Abstract
People facing similar circumstances may function very differently. The stochastic cusp model offers one possible explanation: the same conditions may support two stable levels of functioning. This tutorial shows how to understand, fit, and interpret the model in R, assuming familiarity with regression and basic R. Simple illustrations explain how stable states emerge and how changing conditions can alter them. A synthetic example links personal resources and acute stress to adaptive functioning. Readers learn to specify the model, check the fit, interpret coefficients and uncertainty, and examine its predictions. At average resources, the fitted distribution changes from one peak at low stress to two separated peaks at higher stress. The tutorial also explains which further observations would help distinguish a cusp hypothesis from competing explanations. Publicly available data, code, and archived output allow readers to reproduce the example.