iq is the best metric we have but it isn't perfect, at the extreme ends of the scale it is harder to make any reliable distinctions. you can build great predictive models for different outcomes distinguishing between 90 and 100, but between 150-160 or 40-50 it is impossible.   iq scores also aren't racially normed, nobody wants to confront the question of why a white person with an iq of 60 is a drooling retard while a negro with the same score can still function socially. obviously it has something to do with the baseline being radically different, for a white person to be that dumb they must be genuinely damaged in some way, while for a black they are just bit slower than the average. but nobody in the social sciences wants to address it.
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