Intelligence Quotient (IQ)

What is an Intelligence Quotient (IQ)?

The Intelligence Quotient, commonly known as IQ, is the most recognized metric in the field of psychology for assessing human intelligence. It is not a measure of knowledge or education, but rather a gauge of cognitive potential—specifically, the ability to reason, solve problems, understand complex ideas, and learn from experience.

IQ is a statistical construct derived from standardized tests. It places an individual’s performance on a comparative scale against the general population, typically following a Normal Distribution (Bell Curve) with a mean score of 100 and a standard deviation of 15.

The History: From Binet to Stern

The origins of IQ testing trace back to the early 20th century in France.

• Alfred Binet: Along with Theodore Simon, Binet developed the first practical intelligence test in 1905 to identify school children who needed extra educational support. He introduced the concept of “Mental Age.”
• William Stern: The German psychologist who coined the term “Intelligence Quotient” (IQ). He proposed the original formula: IQ = (Mental Age / Chronological Age) × 100.
• Lewis Terman: An American psychologist at Stanford University who adapted Binet’s work into the Stanford-Binet Intelligence Scales, which standardized the IQ scoring system used today.

Modern Scoring: Deviation IQ

Early calculations based on “mental age” worked well for children but failed for adults. Modern tests, such as the Wechsler Adult Intelligence Scale (WAIS), use Deviation IQ.

In this system, a score is calculated based on how far an individual deviates from the average score of their age group.

• 100: The average IQ.
• 85–115: Represents roughly 68% of the population (one standard deviation from the mean).
• 130+: Typically classified as “Very Superior” or “Gifted” (top 2% of the population).
• 70 or below: Often indicates intellectual disability.

What Does IQ Actually Measure?

While often debated, IQ tests are designed to measure General Intelligence (g). They typically assess a battery of distinct cognitive domains:

1. Verbal Comprehension: Vocabulary, general knowledge, and abstract reasoning with words.
2. Perceptual Reasoning: Visual-spatial skills, pattern recognition, and non-verbal problem solving.
3. Working Memory: The ability to hold and manipulate information in the short term (e.g., mental arithmetic).
4. Processing Speed: How quickly and accurately the brain can perform simple cognitive tasks.

The Validity and Impact of IQ

Decades of research have established IQ as one of the most reliable predictors in social science.

• Academic Achievement: IQ correlates strongly with school grades and educational attainment.
• Job Performance: For complex professions (like medicine, engineering, or law), IQ is a robust predictor of trainability and efficiency.
• Life Outcomes: Higher IQ is statistically associated with better health, higher income, and longevity.

However, IQ is not destiny. While it sets the “cognitive ceiling” or potential, factors like Conscientiousness, Grit, and opportunity play massive roles in actualizing that potential.

Criticism and Controversy

The concept of IQ is not without controversy. Critics argue that tests can be culturally biased or fail to capture the full spectrum of human ability, such as Emotional Intelligence (EQ) or creativity. Despite this, psychometricians maintain that IQ remains the most objective and validated tool we have for understanding cognitive differences.