Figure 3.1: Route through which genes affect behavior Genes build proteins, which form or alter the body’s physiological systems (including brain systems), which, in turn, produce behavior. Each step in this process involves interaction with the environment. Aspects of the internal environment control gene activation, and aspects of both the internal and the external environments act on physiological systems to control behavior. Behavior, in turn, can affect gene activation through direct and indirect effects on the internal environment.