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Penguins jumping off a cliff, economic forecasters and financial advisors speculating against a currency, and farmers using traditional methods in India are all practising social learning. Such learning from the behavior of others may and does lead to herds, crashes, and booms. These issues have become, over the last ten years, an exciting field of research in theoretical and applied economics, finance, and in other social sciences. This book provides both an informal introduction and in-depth insights into the subject. Each chapter is devoted to a separate issue: individuals learn from the observations of actions, the outcomes of these actions, and from what others say. They may delay or make an immediate decision; they may compete against others or gain from cooperation; they make decisions about investment, crop choices, and financial investments. The book highlights the similarities and the differences between the various cases.

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The canonical model of strategic complementarities between individual actions, which exhibits multiple equilibria under perfect information, is extended with heterogeneous agents and imperfect information. Agents observe their own cost of action and the history of the levels of aggregate activity. The distribution of individual characteristics evolves through a random process, and individuals are rational Bayesians. Under plausible conditions, there is a unique equilibrium with phases of high and low activity and random switches. Applications may be found in macroeconomics and revolutions.* I am grateful to Olivier Blanchard and two referees for their remarkable help.