I had to double-check my source and I realize the error was mine. “40% read at or above an eighth grade level” is the correct description of the state-level data I was looking at, which is distinct from the general metric used for literacy.
This led me into a bit of a rabbit hole trying to understand what in fact the official literacy rate is measured by if it’s so wildly different from - indeed almost double — the portion who can read at an eighth grade level.
The data is actually quite interesting. US National Center for Education statistics administer tests to assess “the ability to understand, evaluate, use, and engage with written texts to participate in society” and an individual falls into one of five categories. Official literacy definition considers above category one (“below basic”), but it is category three that maps approximately onto “eighth grade knowledge” (thus four as high school, five as post-graduate). The most interesting thing I found in the data is exploring that gap between two and three, ie states that have a high attainment of official literacy but then very low rates of the higher levels. California, for example, has the highest percentage of people below level two, but a relatively high percentage at level three and above — obviously I haven’t considered the data for long enough to conclude, but that suggests to me largely a question of immigration/non-English speaking populations. The state I’m from does better than California on attainment of level two, but significantly worse at attainment of three or above.
States where level 3+ > levels 1-2: District of Columbia, Washington, Minnesota, Oregon, Massachusetts, North Dakota / Utah / Colorado.
States with lowest level 3 (ie “eighth grade” equivalent) attainment: Mississippi 35%, Louisiana 35%, West Virginia 37%, New Mexico 39%, Nevada 39%, Alabama 39%, Arkansas 39%, Texas 40%, Tennessee 40%, Kentucky 41%.
TLDR that gap looks like an interesting way to separate issues with a state’s educational system from other questions. Whatever the best measure of literacy may be, it seems like the bar should be a bit higher than just “native speaker of the measured language.”