Q. I don’t understand IQ tests and what it is that’s really being measured. Can you explain this?

A. Interesting question. Obviously, given that every brain is unique, each will respond slightly differently to any measure of IQ. My understanding is that Raymond Cattell identified two forms of general intelligence: fluid and crystalized. They are believed to involve separate mental and neural systems.

  • Fluid intelligence, so called, involves the ability to think inductively and deductively in solving problems separate from specific knowledge that you have acquired. This type of intelligence helps you to analyze problems (especially novel or unusual ones), identify factors that contribute to the problems, and come up with solutions.
  • Crystalized intelligence, so called, involves the ability to use knowledge you have gained from educational and cultural experience along with learned skills that are accessed from long-term memory. Because this type of intelligence involves your lifetime intellectual achievement (including vocabulary, general knowledge, and the ability to think and reason using numbers and words) it can improve with age.

Of course, overall, you use both of these types of intelligences in combination. Typical IQ tests are designed to measure both fluid and crystalized intelligences. The overall score is a combination of both these measures. According to some sources, the WAIS (Wechsler Adult Intelligence Scale) is designed to assess crystallized intelligence on the verbal scale and fluid intelligence on the performance scale, the overall IQ score resulting from a combination of these two scales.