“Average joe”. Humanity evolves slowly over long periods. The average joe today is far more educated than one from 200 years ago.
And it's not at all clear if that education does anything other than magnify their intellectual predispositions. The smart people can make great strides, but the stupid will be stupid louder and harder. And the average may well just be... more average.
An increase in blood lead from 10 to 20 micrograms/dl was associated with a decrease of 2.6 IQ
...nobody really knows why
IQ scores were never intended to be used to compare across cohorts. To do so is invalid.