How to Fairly Allocate Easy and Difficult Chores

A major open question in fair allocation of indivisible items is whether there always exists an allocation of chores that is Pareto optimal (PO) and envy-free up to one item (EF1). We answer this question affirmatively for the natural class of bivalued utilities, where each agent partitions the chores into easy and difficult ones, and has cost $p > 1$ for chores that are difficult for her and cost $1$ for chores that are easy for her. Such an allocation can be found in polynomial time using an algorithm based on the Fisher market. We also show that for a slightly broader class of utilities, where each agent $i$ can have a potentially different integer $p_i$, an allocation that is maximin share fair (MMS) always exists and one that is both PO and MMS can be computed in polynomial time, provided that each $p_i$ is an integer. Our MMS arguments also hold when allocating goods instead of chores, and extend to another natural class of utilities, namely weakly lexicographic utilities.