G6PD and thalassemia: We also considered using other polymorphisms, including glucose-6-phosphate dehydrogenase (G6PD) A deficiency and α-thalassemia, as instrumental variables, but, in meta-analyses of published literature (Extended Data Fig. 1 and Supplementary Table 1), the protective effects of these polymorphisms are largely confined to severe malaria, an outcome of limited relevance as a cause of ID at a population level.