Spearman noticed that children’s grades across all school subjects tended to be highly correlated. If a child did well in one subject, they generally also did well in another subject, and vice versa. What did this say about the nature of intelligence?
He devised factor analysis to measure the relationships between seemingly varied cognitive abilities and account for the correlations he saw between scores on different tests.
The result was Spearman’s two-factor theory which attempted to show that all cognitive performance can be explained by two variables: one general ability (g) and the many specific abilities (s) it gave rise to. Later, however, further analysis showed that g alone was enough to explain the correlations between different tests. When people talk about IQ or intelligence, it’s usually this general mental ability that they are referring to.
Psychometrically, g as a construct refers to the overall mental capacity behind a person’s performance on any number of cognitive tasks.
Statistically, g is a way to account for variance. This single factor has been shown to explain 40 - 50% of the variance in individual performance on IQ tests. This is why a composite score of many different tests is assumed to give an estimation of g.
Today, almost all IQ tests are factor models inspired by Spearman’s work on g. As an example, consider the Stanford-Binet test, which measures different areas of performance that contribute to general intelligence, like working memory and visual-spatial reasoning.
Today intelligence is usually understood as a hierarchy: smaller factors manifest in the ability to do highly specific tasks, but those factors can be arranged into broader intermediary categories which in turn are encompassed within the most general factor, g.
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.
The early 1900s saw Charles Spearman using a mathematical approach to the question of measuring human intelligence. Using statistical factor analysis Spearman identified g, a single underlying intelligence factor he believed accounted for the variety of observable abilities.
General intelligence, also known as g factor, refers to the existence of a broad mental capacity that influences performance on cognitive ability measures. Charles Spearman first described the existence of general intelligence in 1904. According to Spearman, this g factor was responsible for overall performance on mental ability tests. Spearman noted that while people certainly could and often did excel in certain areas, people who did well in one area tended also to do well in other areas.
For example, a person who does well on a verbal test would probably also do well on other tests.
Those who hold this view believe that intelligence can be measured and expressed by a single number, such as an IQ score.The idea is that this underlying general intelligence influences performance on all cognitive tasks.
General intelligence can be compared to athleticism. A person might be a very skilled runner, but this does not necessarily mean that they will also be an excellent figure skater. However, because this person is athletic and fit, they will probably perform much better on other physical tasks than an individual who is less coordinated and more sedentary.
Spearman and General Intelligence
Charles Spearman was one of the researchers who helped develop a statistical technique known as factor analysis. Factor analysis allows researchers to a number of different test items that can measure common abilities.
For example, researchers might find that people who score well on questions that measure vocabulary also perform better on questions related to reading comprehension.
Spearman believed that general intelligence represented an intelligence factor underlying specific mental abilities. All tasks on intelligence tests, whether they related to verbal or mathematical abilities, were influenced by this underlying g-factor.
Many modern intelligence tests, including the Stanford-Binet, measure some of the cognitive factors that are thought to make up general intelligence. These include visual-spatial processing, quantitative reasoning, knowledge, fluid reasoning, and working memory.
- Visual-spatial processing involves such abilities as putting together puzzles and copying complex shapes.
- Quantitative reasoning involves the capacity to solve problems that involve numbers.
- Knowledge involves a person's understanding of a wide range of topics.
- Fluid reasoning involves the ability to think flexibly and solve problems.
- Working memory involves the use of short-term memory such as being able to repeat a list of items.
Challenges to the Concept of General Intelligence
The notion that intelligence could be measured and summarized by a single number on an IQ test was controversial during Spearman's time and has remained so over the decades since. Some psychologists, including L.L. Thurstone, challenged the concept of a g-factor. Thurstone instead identified a number of what he referred to as "primary mental abilities."
More recently, psychologists such as Howard Gardner have challenged the notion that a single general intelligence can accurately capture all of human mental ability.
Gardner instead proposed that different multiple intelligences exist. Each intelligence represents abilities in a certain domain such as visual-spatial intelligence, verbal-linguistic intelligence, and logical-mathematical intelligence.
Research today points to an underlying mental ability that contributes to performance on many cognitive tasks. IQ scores, which are designed to measure this general intelligence, are also thought to influence an individual's overall success in life. However, while IQ can play a role in academic and life success, other factors such as childhood experiences, educational experiences, socioeconomic status, motivation, maturity, and personality also play a critical role in determining overall success.
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