Masks have been a cultural flash point since the start of the pandemic, and mask mandates in schools have been especially incendiary. Critics have argued that there is no strong evidence to prove that masks slow the spread of Covid, and that in any case children weren’t wearing the right kinds of masks or weren’t wearing them properly.

Now a research paper details a so-called natural experiment that occurred when all but two school districts in the greater Boston area lifted mask requirements in the spring. Researchers took that opportunity to make a direct comparison of the spread of Covid in masking and non-masking schools.