"Japanese and Dutch adults who were ages 25 to 34 and only completed high school easily outperformed Italian or Spanish university graduates of the same age."
"US adults are dumber than the average human" Actually this is not true. The researchers only tested 166,000 people ages 16 to 65 in 20 western countries. This only means that US Adults are dumber than humans in 20 leading western nations. US adults are actually far above average in math, reading and problem-solving using technology when compared to the entire human population across the planet. In many countries many people can not even read or write much less understand math or science.
I have been interacting with Mexican immigrants for decades while living in Southern California. I was shocked to learn that half the people in Mexico never learn to read in any language at all. A lot of things fell into place after I learned that.