The assessment of inter-group disparities in the distribution of human development is an important component of the Capability Approach to understanding aspects of social justice. In situations where an adverse social outcome affects disadvantaged and advantaged groups in society differently, the rates at which those groups experience favourable or adverse outcomes tend to be systematically related to the overall prevalence of the outcome. Specifically, as the overall prevalence of that outcome reduces (e.g. as a result of a policy measure or social improvement), the adverse outcome may be found to reduce proportionately less among the group with the higher baseline rate (call it the ‘disadvantaged’ group), while concomitantly, the rate of avoiding the unfavourable outcome rises proportionately less in the other (‘advantaged’) group. The propensity for this to happen was first noticed by James P. Scanlan, and is sometimes referred to as Scanlan's Rule. The Rule might be seen as calling into question standard measurement devices for characterizing groups as being relatively disadvantaged or advantaged, and as suggesting that a concern for group inequality could stymie the possibility of social progress. This chapter undertakes a critical examination of how convincing these interpretations of Scanlan’s Rule are.