Not discounting knowledge decay, but fwiw it could be interpreted as the poorly-scoring teenagers who become adults lowering the average, which builds up year by year as their portion of adults steadily increases and the higher-scoring adults die off, rather than the individual adults scoring lower every year. Could totally be existing adults continuing to score lower as years pass, though.
Haven’t looked into the study, just adding my two cents on the graphic.
Literary RPG; novel with RPG elements like classes, levels, stats, skills, etc. with a focus on progression. Can be considered a subgenre of gamelit, which is any story in a game-like world.
Not sure about why the author and artist went for that cover, aside from the obvious of the guy holding what is presumably the eponymous staff.