IQ levels has risen on average because of nutrients, not using lead and education.
An increase in blood lead from 10 to 20 micrograms/dl was associated with a decrease of 2.6 IQ
An increase in blood lead from 10 to 20 micrograms/dl was associated with a decrease of 2.6 IQ
IQ scores were never intended to be used to compare across cohorts. To do so is invalid.
...nobody really knows why