Children with low lead (yes, the metal, not a typo) levels have higher IQ because they are nearsighted.

That is, nearsightedness increases IQ (this is known). But, actual genetic nearsightedness is quite rare rare. Most of the nearsighted people today are nearsighted from the lack if lead, which isn't actually toxic. On the contrary, higher levels of lead prevent nearsightedness, which is why it seems, that children with lower levels have higher IQ.