This belief is plain idiotic. Researchers in the field of psychometrics have had an understanding of "intelligence" for decades. The various attributes related to the common notion of "smartness" can be measured, we can measure reaction times, spatial reasoning ability, verbal ability, the ability to recall information, etc. Those various attributes consistently correlate, those skilled in one area are as skilled in other areas. This allows us to create a factor of general intelligence. We then take the mean score of general intelligence, make it 100, and compare everyone against it.
Unsurprisingly g is a major predictor for various social factors, it correlates positively with income level and job performance while negatively correlating with illegitimacy and criminality. G also correlates with brain size, those with more neurons are generally more intelligent. IQ tests are fairly accurate, highly predictive, and apply to all groups of people.