A clear two‑dimensional structure explains much of the cross‑national variation in party systems.

📊 Examining Seat Shares Across 17 Democracies (1970–2013)

A principal component analysis (PCA) was applied to a party seat share dataset covering 17 advanced democracies from 1970 to 2013 to reduce dimensionality and reveal the core patterns that distinguish party systems.

🔎 Main Patterns Identified

• Two principal dimensions account for the most important differences across party systems:
• "Size of the biggest two parties."
• "Competition between the two biggest parties."
• These dimensions capture how party strength and head‑to‑head rivalry jointly shape the structure of party systems.

📈 Comparing Electoral and Legislative Party Systems

• The PCA results are used to compare changes in electoral and legislative party systems over time, showing how shifts in seat shares map onto the two core dimensions.

🧭 How This Relates to Existing Measures and Typologies

• Existing party system typologies tend to sort countries into categories that reflect variation along both identified dimensions.
• Common quantitative measures in the literature (for example, the effective number of parties) are highly correlated with the first dimension—the size of the largest parties—leaving the competition/opposition dimension under‑captured.

🔑 Why This Matters

• Reliance on single measures like the effective number of parties risks overlooking the role of opposition structure and head‑to‑head competition.
• Indices that explicitly measure opposition structure and competition are recommended for research questions about party system competitiveness and the dynamics of political rivalry.