[Flyndaran]

Idiocracy: for no known reason, each generation of complex animals suffer reduced cognitive function.

It has a nearby reality 100 million BCE where all complex life appears to be gaining intelligence. Some dinosaurs have reached primate levels on par with movie versions.
Hassle, because a rumor that conceiving here will lead to a smarter baby. No evidence for or against that has shown up, but people will still try.

Researchers don't know if these are examples of some kind of environmental or biologically innate cycle.