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Intelligence is an area full of deep and successful psychological scientific research. Yet the amount of myths and misconceptions that are commonly held in society, some of which we debunk in our article about intelligence myths, is staggering.
The huge number of myths is in part due to the excessively technical jargon that psychology researchers use, in part due to the interest of journalists in only publishing discoveries that can become click-bait articles, and as Gottfredson (1998) reminds, in part also because of the societal belief that we are all equal and that any science that challenges that notion should be ignored. And mother Nature is stubborn in showing us every day how different our capacities truly are since we are born. But sometimes, those misconceptions are the reflection of the fact that researchers are still openly debating an issue. And that is exactly what happens with general intelligence.
General intelligence, also called the “g” factor, refers to the concept proposed by early psychologists that there is a global cognitive capacity in each person that can be measured, distinct from each cognitive ability itself, and that influences all other abilities like reasoning, knowledge, perception and other.
In practice this means, as Professor Jensen detailedly explains in “The g factor: psychometrics and biology”, that people who score above average on any given task will tend to score on any other task above average too, while those who score below average will tend most times to score below average.
For example, let's say we focus on persons A and B. If we say A is better in reasoning, but also in knowledge and perceptual tasks, while B is worse than A in all of them, we can say that A has greater general intelligence than B. Because we can see this effect in the population, some researchers think that there must exist some common cause that explains why someone more intelligent tends to perform better in most tasks than someone less intelligent. In the words of Linda Gottfredson (1998), a general ability that “permeates” the rest of cognitive abilities. But not all researchers agree that such a general ability exists, as we shall see.
The difference between factor “g” and IQ
The difference between “g” and IQ is very small but it is important to understand it. When we talk about “g” we refer to the precise and exact general intelligence that someone has. Something we cannot really know because we always measure with some degree of error.
On the other hand, IQ refers to the global level of intelligence that someone has according to a specific IQ test that was administered on a given day under a concrete set of circumstances and compared against a specific sample of persons. All IQ tests suffer a certain degree of error and other factors, like mood, sleep, and others, can influence a little bit the performance of any given day for the better or the worse.
Professor Ortiz (2015) explains that IQ tests are like samples of behavior. So when we talk of IQ, we should talk of IQ in a specific test. Obviously, IQ measurement is trying to predict “g” as precisely as possible. A good approach to get a solid IQ result is to carry out several IQ tests. The more “samples” of intelligence you have, the more powerful the prediction will be, and IQ and “g” will be closer to each other. “G”, like many other psychological variables like fear or love, is impossible to measure directly and that is why psychologists consider it a latent variable or construct.
The historical debate about factor “g”
The first relevant proposal about “g” can be found in the famous two-factor theory of Spearman at the beginning of the 20th century. Spearman, an expert in statistics, proposed that there was a general intelligence factor at the top, and many different specific abilities born from it. Competing theories emerged, and for example Thurstone contradicted Spearman proposing that intelligence was made up of seven independent intelligence abilities and that no single “g” existed. The discussion had just begun.
Spearman’s disciple R. Cattell, whose bifactor theory of fluid and crystallized intelligence paved the way for the most proven theory of intelligence, the CHC model, also rejected the concept of “g” after an initial acceptance. Later, Horn extended Cattell’s “Gf-Gc theory” with numerous abilities like visual processing or memory, and rejected more forcefully the importance of “g”, which he considered nothing else than a statistical meaningless calculation.
Schneider & McGrew (2012) note Cattell’s words on the issue: “Obviously, “g” is no more resident in an individual than horsepower in an engine. It is a concept derived from the relations between an individual and its environment”.
If the most advanced theories were discarding “g”, that would change completely when John Carroll published in 1993 his huge analysis of more than 400 previous intelligence studies in his work “Human Cognitive Abilities”. In his statistical analysis, he observed that the results in the tests were explained nearly 50% by a general intelligence factor that influenced lower level abilities. Hence, he theorized that intelligence had three levels and that at the top sat the factor “g” influencing all other abilities.
Current status of factor “g”
In the same way that there was a debate at the beginning of psychological research around the existence of “g”, there is still discussion today. But the issue is not anymore whether a “g” factor can be computed from the data, which it definitely can, or correlated to external variables, which has been done successfully many times, but whether G is just a statistical calculation without real meaning or reflects the existence of a real psychological global ability.
A reflection of the ongoing debate can be found in the most proven current theory of intelligence, the CHC model, which is a hierarchical theory that states that intelligence is made up of several abilities, and to which most researchers include “g” in the model, but not all.
Another important theory nowadays is the one proposed by Johnson & Bouchard in 2005, which states that intelligence can be better conceived as a “g-VPR model”. According to it, there is a general intelligence factor and three middle-level factors: verbal, perceptual, and rotational/kinesthetic. Again you could also evaluate the middle-level abilities ignoring the G factor.
From a clinical point of view, most intelligence tests are prepared for the calculation of a global ability, but its importance has been greatly reduced and most psychologists pay more attention to the differential profile of intelligence abilities.
The lesser importance given to G, should not lead us into the trap of thinking that G is not important, because it is. As Brody (2000) explains, several studies have found G to be very predictive of many relevant outcomes in life, such as academic success, income or even probability of divorce, an issue which we explain in depth in our article about IQ & success. And studies of twins reared apart have found that ⅔ of the correlation between IQ & outcomes was due to genes, which represents a huge indication that a general intelligence factor based on genes is responsible.
Animal intelligence gives us some clues
As Professor Anderson (2000) explains, when scientists have studied the intelligence of rats using different types of tasks, they have found that the rats that did well on one type of task (let us say for example in reasoning abilities applying previous knowledge to new tasks) usually did well on other tasks (like attention for novelty or response flexibility).
The same happened when researchers Shaw, Boogert, Clayton, and Burns (2015) developed a bird battery of tests to measure different cognitive abilities of ribbons (with tasks like recognizing symbols or memorizing positions), finding that those birds that did better on one task tended to do so in others. In other words, animal research supports the idea that a global ability like G was in play and explains many of the sub-abilities. If you want to learn more about the fascinating intelligence of animals, check our article about animal intelligence.
The fact that both human and animal studies support the existence of a general cognitive ability that influences all cognition has led many researchers to think that further research will discover the causes behind factor G, which could probably lie in neurology. As Professor Jensen (2000) puts it: “Understanding [the G factor]..., at a causal level, calls for the involvement of molecular genetics, the brain sciences (including animal models) and evolutionary psychology”.
The observed relationship between G and neurological factors such as relative brain size, speed of signal transmission, the number of connections of the neurons, amplitude and latency of brain waves, and others that you can learn about in our article “Where is intelligence in the brain”, indicate that one or more biological features of the brain could be the cause of general intelligence in humans and animals.
Wrapping up
In our fast paced review of general intelligence, we have seen that factor “G” is an important and predictive measure of our intelligence which is not completely understood. Conceived as a global cognitive capacity that permeates all abilities, it has been found both in humans and animals.
The current scientific debate centers around whether factor G is just a statistical computation without real psychological meaning, or if a general intelligence capacity really exists. Some researchers point to the strong association between G and outcome variables such as academic and job success as proof of its existence, and many think that it is likely explained by one or more neurological factors that affect all abilities.
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