Envy-Free Randomized Apportionment

Abstract

The United States allocates representatives to each state in proportion to its population, while many parliamentary democracies select representatives from party lists in proportion to the number of votes each party receives. In both settings, the legislature's small size necessitates rounding to the nearest whole number of seats. Recent work has shown a plethora of ways to randomly round seat shares to achieve perfect ex ante fairness. As a result, party (state) $i$ may have a larger number of votes (population) than party $j$ but receive fewer seats. In this case, we say that party $i$ envies party $j$. We investigate the extent to which such envy is avoidable. Specifically, we seek to design apportionment rules that minimize the expected number of envious pairs $(i, j)$. If the expectation is zero, we say that the seat allocation is envy-free. On the negative side, we show that any $n$-party apportionment rule that ignores lower quotas (the floors of the deserved number of seats for each party) suffers from a worst-case envy of $Θ(n^2)$, even on instances where an envy-free allocation exists. All previously studied apportionment rules fall into this class and thus do a poor job of minimizing envy. In the case where all lower quotas are equal, we prove that all allocations have the same expected envy. In the general case, we we introduce a general polynomial-time LP framework for a large class of apportionment objectives, including minimizing the expectation and variance of envy, by efficiently solving the separation problem on the dual. Finally, we empirically demonstrate that envy-free allocations exist very frequently, including in every historical instance from the US decennial census. Taken together, these results provide a clear recipe for implementing randomized apportionment in a less objectionable way in practice.