This dissertation consists of three empirical research papers on the investment behavior of private investors. All research papers are based on the same data set from a major German online broker, which covers portfolio, transaction and demographic data of more than 70,000 private investors. The focus of the first paper is the influence of financial advice on the disposition effect, which is the tendency of investors to sell winning investments too early and to hold on to losing investment for too long. The main research question is whether financial advice helps private investors to overcome this investment mistake. The second and the third paper deal with the topic of financial product innovations. The author analyzes so-called »certificates« - structured products sold to retail investors in Germany - which have grown dramatically over the last decade and now make up more than 10% of private investors' portfolios. The main research question of the second paper is: Does a financial product innovation lead to excessive trading of private investors? Finally, the third research paper deals with the question whether word-of-mouth communication plays a role in the diffusion of a financial product innovation.