What Is IQ?

IQ, short for intelligence quotient, is a standardized score from a test of reasoning, memory and related mental skills, scaled so that the average person scores 100 and about two thirds of people score between 85 and 115. It compares a person with others of the same age rather than measuring intelligence in absolute units. This guide explains where IQ came from, how scores are built and what the number can and cannot tell you.

Plain-language disclaimer: online tools and articles can only estimate where a score sits on a bell curve. A real IQ score comes from a standardized test given and interpreted by a licensed psychologist. Nothing here is a diagnosis or a claim about giftedness.

What does IQ mean?

“Intelligence quotient” originally described a ratio: mental age divided by chronological age, multiplied by 100. Today the word survives, but modern tests use a deviation score. A raw result is compared with the results of a large norming sample of people of the same age, then converted to a scale with a mean of 100. The score shows relative standing, such as above average or below average, not an amount of intelligence.

A short history of the IQ score

French psychologist Alfred Binet and Theodore Simon published the first practical intelligence scale in 1905 to identify schoolchildren who needed extra help. In 1912 William Stern proposed expressing results as a quotient of mental age to chronological age. Lewis Terman at Stanford revised the Binet scale in 1916 and popularized the IQ number in the United States. David Wechsler introduced the deviation IQ with the Wechsler-Bellevue scale in 1939, and Wechsler scales (WAIS for adults and WISC for children) remain among the most widely used tests today.

How IQ scores are scaled

Most modern IQ scores are standard scores with a mean of 100 and a standard deviation of 15. The scores follow an approximate bell curve, which makes it possible to translate any score into a percentile. About 68 percent of people score within one standard deviation of the mean (85 to 115), about 95 percent within two (70 to 130) and about 99.7 percent within three (55 to 145).

IQ score	Approximate percentile
70	2nd
85	16th
100	50th
115	84th
130	98th

Some older tests used other standard deviations, such as 16 for the 1937 to 1960 Stanford-Binet forms and 24 for Cattell scales, so the same number can mean different things. Use the IQ score converter to translate between them.

Is there one general intelligence?

Charles Spearman noticed in 1904 that people who do well on one mental test tend to do well on others. He proposed a general factor, g, behind these positive correlations. Many researchers still use g as a statistical summary of shared variance across tests, while others argue that intelligence is better described as several distinct abilities, or that g is a convenient statistic rather than a single mental thing. Both views are active areas of research.

What IQ predicts, and how much

IQ scores correlate with school achievement, years of education and some measures of job performance. Reported correlations are typically moderate, often around 0.3 to 0.6 depending on the outcome. A correlation of 0.5 means that most of the variation in the outcome is explained by other things, such as motivation, health, opportunity, teaching and chance. IQ is one useful predictor and not a verdict on a person’s future.

What IQ does not measure

IQ tests do not measure creativity, wisdom, practical skills, emotional understanding, character, curiosity or effort. They sample a limited set of cognitive skills in a particular testing situation, with norms built from a particular population and time. A person can score lower than they could on a different day because of fatigue, anxiety, language or health.

Can IQ change?

Scores are fairly stable in adults over short periods, but they are not fixed. Childhood scores shift as children develop and as test measurement error plays out. Education, health, nutrition and environment influence cognitive development, and the population average has risen over the 20th century, a trend known as the Flynn effect.

The Flynn effect

New Zealand political scientist James Flynn documented that raw scores on intelligence tests rose by about three points per decade across many countries during much of the 20th century. Because tests are periodically renormed back to a mean of 100, the trend is hidden unless old and new norms are compared. Proposed explanations include better nutrition and health, more schooling, smaller families and a more cognitively demanding environment. In some countries the gains have slowed or reversed in recent decades.

Genes and environment

Twin and adoption studies suggest that differences in IQ among people in a population are influenced by both genetic and environmental factors, with estimates of heritability commonly reported to rise from childhood into adulthood. Heritability describes variation within a group in a given environment. It does not mean a trait is fixed or that group differences share the same causes.

How to read an IQ score responsibly

Treat a score as a range, not a point. Test results have a standard error of measurement of roughly three to five points. Ask which test was used, who gave it and what the standard deviation is. Use a confidence interval calculator to see the plausible range, and treat quiz results from the internet as entertainment.

Try the IQ tools

Frequently asked questions

What does IQ stand for?

Intelligence quotient.

What is a normal IQ?

Scores from 90 to 109 are classed as average on Wechsler scales, and about half of people fall in that range.

What is the average IQ?

100, by design. Tests are normed so that the mean of the reference population is 100.

Who invented the IQ test?

Alfred Binet and Theodore Simon created the first practical intelligence scale in 1905. The IQ number came later through William Stern, Lewis Terman and David Wechsler.

Is IQ the same as intelligence?

No. IQ is a score from a particular set of tasks. It captures part of what people call intelligence.

Can you raise your IQ?

Education and a healthy environment support cognitive development, but there is no proven quick method to raise a measured IQ, and practice on a test mostly improves performance on that test.

Are online IQ tests accurate?

Most are not standardized or normed, so they give rough entertainment-level estimates.

Sources

Back to the IQ Online Center

Effortless Math Team

11 minutes ago