本文以控制股東代理成本解釋多角化剝奪效應之原因，驗證全球的銀行其股權集中度對於多角化策略的重要性。銀行資產應集中在放款與其他生利資產，才能有效降低銀行營運風險並提升績效；收益多角化能顯著帶來多角化溢酬，雖然風險會隨非利息收入比率提高而增加，但結果仍舊支持收益多角化有助於銀行績效提升。控制內部公司治理後，股權集中度高的銀行，控制股東會利用資產多角化剝奪少數股東的利益，特別是透過增加放款或其他生利資產比率，結果支持剝奪效果理論。考量股權集中度及外部公司治理因素後，較低的外部市場佔有率集中度和較高的政府監督及業務限制，為有效的外部監理機制以降低股權集中之剝奪效果。同時考慮內、外部公司治理後，外部市場競爭較弱的環境中，銀行因外部監督能力較差，管理者較無積極治理動機，多角化策略會帶來銀行績效之減損。

This study examines the entrenchment effect of the diversification in terms of the agency cost of the controlling shareholders. Banks should concentrate their assets on loans and other earning assets to reduce risk and enhance their performance. The results of bank revenue diversity show significant diversification premium even though non-interest income increases bank risk. The study elaborates the importance of the ownership concentration to the diversification strategy of the banks in the world. Taking internal corporate governance into consideration, controlling shareholders of the banks with high concentrated ownership structure expropriate minority shareholders’ benefits by increasing the degree of asset diversification, especially by increasing the loan and other earning assets. This result supports the entrenchment effect. After controlling the level of ownership concentration and the external factors of corporate governance, higher market competition, higher official supervisory power and higher activities restriction are the effective external monitoring mechanisms to reduce the entrenchment effect of concentrated controlling shareholders of the banks. After controlling internal and external corporate governance, this study finds that because managers have no incentive to operate actively, the bank in weak external product competition market will bring diversification discount. Overall, corporate governance issue plays an important role on bank performance.