This study focuses on the question of rationality. Interest in decision making has been preoccupying researchers and public from many different fields and perspectives. Assumptions about consumer behavior shape companies' strategy and marketing activities, laws and public policies, bank products, etc. It affects the society as a whole. The discipline of economics largely contributed to defining (rational) preferences and choices of most (rational) individuals. The theory of rational choice has shaped classical, neoclassical, and mainstream economics in general. Although it has been widely contested, it had far-reaching effect in sociology, political science, philosophy, and even evolutionary theory (Browning, Halcli, & Webster, 1999). Since the middle of the 20 th century, however, many premises about rational consumer have been shattered. The work of Simon (1955), Tversky and Kahneman (1974, 1981, and Kahneman and Tversky (1979, 1992) contributed to more real and descriptive understanding of how people really reason.Recent economic crisis brought into light these rationalbehaving-people presumptions and gave more space to alternative explanations. One of them came from the field of Andrijana Mušura Gabor, Zagreb School of Economics and Management, Jordanovac 110, 10 000 Zagreb, Croatia. E-mail: amusura@zsem.hr (the address for correspondence);Lidija Gamulin, ZBInvest, Croatia.Breaking the myth about rational investor: Investors' susceptibility to heuristical and biased reasoning ANDRIJANA MUŠURA GABOR and LIDIJA GAMULIN The theory of rational investors has empirically proven not to be an accurate model for describing how investors in reality behave. However, there is a great number of finance theorists and orthodox economists that advocate the theory of homo economicus. In this paper, we take closer look at susceptibility of investors to heuristics and biases in general, and look at some individual differences with respect to committing to these biases. Individual differences include educational background, professional experience, and years of investing experience. We found that susceptiveness to heuristics and biases is present in majority of hypothetical scenarios. With regard to individual differences, we found that educational background can play a role in susceptiveness to heuristics and biases.Key words: behavioral finance, heuristics, biases, investors' rationality behavioral economics and behavioral finance. These disciplines explain how human emotional and cognitive system shape economic reasoning and economic decision making. Relevant findings from these fields point to systematic biases in decision making. It seems that people are hardwired to make certain errors in judgment, to make irrational decisions under the influence of emotions, and to use heuristics to make judgments faster and efficiently but not necessarily correct ones. Taking that into account, our motivation behind this research is to explore the degree to which the investors are susceptible to different biases and heuristics. Also, w...