Deciding how to allocate the seats of a deliberative assembly is one of the most fundamental problems in the political organization of societies and has been widely studied over two centuries already. The idea of proportionality is at the core of most approaches to tackle this problem, and this notion is captured by the divisor methods, such as the Jefferson/D’Hondt method. In a seminal work, Balinski and Demange extended the single-dimensional idea of divisor methods to the setting in which the seat allocation is simultaneously determined by two dimensions and proposed the so-called biproportional apportionment method. The method, currently used in several electoral systems, is, however, limited to two dimensions and the question of extending it is considered to be an important problem both theoretically and in practice. In this work we initiate the study of multidimensional proportional apportionment. We first formalize a notion of multidimensional proportionality that naturally extends that of Balinski and Demange. By means of analyzing an appropriate integer linear program we are able to prove that, in contrast to the two-dimensional case, the existence of multidimensional proportional apportionments is not guaranteed and deciding their existence is a computationally hard problem (NP-complete). Interestingly, our main result asserts that it is possible to find approximate multidimensional proportional apportionments that deviate from the marginals by a small amount. The proof arises through the lens of discrepancy theory, mainly inspired by the celebrated Beck–Fiala theorem. We finally evaluate our approach by using the data from the recent 2021 Chilean Constitutional Convention election.
Keywords: apportionment; integer programming; social choice.