Legislative effects of single-member vs multi-member districts

Theory: This project uses mathematical logic and results from spatial models to explain how parties in a legislature elected under a multi-member district system will differ from those elected under single-member plurality, holding all else constant. Hypothesis: Under most circumstances, parties elected under multi-member districts will be more ideologically diverse than those elected under single-member plurality, all else being equal. Methods: The above hypothesis is tested using interest group ratings for members of the Illinois General Assembly, a legislative body that has used both single-member and multi-member systems to elect its members. Results: During the time when the Illinois House was elected by multi-member districts and the Senate was elected by single-member districts, parties in the House were consistently more ideologically diverse than their counterparts in the Senate. This difference disappears after the House adopts a single-member district system.