The most important part of the abstract is this: > The analysis controls for all factors shared by siblings and finds no evidence for prominent causal hypothes

The most important part of the abstract is this: > The analysis controls for all factors shared by siblings and finds no evidence for prominent causal hypotheses of the decline implicating genes and environmental factors that vary between, but not within, families. My first thought was the decline might be due to rising number of immigrants, but this suggests that is not true. Instead it appears the decline is due to some broader environmental factor that is common across all families. It also suggests that this is likely occurring in the rest of the developed world as well, since I can't think of anything obvious that's occurring in Norway that isn't also occurring in the rest of the developed world.