Why or why not? I think it is, because IQ is the best predictor of success or failure in a range of tasks considered to require "intelligence". Further, those with low IQs are rarely or never found in high IQ occupations, such as hard science academics. This is found across cultures, and no-one has succeeded in making a test which produces equal results between races and nations, yet has predictive validity for intellectual activities as defined in each culture. Despite claims to the contrary what it means to be "intelligent" is pretty consistent across cultures. Also the positive manifold of correlations between IQ subtests yields a general factor which itself correlates with a number of biological brain measures.