Intelligence tests are as much a measure of motivation as they are of mental ability, says research from the US.
Researchers from Pennsylvania found that a high IQ score required both high intelligence and high motivation but a low IQ score could be the result of a lack of either factor.
Incentives were also found to increase IQ scores by a noticeable margin.
The study is published in Proceedings of the National Academy of Sciences.
Firstly, it analysed previous studies of how material incentives affected the performance of more than 2,000 people in intelligence tests.
Researchers from the University of Pennsylvania, Philadelphia, found that incentives increased all IQ scores, but particularly for those of individuals with lower baseline IQ scores.
Then the same researchers tested how motivation impacted on the results of IQ tests and also on predictions of intelligence and performance in later life.
By using data from a long-term study of 250 boys from adolescence to early adulthood, they were able to conclude that some individuals try harder than others in conditions where the stakes are low.
Therefore, the study says, "relying on IQ scores as a measure of intelligence may overestimate the predictive validity of intelligence."
Getting a high score in an IQ test requires both high intelligence and competitive tendencies to motivate the test-taker to perform to the best of their ability.
Dr James Thompson, senior honorary lecturer in psychology at University College London, said it had always been known that IQ test results are a combination of innate ability and other variables.
"Life is an IQ test and a personality test and an IQ result contains elements of both (but mostly intelligence).
"If an IQ test doesn't motivate someone then that is a good predictor in itself."