examinees who did well on one test tended to perform well on all of the others -- even though some of the tests didn’t seem very academic or resemble school tests at all. To try to explain this tendency, Spearman invented a new mathematical procedure called “factor analysis” and used it to argue that the consistent performance across tests had a common cause: an unobserved variable or “factor” underlying all the tests. Spearman called this factor a “general factor,” which was later abbreviated to g. Sometimes it is also called “Spearman’s g” in his honor.

In his article, Spearman equated g to general intelligence and stated that the reason some students did better on the tests were because they had higher intelligence. (Conversely, those who did not do as well had lower general intelligence.) The claim that g and intelligence are the same thing has been a point of contention among psychologists, but many argue that the characteristics of g and intelligence are so similar that they are functionally the same.

Does g Exist?

Finding a factor in a dataset is not sufficient to prove that the factor corresponds to a real entity. Fundamentally, a factor is just a group of variables that correlate strongly enough to form a group. Additional data must be gathered to show that the factor is measuring something real.

The evidence is overwhelming that g is real. The most compelling evidence is that scores that measure g, such as IQ scores, correlate with biological variables, including brain size, brain functioning, reaction time, and DNA variants. If g were a statistical artifact or an illusion, it shouldn’t correlate with any of those things. And yet it does.

Limits of IQ

While almost all experts on intelligence agree that g is real and that IQ measures it well, they also acknowledge that other cognitive abilities matter. General intelligence is just that: general. IQ is good for predicting general life outcomes, but in both school and the workplace, other cognitive abilities matter, too. This is why many IQ tests, including the Reasoning and Intelligence Online Test (RIOT), report an overall IQ score to measure g and subscores to measure other abilities. The RIOT, for example, reports scores for Verbal Reasoning, Fluid Reasoning, Spatial Ability, Working Memory, Processing Speed, and Reaction Time -- in addition to IQ.

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