Separable Social Welfare Evaluation for Multi-Species Populations ∗ St´ ephane Zuber a Dean Spears b Mark B. Budolfson c March 30, 2022 Abstract If non-human animals experience wellbeing and suffering, such welfare conse- quences arguably should be included in a social welfare evaluation. Yet economic evaluations almost universally ignore non-human animals, in part because axiomatic social choice theory has failed to propose and characterize multi-species social welfare functions. Here we propose axioms and functional forms to fill this gap. We provide a range of alternative representations, characterizing a broad range of possibilities for multi-species social welfare. Among these, we identify a new characterization of additively-separable generalized (multi-species) total utilitarianism. The multi- species setting permits a novel, weak species-level separability axiom with important consequences. We provide examples to illustrate that non-separability across species is implausible in a multi-species setting, in part because good lives for different species are at very different welfare levels. Finally, we explore the consequences for evaluating climate policy and understanding speciesism and non-climate environ- mental goals, such as biodiversity. Keywords: animals, animal welfare, social welfare functions, population ethics, utilitarianism JEL Classification numbers: D60, Q57 * Zuber acknowledges support by the Investissements d’Avenir program (PGSE-ANR-17-EURE-01). a Paris School of Economics – CNRS, France. E-mail: stephane.zuber@univ-paris1.fr b Department of Economics and Population Research Center, University of Texas at Austin, USA. IZA, Germany. E-mail: dspears@utexas.edu. c Department of Environmental and Occupational Health and Justice, Center for Population-Level Bioethics, and Department of Philosophy, Rutgers University, New Brunswick, NJ, 08901, USA E-mail: budolfson@cplb.rutgers.edu