US adults aren't necessarily dumber, they're just willfully ignorant.
It's like an aberrant offshoot of the Dunning-Kruger effect*. We're so busy shouting "U-S-A, U-S-A, U-S-A" that we haven't noticed that were falling behind in Science, Math, Health Care, Social Welfare, Infrastructure.... etc.etc.etc.
It's this sort of jingoistic, false patriotism that keeps us from addressing the real problems that the U.S. faces.
Heck, half our problems are caused by our willful ignorance, general suspicion of science and anybody who seems like they're smarter than we are, and our willingness to dismiss anything that we can't understand.
Sorry, for the rant... I live in the U.S. and I have a deep and abiding love for my country... but it doesn't extend to those who mistake their own self-interest and ignorance for nationalist fervor. I hate feeling like I have to apologize to the rest of the world, the scientific community, and civilization at large for the vocal minority of yahoos that live here.
BTW... Average U.S. IQ is just 98. So yes. Science says that on the whole we're dumber than the median IQ.
www.statisticbrain.com/countries-with-the-highest-lowest-average-iq/
*(Please excuse the blatant quote from wikipedia.)
"The Dunning–Kruger effect is a cognitive bias in which unskilled individuals suffer from illusory superiority, mistakenly rating their ability much higher than is accurate. This bias is attributed to a metacognitive inability of the unskilled to recognize their ineptitude."