What does an IQ test measure?

Fluid and crystallized intelligence

In 1963 Raymond Cattell drew a distinction that is still central to how tests are built (Wikipedia: Fluid and crystallized intelligence):

• Fluid intelligence is reasoning with new material: seeing the rule in an unfamiliar pattern, solving a problem you have not met before.
• Crystallized intelligence is what you have learned and can use: vocabulary, facts, methods.

Clinical batteries usually measure both, through a mix of verbal and nonverbal tasks. Nonverbal tests, such as matrix puzzles, lean heavily toward fluid reasoning because they give you material nobody has studied. That is the approach IQ Octopus takes, and it is explained further in our guides to culture-fair tests and matrix reasoning.

What IQ scores are linked to

IQ is one of the most studied measurements in psychology, and across large groups it is linked to real outcomes:

• School. In a sample of about 70,000 English children, general intelligence at age 11 correlated 0.81 with total exam points at age 16 (Deary, Strand, Smith & Fernandes, 2007).
• Work. A review of 85 years of personnel research estimated the correlation between general mental ability and job performance at 0.51 (Schmidt & Hunter, 1998).
• Life outcomes. A 2007 meta-analysis reported correlations with education of about 0.56, with occupational level of about 0.45, and with income of about 0.23 (Strenze, 2007).

A correlation describes how two things move together across many people. A correlation of 0.81 is strong, and one of 0.23 is modest, but neither tells you what will happen to a particular person. Many people do much better or worse than their score would suggest, because so much else matters: effort, health, opportunity, interests, luck and the people around them.

What an IQ test does not measure

An IQ test samples reasoning on short, well-defined problems with one right answer. It does not ask about:

• creativity, or how many good ideas you produce when there is no single right answer;
• motivation, persistence or how you handle setbacks;
• social and emotional judgment;
• expertise in a particular field, from plumbing to cardiology;
• character, values or kindness.

These things matter for most of what people care about, and an IQ score is silent on all of them. That is not a flaw in the test. It is the limit of what any short measurement can do.

Is IQ fixed?

Not entirely. Twin and family studies find that the heritability of intelligence rises with age, from about 20% in infancy to perhaps 80% in later adulthood (Plomin & Deary, 2015). Heritability is a statement about what explains differences across a population, not a measure of how much of any one person's ability is “genetic”, and it does not mean a score cannot change.

Scores do move. Average measured IQ rose through much of the 20th century, by about 2.8 points per decade across 31 countries (Pietschnig & Voracek, 2015), which is called the Flynn effect. Individual scores shift with practice and test conditions. And because IQ is age-normed, a person whose raw speed slows with age does not lose IQ points for that reason alone. Our guide to IQ and age covers what does change.

Where IQ Octopus fits

IQ Octopus is a short online test of fluid reasoning: 25 generated visual and number puzzles in five sections, scored with a Rasch model and reported as an estimated IQ with a confidence range. It is designed to give you an honest estimate of the general reasoning ability described above. It is not a clinical test and cannot replace one. The how it works page explains the method in full, and online vs clinical tests explains when you need which.