Okay, I see what you're saying, so we take two sample groups, use one as a baseline, then lay the second group over that curve.

So now we need to be really careful about bias in the sample groups.

To say IQ scores are dropping and we don't know why then means to me we don't know what variables were the key drivers of the first groups results so we therefore cannot possibly create a consistent second group to measure against the baseline.

I'm still stuck but I'm often dense and skeptical on these type of analysis.