Variable population welfare and poverty orderings satisfying replication properties

We discuss and compare the variable population axioms of Critical Level (CL) and Population Replication Invariance (PRI) introduced in the economic and philosophical literature for evaluating distributions with different population size. We provide a common framework for analyzing these competing views considering a strengthening of the Population Replication Principle (PRP) based on Dalton's (1920) "principle of proportionate additions to persons" that requires an ordering defined over populations of the same size to be invariant w.r.t. replication of the distributions. The strong version of PRP extends the invariance condition to hold also when distributions of different population size are compared. We suggest ethically meaningful general specifications of the invariance requirement underlying the Strong PRP and characterize the associated classes of parameterized evaluation functions that include CL principles and PRI properties. Moreover, we identify a general class of evaluation functions satisfying the Strong PRP: the social evaluation ordering will be represented by the simple formula considering the product of the population size times a strictly monotonic function of the Equally Distributed Equivalent Income (EDEI). Interesting ethical properties are shown to be associated with the shape of the function transforming the EDEI. Implications for poverty measurement are investigated.