However, as explained above, IQ tests were highly successful in assessing recruits during wartime. Therefore, "it must be true that they are measuring a relevant mental capability". Therefore, IQs are not simply a mathematical fiction: they relate to the ability of individuals to perform certain functions. Even if experts do not agree on a definition of intelligence, that does not disprove the usefulness (or otherwise) of the tests. In every day life people do notice the relative intelligence of others. The issue is central to human nature and evolutionary psychology, because humans evolved the characteristics which helped them survive and reproduce.