The zero optimal capital tax relies on the assumption of preference homogeneity. Both Mirrlees (1976) and Saez (2002) argue that high-ability might have higher saving rates due to different preferences. If this is the case, then capital income taxation is optimal for income redistribution as the individual savings level reveals information about individuals' ability, thereby facilitating the redistribution of income from high-ability to low-ability individuals. This argument is empirically borne out by research on the correlation between individuals' willingness and earnings ability.
