Education!
"It garnered this attention for its intriguing yet simultaneously obvious finding: over the past 80 years, as a far greater proportion of North Americans attended college, the average IQ of college undergraduates dropped from around 120 to 102, just slightly above the average of 100."
https://www.zerohedge.com/political/x-users-didnt-papers-tone-and-findings-so-they-got-it-rejected
Discussion
I think there are simply more subjects being taught there that used to be vocational training, and the exam difficulty is lower.