Final answer:
According to Reinforcement Sensitivity Theory, the amygdala is the active part of the brain in a cautious person like Jacob, as it processes emotional responses, especially fear, and can trigger a fight-or-flight response.
Step-by-step explanation:
According to Reinforcement Sensitivity Theory, the amygdala part of Jacob's brain is likely very active if he is a very cautious person who often finds reasons to avoid new situations. The amygdala is responsible for processing emotional information, particularly fear and threat-related stimuli. When a person is anxious or scared, signals from the amygdala prompt the hypothalamus to stimulate the sympathetic fight-or-flight response, and in the case of continuous cautious behavior, it may indicate an overly active amygdala. Though other brain regions like the prefrontal cortex are involved in regulating these responses and promoting flexibility in cognition and affect, the amygdala's role in emotional reaction is central to the experience of fear and anxiety that underlies cautious behavior.