Efficiency or equality? The utilitarianism-egalitarianism trade-off determines carbon allocation preference.

International carbon allocation confronts the conflict between efficiency and equality. Previous research based on the intergroup bias perspective has attributed carbon allocation preference to the defence of ingroup interests (i.e., national interests) while overlooking the critical role of trade-offs between competing moral values. Integrating the contingency theory of justice and moral philosophical theories of utilitarianism and egalitarianism, we proposed that the moral-values trade-off between utilitarianism and egalitarianism determines carbon allocation preference through justice reasoning. Analysis of large-scale survey datasets (Study 1) revealed that aggregated national endorsement of utilitarianism over egalitarianism predicted greater efficiency preference in total and per capita carbon emission levels. Study 2 demonstrated that experimentally manipulating endorsement of utilitarianism versus egalitarianism boosted efficiency (vs. equality) preference in carbon allocation, and justice reasoning characterized by enhanced efficiency-focused justice and diminished equality-focused justice accounted for these effects. Using a 'manipulation-of-mediator' design, Study 3 further confirmed the causal link in the mediation model. By highlighting the significance of moral trade-offs in shaping carbon allocation preference, this research not only provides a novel moral perspective in understanding debates on international carbon allocation but also has important implications for fostering international carbon abatement cooperation.