We construct a simple Keynesian business cycles model in which animal spirits is incorporated into the model. We assume that each firm possesses the Keynesian investment function depending on demand. Like the well-known Kaldor model, we connect the animal spirits with the degree of response of investment to demand. However, for each firm we assume that even if the level of demand is the same, the strength of it is different between upswing and downswing of income. That is, we assume that the animal spirits of each firm is strong (resp. weak) in the case where demand increases (resp. decreases). This assumption implies that the Keynesian investment function of each firm is asymmetry. Unlike many Keynesian business cycles models, we introduce a certain kind of homogeneity among such a type of investment function. Moreover, we construct a statistical model that builds a bridge between microinvestment and macro-investment and derive a nonlinear macro-investment function. By using the investment function, we construct a simple business cycles model. We demonstrate that the nonlinearity yields a limit cycle in our model and detect the occurrence of a generalized Hopf bifurcation
