I develop a behavioral macroeconomic model in which agents have cognitive limitations. As a result they use simple but biased rules (heuristics) to forecast future output and inflation. Although the rules are biased, agents learn from their mistakes in an adaptive way. This model produces endogenous waves of optimism and pessimism (“Animal Spirits”) that are generated by the correlation of biased beliefs. I identify the conditions under which animal spirits arise. I contrast the dynamics of this model with a stylized DSGE-version of the model and I study the implications for monetary policies. I find that strict inflation targeting is suboptimal because it gives more scope for waves of optimism and pessimism to emerge thereby destabilizing output and inflation.

Keywords : DSGE-model, imperfect information, heuristics, animal spirits