本文探討在上下游產業中，非對稱製造商在異質產品之連續寡占的產品專業化過程。我們分析替代性（產品間與廠商間）以及相對潛在利潤（高品質商品相對於低品質商品的市場潛在需求）如何推動企業趨向生產單一產品，特別是朝高品質產品的方向。這種轉變在下游廠商尤為明顯，因為多產品廠商產品間的蠶食效果（cannibalization effect）較強。此專業化過程發生於替代性足夠高，且相對潛在利潤也夠高的時候。這個結果揭示了既有假設：各廠商提供單一但相異品質商品的局限性。隨著替代性越高，生產低品質產品的廠商將退出市場，推動該產業朝著連續雙占發展。在極高的相對潛在利潤以及替代性的推動下，這一轉變挑戰了同質產品的既有假設。這種專業化的過程也可能闡明，由於產品特徵或廠商日漸相似，導致替代性增強的影響。此外，儘管下游多產品廠商的專業化為上游製造高階中間財的供應商帶來了好處，但降低了消費者剩餘和社會福利，突顯了制定策略政策以平衡企業成就與整體經濟福祉的重要性。

This thesis examines the product specialization process within upstream and downstream industries using a model of successive oligopoly, featuring asymmetric manufacturers and differentiated goods. We analyze how both substitutability (among products and firms) and relative potential profitability (the market’s capacity for high-quality goods over low-quality ones) drive the transition from multiproduct to single-product firms, with a particular focus on the shift toward high-quality production. This shift is more pronounced in the downstream dual-quality goods firm, where the cannibalization effect is stronger. This process occurs when sufficiently high substitutability is combined with substantial relative potential profitability, exposing the limitations of existing assumptions that each firm offers a single distinct quality level. As substitutability escalates, the market phases out low-quality producers, steering industries toward a standard successive duopoly configuration. Fueled by significant profitability and substitutability, this shift challenges the prevailing assumptions of uniform product quality. This specialization trajectory might also clarify the impacts of increased substitutability due to different quality product features or firms becoming more similar. Furthermore, while the downstream multi-product firm’s specialization benefits upstream suppliers producing high-quality materials, it also reduces consumer surplus and social welfare. This highlights the need for strategic policies to balance firm achievements with overall economic well-being.

Abstract (Chinese) i
Acknowledgements (Chinese) ii
Abstract iii
Acknowledgements iv
Contents v
List of Figures viii
List of Tables ix
1 Introduction 1
2 Literature 5
3 Model 9
4 Analysis 13
4.1 Static Analysis 14
4.1.1 Scenario 1: Upstream and Downstream Dual-Quality Producers Offering Varied Quality Goods 14
4.1.2 Scenario 2: Downstream Dual-Quality Producer Specializing in High-Quality Goods 23
4.1.3 Scenario 3: Upstream Dual-Quality Producer Specializing in High-Quality Goods 30
4.1.4 Scenario 4: Standard Successive Duopoly Game 34
4.2 Comparative Static Analysis 39
4.2.1 Quantities and Prices 39
4.2.2 Profits 52
4.2.3 Welfare Analysis 57
4.2.4 Numerical Analysis 59
5 Conclusions 67
Bibliography 71
Appendix 75
A Detailed Derivation Process of Static Analysis 75
A.1 Upstream and Downstream Dual-Quality Producers Offering Varied Quality Goods 75
A.2 Downstream Dual-Quality Producer Specializing in High-Quality Goods 79
A.3 Upstream Dual-Quality Producer Specializing in High-Quality Goods 82
A.4 Standard Successive Duopoly Game 83
B Proof 85
B.1 Proof for Proposition 1 85
B.2 Proof for Remark 1 86
B.3 Proof for Relations 87
B.3.1 Positive Production Conditions in the Upstream: Scenario 1 87
B.3.2 Comparison of the Quantities of Intermediate Goods: Scenario 1 87
B.3.3 Positive Production Conditions in the Downstream: Scenario 1 87
B.3.4 Comparison of the Quantities of Final Goods: Scenario 1 87
B.3.5 Positive Production Conditions in the Upstream: Scenario 2 88
B.3.6 Comparison of the Quantities of Intermediate Goods: Scenario 2 88
B.3.7 Comparison of the Profits of Upstream Firms: Scenario 2 88
B.3.8 Comparison of the Quantities of Final Goods: Scenario 2 89
B.3.9 Comparison of the Prices of Final Goods: Scenario 2 89
B.3.10 Comparison of the Profits of Upstream firms between Scenario 1 and 2 89
B.3.11 Comparison of the Quantities of Intermediate Goods: Scenario 3 90
B.3.12 Positive Production Conditions in the Upstream: Scenario 3 90
B.3.13 Comparison of the Prices of Intermediate Goods: Scenario 3 90
B.3.14 Equilibrium Quantities of Intermediate Goods and Their Relationship
to Substitutability: Scenario 1 90
B.3.15 Equilibrium Quantities of Final Goods and Their Relationship to Substitutability: Scenario 1 91
B.3.16 Equilibrium Quantities of Intermediate Goods and Their Relationship to Substitutability: Scenario 2 91
B.3.17 Producer Surplus and Their Relationship to Substitutability: Scenario 3 91
B.3.18 Producer Surplus and Their Relationship to Substitutability: Scenario 4 92
B.3.19 Consumer Surplus and Their Relationship to Substitutability: Scenario 1 92
B.3.20 Consumer Surplus and Their Relationship to Substitutability: Scenario 2 92
B.3.21 Consumer Surplus and Their Relationship to Substitutability: Scenario 3 93
B.3.22 Consumer Surplus and Their Relationship to Substitutability: Scenario 4 93
C Heat Map 95

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