An outcome is Pareto efficient when no feasible alternative weakly improves every agent's utility and strictly improves at least one. For strict preference orders over distinct alternatives, an outcome unanimously ranked below another is inefficient. Onto strategyproof social choice functions on an unrestricted domain are Pareto efficient: rank-raising monotonicity and unanimity rule out selecting a unanimously dominated alternative.
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Pareto efficiency, also known as Pareto optimality, is an economic concept that describes a situation in which resources are allocated in a way that no reallocation can make one individual better off without making at least one other individual worse off. In simpler terms, an allocation is Pareto efficient if there are no possible changes that could improve someone's situation without harming someone else's situation.