General intelligence is a broad mental capacity involving the ability to reason, solve problems, think abstractly, and learn from experience. It is not about possessing a large amount of knowledge, but the ability to acquire and apply it effectively. This cognitive ability influences performance across a wide spectrum of mental tasks, allowing individuals to understand complex ideas and adapt to new situations.
The General Intelligence Factor (g)
The concept of a general intelligence factor, or “g,” was proposed by psychologist Charles Spearman in the early 20th century. Spearman observed that individual performances on a variety of seemingly unrelated cognitive tests were positively correlated. For instance, a person who performed well on a verbal ability test was also likely to do well on a mathematical skill test, suggesting a single, underlying mental ability influenced all cognitive tasks.
Spearman developed factor analysis to study these correlations and proposed his two-factor theory of intelligence. This theory states that every mental task involves two components: the general factor (“g”) common to all tasks, and a specific factor (“s”) unique to that task. A person’s success on a math problem is thus influenced by their general intelligence and their specific mathematical aptitude.
The g factor can be understood through an analogy to general athleticism. A gifted athlete possesses a high level of overall physical conditioning that allows them to perform well across different sports, like basketball or track. While they may have more skill in one sport, their underlying athleticism contributes to success in all of them, much as the g factor contributes to performance across cognitive domains.
Measuring General Intelligence
General intelligence is an abstract construct and cannot be measured directly. Instead, it is assessed indirectly through psychometric tools, most notably Intelligence Quotient (IQ) tests. While not a direct measure of “g,” scores on a comprehensive IQ test are considered strong estimates of a person’s standing on the g factor.
Widely used IQ tests, like the Wechsler Adult Intelligence Scale (WAIS), use numerous subtests to assess specific cognitive abilities that combine into an overall score. The WAIS-IV yields scores across four domains: Verbal Comprehension, Perceptual Reasoning, Working Memory, and Processing Speed. For example, subtests may ask individuals to define words or repeat a sequence of numbers.
Test scoring is standardized by administering them to a large, representative sample of the population, with an individual’s performance compared to others in their age group. The average score is set at 100, and most people score between 85 and 115. This approach provides a consistent estimate of an individual’s cognitive ability relative to their peers.
Genetic and Environmental Influences
Both genetics and environment play significant roles in determining intelligence. Evidence for heritability comes from twin studies, where identical twins reared apart show highly correlated IQ scores. Adoption studies further support this, as adopted children’s IQ scores correlate more strongly with their biological parents than with their adoptive parents.
Heritability estimates for intelligence range from 40% to 80%, meaning much of the variation in IQ scores within a population is due to genetic differences. Heritability is a population statistic that explains variance; it does not mean an individual’s intelligence is fixed by their genes. The influence of genes on intelligence also appears to increase with age.
Environmental factors also have a profound impact on cognitive development, starting with prenatal conditions like maternal nutrition. After birth, factors such as nutrition, quality education, family income, and home environment continue to shape intelligence. Children in deprived environments with poor nutrition and limited educational opportunities may not reach their full genetic potential, demonstrating the complex interplay between genes and environment.
Real-World Predictive Value
Measures of general intelligence consistently predict a wide range of life outcomes. In academic settings, general cognitive ability is a strong predictor of success at all levels of education. Higher IQ scores are correlated with better grades and higher levels of educational attainment.
This predictive power extends to the world of work, where general intelligence is a significant predictor of job performance, especially in complex roles. Individuals with higher cognitive ability tend to acquire job knowledge more quickly, which translates to better performance. This also extends to occupational attainment and income.
General intelligence has also been linked to health and longevity, as individuals with higher IQ scores tend to have better health outcomes and live longer. It is important to view these findings as correlations and probabilities, not deterministic guarantees. General intelligence is one of many factors that contribute to life outcomes.
Alternative Intelligence Theories
While the g factor is a well-established concept, some psychologists argue it presents an incomplete picture of human intellect. They propose that intelligence is a collection of multiple, distinct abilities, not a single general one. These alternative models challenge the idea that one IQ score can capture the breadth of human capabilities.
Howard Gardner’s theory of multiple intelligences is a prominent alternative model. He proposed several distinct intelligences beyond the linguistic and logical-mathematical abilities measured by IQ tests. These include:
- Musical
- Spatial
- Bodily-kinesthetic
- Interpersonal (understanding others)
- Intrapersonal (understanding oneself)
- Naturalist
Gardner’s theory suggests a person can be highly skilled in one area without necessarily excelling in others.
Robert Sternberg’s triarchic theory of intelligence is another influential model, proposing three types of intelligence: analytical, creative, and practical. Analytical intelligence is the academic problem-solving measured by IQ tests. Creative intelligence is the ability to generate new ideas and handle novel situations, while practical intelligence (“street smarts”) is the ability to manage everyday tasks. Sternberg argues all three are necessary for “successful intelligence.”