"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.