This study aims to examine the effect of the efficient market hypothesis, gambler's fallacy, familiarity effect, risk perception, and economic factors on investment decisions. This research is quantitative research with a descriptive approach. The population in this study were all capital market investors in Medan City. Determination of the research sample carries out by using judgment sampling technique and Malhotra theory so that 270 samples obtain. Data analysis using multiple linear regression analysis. The results of the multiple linear regression analysis showed that the efficient market hypothesis, gambler's fallacy, familiarity effect, risk perception, and economic factors partially had a positive and significant impact on investment decision making. Other results, the efficient market hypothesis, gambler's fallacy, familiarity effect, risk perception, and economic factors simultaneously have a positive and significant impact on investment decision making.

Keywords : Efficient Market Hypothesis, Gambler’s Fallacy, Familiarity Effect, Risk Perception, Economic Factors, Investment Decisions