A system dynamics model of individual performance is developed and simulated.  Performance, a behavioral factor, depends upon and influences emotional and cognitive factors: stress, mood, and motivation.  Activation, found in both stress and motivation literatures, is treated separately.  Each causal relationship is assumed to be simple and unambiguous.  Analysis of the model output under a range of work conditions shows that ambiguous or complex relationships would be supported by traditional research.  Complex relationships between stress, motivation, and individual performance emerge from model structure and interactions, rather than from assumed causality.  This work demonstrates the benefit of simulation in theorizing when multiple factors operate in tandem.