 The usefulness of the method of extrapolation by causal capacities depends, among other things, on the extent to which biomedical factors can be characterized as having capacities. Many biomedical causes do indeed have some degree of stability. The sickle cell trait is 50% protective against mild clinical malaria, 75% protective against admission to the hospital for malaria, and almost 90% protective against severe or complicated malaria (Williams et al. 2005). These figures suggest a reading along the lines of, 