Brain Rule #4 states, “Stressed brains don’t learn the same way” (59). Medina explains that there are many different types of stress. Some types can help boost learning, while others can debilitate it. Stress is defined in three parts. First, stress produces a physiological response. Medina provides the example of his young son encountering carrots on his plate for the first time. His son cried and screamed—a clear physiological response. Second, stress produces a desire to circumvent the stressor. Third, it creates a sense that control has been lost. Humans feel stress about things that they cannot control. The more pronounced this loss of control is, the more developed the stress.

Human brains have evolved to handle stress that lasts for a short burst of time. As the body exhibits the physiological response, the brain enacts a fight or flight response. The body’s glands release adrenaline and cortisol. For early humans, predators, physical injury, and weather were major sources of stress. Each of these were immediate stressors. Modern humans experience stressors that last for longer and are less discernable. As stress builds up, it can have a detrimental effect on human brains and bodies.