Yeah, it is a single one dimensional number that attempts to represent a multidimensional system. But also you can pretty well predict whether someone is capable of, like, standing trial or something if that single number is particularly low. It does correlate with a real phenomenon, if not perfectly.

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Lots of things are used to justify bigotry. If you assume that subjectively negative attributes are a result of heritage rather than upbringing, you can use anything to justify bigotry.

For native English speakers, IQ largely correlates with general intellectual ability. People with higher IQs are generally more capable of understanding more complicated topics. It’s not a moral failing to have a low IQ, and there’s no reason to assume it’s genetically heritable or innate and immutable.