Shift the Blame. A few people have attempted to shift the blame onto technology, and how we’ve become so reliant on it. But that theory has been stomped out since the decline first started in the 1970s.
Quality. It could also have to do with the fact that people are reading less these days, and instead filling their time absorbing more “trashy media.” And the quality in education has been declining pretty rapidly.
Outdated. Another problem may be that IQ tests are kind of outdated. They tend to measure our crystallized intelligence--all the things we’ve been taught and remember--and education has evolved and doesn’t focus as much on memorization now. Scientists are still working hard to figure out the exact reason though.