People are always advised to "save for a rainy day," but University of Haifa researchers advise investors not to estimate financial returns of their investments on cloudy days or even during autumn. Dr. Doron Kliger and Dr. Ori Levy, who studied the effects of mood on investors, found that people who play the stock market have a greater tendency to exhibit bad judgment during autumn compared to other seasons and on cloudy days rather than sunny ones. The two analyzed US stock market data between 1987 and 1995 to assess the influence of subjective factors like mood swings on the ability of investors to correctly estimate their market returns. They examined connections between market fluctuations and the theoretical model of decisionmaking theory developed by Profs. Daniel Kahneman and Amos Tversky. The Israeli-born Kahneman, who studied and lectured at the Hebrew University and became a key figure in the discovery of systematic human cognitive bias and risk handling, is now at Stanford University and won the 2002 Nobel Prize in Economics; Tversky, with whom he worked closely on developing a cognitive basis for common human errors and prospect theory to explain irrational human economic choices, would surely have shared the Nobel if he had not died of cancer in 1996. While classic economic models take into consideration "objective" causes only and calculate events linearly with the probability of their occurrence, the theoretical model recognizes the possibility of "subjective" adjustments. The Haifa researchers examined the actual behavior of investors and investment returns and showed that the level of distorted perception and bad judgment was linked to circumstances that affected the investors' moods. Using psychological research that showed the length of the day, the amount of natural light and weather conditions influence people's moods and produce seasonal affective disorder (SAD) in many people, the researchers analyzed the influence of the length of the day and level of cloudiness on market yields. The findings show that as daylight hours decrease, and during the fall - when days are already shorter - significant distortions were found compared to other seasons of the year. In addition, on cloudy days major distortions in judgment were clearly identified, compared to sunny or less cloudy days. The implication is that during these periods, investors' assessments deviated from classic economic models. The University of Haifa researchers thus advise economists and stock market analysts to take into account not only rational considerations but also psychological causes when applying economic models and making decisions.