Psychometric theories have generally sought to understand the structure of intelligence: What form does it take, and what are its parts, if any? Such theories have generally been based on and established by data obtained from tests of mental abilities, including analogies (e.g., lawyer is to client as doctor is to __), classifications (e.g., Which word does not belong with the others? robin, sparrow, chicken, blue jay), and series completions (e.g., What number comes next in the following series? 3, 6, 10, 15, 21,_).

Psychometric theories are based on a model that portrays intelligence as a composite of abilities measured by mental tests. This model can be quantified. For example, performance on a number-series test might represent a weighted composite of number, reasoning, and memory abilities for a complex series. Mathematical models allow for weakness in one area to be offset by strong ability in another area of test performance. In this way, superior ability in reasoning can compensate for a deficiency in number ability.

One of the earliest of the psychometric theories came from the British psychologist Charles E. Spearman (1863–1945), who published his first major article on intelligence in 1904. He noticed what may seem obvious now—that people who did well on one mental-ability test tended to do well on others, while people who performed poorly on one of them also tended to perform poorly on others. To identify the underlying sources of these performance differences, Spearman devised factor analysis, a statistical technique that examines patterns of individual differences in test scores. He concluded that just two kinds of factors underlie all individual differences in test scores. The first and more important factor, which he labeled the “general factor,” or g, pervades performance on all tasks requiring intelligence. In other words, regardless of the task, if it requires intelligence, it requires g. The second factor is specifically related to each particular test. For example, when someone takes a test of arithmetical reasoning, his performance on the test requires a general factor that is common to all tests (g) and a specific factor that is related to whatever mental operations are required for mathematical reasoning as distinct from other kinds of thinking. But what, exactly, is g? After all, giving something a name is not the same as understanding what it is. Spearman did not know exactly what the general factor was, but he proposed in 1927 that it might be something like “mental energy.”

Components of General Intelligence

There are several key components that are believed to make up general intelligence. these include:

How Is It Measured?

Many modern intelligence tests measure some of the cognitive factors that are thought to make up general intelligence. Such tests propose that intelligence can be measured and expressed by a single number, such as an IQ score.

The Stanford-Binet, which is one of the most popular intelligence tests, aims to measure the g factor. In addition to providing an overall score, the current version of the test also offers a number of score composites as well as subtest scores in ten different areas.

Alternatives and Criticisms

The existence of a single quantifiable factor for human intelligence has been hotly debated ever since Spearman proposed it.

Criticism came from one of Spearman’s own students, Raymond Cattell, who thought that intelligence could be understood as two main capacities: “fluid" (Gf) and “crystallized" (Gc).

Cattell thought that crystallized intelligence was a kind of cemented knowledge bank acquired over time, representing all those abilities that were already familiar from previous learning. On the other hand, fluid intelligence was the ability to acquire that knowledge in the first place, i.e. to learn in the moment. He saw g as more accurately Gc, and that tests focusing only on g would omit an important developmental factor in human intelligence.

Others were similarly critical for the reductive nature of g, including psychologist L.L. Thurstone and J. P Guilford. Both believed that there were several, irreducible and independent domains of intelligence, however many have since found correlations between their tests which strongly suggest a general factor.

Still more criticism came from Howard Gardener who proposed nine domains of intelligence, including some decidedly non-cognitive ones like musical, existential and bodily-kinesthetic intelligence. Almost everyone can think of a person who performed poorly at school but excelled in sport or dance, perhaps, or a person with musical genius that didn’t translate to any other area in their life.

Gardner argued that the academic environment over-emphasized verbal and logical skill while ignoring these other forms of intelligence. However, his critics have responded that we think of something like athletic skill as just that – a skill and not strictly intelligence.

Currently, the g factor theory of intelligence is largely undisputed and has been established through experimental cognitive research, brain anatomy and molecular genetics – where it has also been shown to have a strong heritable component. Though it is taken as true that there is a high correlation between performance on different skills tests, research is still underway to determine what causes that correlation and how.