⚠️ What Conventional Ideal-Point Models Overlook

Most ideal-point estimation assumes legislators vote according to spatial (policy) preferences. That assumption ignores non-policy, tactical motives and can produce implausible results in many legislatures—especially in parliamentary systems where party strategy and motion origin matter.

🧭 A Model That Separates Policy and Tactical Motives

A roll-call voting model is developed that explicitly distinguishes between policy incentives and non-policy (tactical) incentives. The model allows the relative weight of these two motives to vary systematically with who moves a motion (the mover) and with key characteristics of the motion itself.

📊 Evidence From 5,469 Parliamentary Votes

• Data: 2,174 roll call votes from German state legislatures and 3,295 roll call votes from the British House of Commons.
• Design: Estimation of legislators' revealed preferences while accounting for both policy and tactical drivers and for heterogeneity linked to mover identity and motion features.

🔑 Key Findings

• Tactical incentives can outweigh policy incentives. In many roll calls, non-policy motives are more important than ideological distance for explaining votes.
• The influence of tactical incentives is conditional on the importance of the motion: tactical weight varies with motion salience and characteristics.
• Important exceptions and twists:
• Backbench private members' bills can reverse typical tactical incentives, producing voting patterns that differ from party-led motions.
• Proposals from anti-system parties are almost always rejected by moderate parties, which makes those votes effectively uninformative about those moderates' policy positions.

🌍 Why This Matters

These results imply that standard spatial ideal-point estimates in parliamentary systems may be biased when tactical motives are ignored. Accounting for mover identity and motion characteristics improves interpretation of roll-call behavior and has implications for comparative work on ideal points and for studies contrasting parliamentary and separation-of-powers systems.