Master's Thesis from the year 2022 in the subject Business economics - Investment and Finance, grade: 1.3, University of Marburg, language: English, abstract: The goal of this master's thesis is to investigate whether the ambiguous relationship between the sustainability of an ESG mutual fund and its performance is due to the fact that ESG mutual funds are subject to behavioral biases. The structure of this master's thesis is divided into three parts. First, it will be investigated if global ESG equity mutual funds domiciled in Europe are subject to the home bias, that is that a too large fraction of the equity investments is invested in domestic stocks. Furthermore, it will be investigated whether ESG mutual funds are subject to a Europe bias (whether a disproportionately large fraction of the equity investments is allocated to stocks from Europe) and finally whether ESG mutual funds are subject to the industry bias (their holdings are heavily allocated to individual industry sectors). Secondly, the financial performance of ESG mutual funds in the period from August 2017 to August 2021 was examined using the Capital Asset Pricing model (CAPM) and the Carhart four-factor model like in the most important works on ESG mutual fund performance. Furthermore, it will be investigated whether the ESG mutual funds perform worse than their unsustainable counterparts (i.e. conventional investment funds via a matched-pair sample). Thirdly, it will be investigated whether behavioral biases have an impact on the performance of ESG mutual funds and lead to possible under-diversification.