I recall when I was in college, one of our textbooks said that ancient Athenians, or maybe it was ancient Greeks had a much higher IQ than modern man.
I have no idea how that was calculated but I can believe it considering it was at most a city of 100,000 and produced so many brilliant men.
The free and literate were high IQ, the top 20-30% of society. The slaves, generally not literate or part of recorded culture except in property lists, were the other 70-80%. An assessment of the literate class thus skews upward in IQ estimates. The low IQ just weren’t generating records for us to review later.