R package cusp: Cusp-Catastrophe Model Fitting Using Maximum Likelihood. Cobb’s maximum likelihood method for cusp-catastrophe modeling (Grasman, van der Maas, & Wagenmakers, 2009, JSS, 32:8; Cobb, L, 1981, Behavioral Science, 26:1, 75–78). Includes a cusp() function for model fitting, and several utility functions for plotting, and for comparing the model to linear regression and logistic curve models.