垂直持股與差別關稅

Vertical Ownership and Discriminatory Trade Policy

本文建立一個三國兩廠商之垂直市場三階段賽局模型，探討垂直持股對進口國關稅政策之影響。模型延伸Brander and Spencer（1985）之第三國市場架構，並結合Bernhofen（1997）之垂直市場設定。在無垂直持股情況下，最適政策為課徵正關稅，且在成本不對稱下呈現差別關稅配置。然而，當上游廠商持有下游廠商股權時，利潤回流效果將降低中間財價格，內生改變下游成本結構，使市場競爭轉由投入價格調整。本文發現，隨持股比例提高，最適關稅將下降；更重要的是，在垂直持股下，即使存在成本差異，最適關稅仍趨於一致，顯示傳統「高成本、低關稅」之原則不再適用。

This paper develops a three-country, two-firm, three-stage game in a vertically related market to examine how vertical ownership affects optimal tariff policy. The model extends the framework of James Brander and Barbara Spencer (1985) and incorporates the vertical structure of Martin Bernhofen (1997). Without vertical ownership, the optimal policy involves positive tariffs and discriminatory tariffs arise under cost asymmetry. With vertical ownership, however, profit internalization induces the upstream firm to lower input prices, thereby endogenously altering downstream cost structures and shifting competition toward pricing. We show that optimal tariffs decrease with the ownership share. More importantly, tariffs become symmetric even in the presence of cost asymmetry, implying that the conventional discriminatory tariff rule no longer holds.