Gerd Gigerenzer (German: [ˈɡiːɡəˌʁɛntsɐ]; born 3 September 1947) is a German psychologist who has studied the use of bounded rationality and heuristics in decision making. Gigerenzer is director emeritus of the Center for Adaptive Behavior and Cognition (ABC) at the Max Planck Institute for Human Development, Berlin, director of the Harding Center for Risk Literacy, University of Potsdam, and vice president of the European Research Council (ERC).

Gigerenzer investigates how humans make inferences about their world with limited time and knowledge. He proposes that, in an uncertain world, probability theory is not sufficient; people also use smart heuristics, that is, rules of thumb. He conceptualizes rational decisions in terms of the adaptive toolbox (the repertoire of heuristics an individual or institution has) and the ability to choose a good heuristics for the task at hand. A heuristic is called ecologically rational to the degree that it is adapted to the structure of an environment.

Gigerenzer argues that heuristics are not irrational or always second-best to optimization, as the accuracy-effort trade-off view assumes, in which heuristics are seen as short-cuts that trade less effort for less accuracy. In contrast, his and associated researchers' studies have identified situations in which "less is more", that is, where heuristics make more accurate decisions with less effort. This contradicts the traditional view that more information is always better or at least can never hurt if it is free. Less-is-more effects have been shown experimentally, analytically, and by computer simulations.

Gerd Gigerenzer was born on 3 September 1947 in Wallersdorf, Germany.

Gigerenzer received a Master of Arts and a Doctor of Philosophy in psychology from the University of Munich in 1974 and 1977, respectively. He received the postdoctoral degree of habilitation (full professor qualification) at the university's department of psychology in 1982.

Previously working at the University of Munich, Gigerenzer moved to the University of Konstanz in 1984 and to the University of Salzburg in 1990. From 1992 to 1995 he was Professor of Psychology at the University of Chicago and has been the John M. Olin Distinguished Visiting Professor, School of Law at the University of Virginia. In 1995 he became director of the Max Planck Institute for Psychological Research in Munich, and in 1997 director of the Max Planck Institute for Human Development in Berlin. Since 2009 he has been director of the Harding Center for Risk Literacy in Berlin, which moved in 2020 to the University of Potsdam.

Gigerenzer argues that heuristic reasoning should not lead us to conceive of human thinking as riddled with irrational cognitive biases, but rather to conceive rationality as an adaptive tool that is not identical to the rules of formal logic or of probability calculus. This is in contrast to other leading experts on cognitive heuristics such as Daniel Kahneman and Amos Tversky. He and his collaborators have theoretically and experimentally shown that many behavioral patterns claimed to demonstrate cognitive fallacies are better understood as adaptive responses to a world of uncertainty, including the conjunction fallacy, the base rate fallacy, and overconfidence.

With Daniel Goldstein he first theorized the recognition heuristic and the take-the-best heuristic. They proved analytically conditions under which semi-ignorance (lack of recognition) can lead to better inferences than with more knowledge. These results were experimentally confirmed in many experiments, e.g., by showing that semi-ignorant people who rely on recognition are as good as or better than the Association of Tennis Professionals (ATP) Rankings and experts at predicting the outcomes of the Wimbledon tennis tournaments. Similarly, decisions by experienced experts (e.g., police, professional burglars, airport security) were found to follow the take-the-best heuristic rather than weight and add all information, while inexperienced students tend to do the latter.