Pretty sure that decline started much earlier.

By their words ye shall know them: Evidence of genetic selection against general intelligence and concurrent environmental enrichment in vocabulary usage since the mid 19th century

https://archive.ph/Zja1a

Also see: MIT admissions test 1869

I'd start curving the line down a lot earlier - like in 1950 rather than 1980, maybe even as early as 1939 or 1914.