IQ tests are controversial because they reduce an entity as complex as human intelligence to a single number. How can one number capture all of intelligence when humans display such diverse mental talents such math aptitude, social savvy, linguistic skills, musical talent etc? The answer is that all mental abilities are positively correlated, so people who are smart in one area, often are smart in another. The intercorrelation of cognitive abilities is caused by g (general intelligence). Tests of abstract reasoning tend to load high on g while tests of psyco-motor speed have low g loadings. IQ tests measure intelligence because they try to sample the most g loaded mental abilities. IQ tests are further validated by the fact that people who score high tend to have more positive life outcomes (higher incomes, more prestigious occupations, longer life span). IQ tests are further validated by their correlation with brain anatomy (high IQ people often have larger brains that metabolize glucose more efficiently with faster nerve conduction speed).
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