The basic tenet of standard finance is rational behaviour. It states that individual investors are assumed to be always rational in making decision. Individual investors  are assumed to have an objective and purpose in achieving their utility. One of standard economic theory, Efficient Market Hypothesis (EMH), states that share prices fully reflect all available information and are closed to its intrinsic value. Modern Finance paid great attention to the study of behavioral finance. The study discusses the psychological influence of investor behavior in investment decision making. Some research shows that one aspect that causes market inefficiency is the presence of behavioral factors. Investors often underreact or overreact against bad or good news that appears on the market. Investors will fail in investing if the decision taken is still strongly influenced by psychological bias. Psychological bias is the aspect that influence investment behavior. Both investors who are accustomed to the world of investment and novice investors, the possibility of a perception bias appears to be unavoidable. Perception bias is a psychological tendency of a person who loses objectivity to a perception and situation. Investors are often influenced by biases that will affect the investment decision making process. Psychological biases shown in this study includes overconfidence, representativeness, anchoring, mental accounting, loss aversion, and regret aversion. The rapid development of financial behavior is supported by psychology. Financial behavior is used to understand how human behavior influences individual decisions, professional investors, markets, and managers. The purpose of this study is to determine and discuss the influence of psychological biases and stock investment decision making of individual investors.

This study was conducted on individual stock investors, namely customers of PT Indo Premier Sekuritas Branch Office in Pondok Indah, South Jakarta, Indonesia. The research method is descriptive because it aims to describe the behavior of investors in investment decision making. The type of data in this study is a quantitative with primary data sources obtained by questionnaire techniques. The population in this study were customers of PT Indo Premier Sekuritas Branch Office in Pondok Indah totaling 757 people. The number of samples in this research is 262 respondents with method of sampling using non-probability sampling method or non-random sampling with purposive sampling types. Purposive sampling method is a sampling technique based on the objective and criteria that have been determined. The data processing method in this study is the SPSS version 21.0 for Windows computer program.

The results showed that the normality test with the regression model using all variables has fulfilled the assumption of normality. Kolmogrov-Smirnov's significance value is greater than 0.05. There are no multicollinearity problems and heteroscedesticity free in all variables. T statistic test and determination from this research have different percentages among variables. The data were analyzed using multiple regression analysis technique and the result showed that four of the six variables of psychological biases have a significant effect on investment decision of individual stock investor. Significant variables of psychological bias are overconfidence (0,224), representativeness (0,222), anchoring (0,193), and loss aversion bias (-0,249). While the other two variables, mental accounting (0,093) and regret aversion (-0,025), do not have a significant effect on stock investment decision.

Keywords: Investment, Psychological Bias, Behavioral Finance, Investment Decision Making