有關外部性行為所造成的市場失靈與社會損害，經濟學者早已提出相當完備的矯正措施。其中，Pigouvian tax 改變當事人的經濟誘因，將其外部性內部化。另方面，社會規範引導個人形成道德情操，同樣構成個人的行為誘因，進而對外部性行為產生約束效果。儘管Adam Smith (1790) 指出道德情操對行為動機的重要性，跨領域文獻也多有評量道德與法律在約束人類行為上的優劣，然則，道德所促成的行為誘因卻甚少被引進外部性的經濟理論中，遑論建立最適道德體系的福利分析。本研究欲將道德情操與租稅兩種誘因機制，一併納入個人的行為模式；強調從善價值感與罪惡感對個人效用的正反面影響，並闡述人格因素與道德情操的關連性。首先擬針對具敵對性與非敵對性兩種外部性行為，探討租稅與道德的效果，並檢視兩者的關係。再者，考量個人道德觀養成的社會教化成本，以及在政府對從事外部性行為者沒有充分訊息的情況下，租稅課徵的施行成本升高。本研究將從社會的觀點，基於社會淨福利的期望值最大之目標，建立最適道德體系與最適租稅，並進一步比較最適道德體系與最適租稅的福利效果。最後，本研究將利用傳統的實證方法與新興的實驗設計，進行個人道德情操的形成與個人外部性行為動機的相關命題檢定。 Economists have developed a system of correction measures to solve the problem of market failure caused by the externalities of human behavior. One way to deal with an externality is the imposition of Pigouvian taxes which give the individuals incentive to internalize the external costs. So too, social norms accomplish this through the threat of moral sanctions. Adam Smith (1790) emphasized the motivational importance of moral sentiments. A few studies compare the character, the effectiveness, and the social costs associated with law and morality. However, the role of morality serving to channel human behavior has rarely been incorporated in a theory of externality, let alone the welfare analysis of optimal moral system. This study employs a behavior model incorporating both incentives of morality and taxation to analyze how moral sentiments and Pigouvian taxes affect the externalities (both rival and nonrival) an individual’s behavior generates. In the model guilt is a disincentive to act, while virtue is an incentive. An individual’s feelings of guilt and virtue depend on the preferences of different selves. Guilt and virtue are costly to inculcate. With incomplete information about the violators, the costs of enforcement of taxes increase substantially. This study will explore optimal moral system and optimal Pigouvian taxation and assess their welfare effects. Consideration is also given to the theoretically optimal domains in which each mean is desirable to utilize from the viewpoint of social welfare. In empirical studies, both traditional econometrics and lab experiments will be undertaken to explore the formation of moral sentiments and to investigate the response of an individual’s externality to the inward and the outward incentive mechanisms, respectively.