" I've found that people who make fewer mistakes on a grammar test also make fewer mistakes when they are doing something completely unrelated to writing — like

" I've found that people who make fewer mistakes on a grammar test also make fewer mistakes when they are doing something completely unrelated to writing — like stocking shelves or labeling parts." I take issue with this. Assuming you could, assuming they were available to work because they couldn't find any other job, would you hire a Rhodes Scholar? If you are hiring for a blue collar type position like "stocking shelves" you want someone who is qualified enough and happy to be in that job and planning to stay at that job. Not thinking it's an interim job until they find something better. A person who is academic enough to have perfect english many times will be an under achiever who might have issues and that is why they are only "stocking shelves". There is a reason why companies often say "you are over qualified for the job" as a reason someone isn't hired. The job has to fit the person roughly. As an aside the OP didn't exactly define what is meant by "poor grammar" or place any links to the "mandatory grammar test" so readers can even decide for themselves.