What is IQ (intelligence quotient)?

Updated October 26, 2026 · QI Agora team

IQ stands for intelligence quotient: a number that sums up how a person did on a reasoning test compared with other people. It doesn't measure everything someone knows or can do, but it is one of the most studied measures in psychology for skills like logic, working memory and abstract reasoning.

Where IQ came from

The story starts in France in 1905, when Alfred Binet and Théodore Simon created one of the first practical intelligence scales. The goal was to identify children who needed more support at school. In 1912, German psychologist William Stern proposed the term "intelligence quotient": mental age divided by real age. A few years later in the United States, Lewis Terman multiplied this value by 100, and the number started to look like the one we know today.

The modern form came with David Wechsler, who created his scales from 1939 onwards. Instead of comparing mental age with real age, he compared each person with others in the same age group. This is called the deviation IQ, still used today.

How the scale works

On the deviation scale, the average is 100 and the standard deviation is 15. In practice, this means that:

about 68% of people fall between 85 and 115;
about 95% fall between 70 and 130;
only about 2% score above 130.

So an IQ of 115 doesn't mean "15% smarter": it means the person did better than roughly 84% of the comparison group. See the full table in Average IQ and scale.

What an IQ test measures

Tests usually combine different tasks, because intelligence isn't just one thing. The most common are:

Abstract reasoning: finding patterns in figures, as in 3×3 matrices.
Numerical reasoning: spotting rules in sequences and solving number problems.
Working memory: holding and using information for a few seconds.
Spatial reasoning: picturing shapes as they rotate or move.
Verbal reasoning: analogies, deductions and relationships between words.

QI Agora covers all of these areas. See examples in types of reasoning.

What IQ doesn't measure

A number doesn't sum up a person. Creativity, emotional intelligence, accumulated knowledge, practical skills, persistence and artistic talent are largely left out. Results also vary with tiredness, anxiety, distractions and familiarity with this kind of question.

Does IQ change? A person's result can vary by a few points from one test to another. Throughout the 20th century, average IQ test scores rose in many countries, by about 3 points per decade. This became known as the Flynn effect, after researcher James Flynn, and it's one of the reasons tests need to be recalibrated from time to time.

Online test vs. psychological assessment

A psychological assessment is carried out by a licensed psychologist, using approved tests, in a controlled setting and with professional analysis. It can be used for diagnoses, formal reports and important decisions.

An online test like QI Agora is different: it is unsupervised and meant for self-knowledge and entertainment. The result is an estimate, calculated with sound statistical methods, but it does not replace a professional assessment. Read the disclaimer.