Nothing more to say about variation arising from geography around the US, such as urban vs rural, high vs low education, by age, etc, etc. And have other developed countries seen similar shifts?
It's like the authors overheard a single sentence on the topic in a bar and decided to turn it into an entire article without looking into it any further.
Seriously though, they observed a reverse Flynn effect by taking a thin slice of negative test results from a test that doesn’t even measure IQ and finding it statistically significant. This kind of reminds of when climate deniers take a tiny slice of climate records and claim the earth is actually getting cooler.
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Unsurprisingly you’ll actually find many IQ researchers in institutions which are still trying to deny climate change. E.g. Charles Murray is now working for the American Enterprise Institute.
It's like the authors overheard a single sentence on the topic in a bar and decided to turn it into an entire article without looking into it any further.