Is there any strong relationship between IQ scores and innate intelligence, as opposed to mental agility gained through education?
Besides the declining groups have the same education with the earlier ones.
Which doesn't matter, since they measured rich and middle class, and poor and discriminated against both before and after.
Did you think the new measurements were done at some ghetto and the earlier higher ones at Martha's Vineyard?
Are you claiming social factors have remained constant during the measuring period? Because they very obviously haven't.
If you're aware of the Peter principle, and how inequality compounds over time, then you know that the rate at which social factors change is correlated with their quantile values.
What do you propose as a replacement?