Knowing what you should fear, and quickly recognizing the biological changes in your body that indicate fear, can save your life. This critical task is processed by a small almond-shaped structure, the amygdala, which lies deep within the bottom of the brain, not far from your ears. The amygdala receives information from many brain regions, your internal organs, and external sensory systems, such as your eyes and ears. The amygdala integrates this information with various internal drives, such as whether you are hungry or thirsty or in pain; it then assigns a level of emotional significance to whatever is going on. For example, when the amygdala becomes aware that you are alone and hearing unfamiliar sounds in the dark, it initiates a fear response, such as panic or anxiety. It then activates the appropriate body systems, the release of hormones, and specific behaviors to respond to the (real or imagined) threat. The amygdala also is activated by sensory stimuli that seem ambiguous or unfamiliar to us, such as unfamiliar sounds or people. In response to ambiguous or unfamiliar stimuli, we become vigilant and pay closer attention to what is happening in our immediate environment. If you were a dog, your ears would perk up. Your amygdala gathers as much sensory information as possible, compares it to what you already know, and then instructs other brain regions to respond.
Almost without fail, and regardless of the nature of the information gathered by your vigilant brain, the amygdala usually comes to the same conclusion: be afraid. If a sensory event, such as a sight or sound or taste, is unfamiliar; your brain almost always assumes that the situation is potentially dangerous and should be treated as such. If everything is assumed to be dangerous until proven otherwise, you are much more likely to survive the experience and pass on your be-fearful-first genes. Thus, humans fear everything that is unfamiliar or not-like-me: we fear unfamiliar dogs, people who look or dress differently, unfamiliar places, unfamiliar odors, things that go bump in the night, people who stare at us for too long, heights, enclosed small spaces, dark alleys, unknown people who follow us, etc. You get the idea. We all have witnessed the consequences of fear: we hide behind closed doors, we hide in protected or gated communities, we keep a loaded gun by every door and under the pillow, we hire bodyguards, we install security systems, we build walls. Brains evolved to perform one primary function: survival of the individual and the species; fear plays a critical role in survival. Unfortunately, your fear-inducing amygdala occasionally overreacts to trivial or harmless stimuli. Sometimes the amygdala induces behaviors that may get a person mentioned on the evening news.
Your brain evolved to help you survive to pass on your genes to the next generation. The best way to achieve this goal is to induce a response immediately to imagined threats regardless of whether that response is appropriate or not. By now you have clearly gotten the point that being frightened of everything all of the time is a safe and effective way to maintain our species. Unfortunately, it is also quite stressful, and chronic stress ultimately will have negative consequences upon our health. The brain, due to the impact of evolution, does not concern itself with the long-term effects of chronic stress on the body because these negative consequences usually appear long after you have finished reproducing and passing on your be-fearful-first genes to the next generation.
The amygdala also controls how your brain processes sensory inputs that are associated with emotional experiences. This is an extremely important function because it determines whether you will remember the details of fearful events. For example, mugging victims tend to distort the details of the tragic event by “remembering” that the mugger was bigger and uglier, the gun was bigger, the alley was darker, etc. The influence of the amygdala makes it less likely that you will walk down that alley alone again. Your amygdala has succeeded again and your be-fearful-first genes live to breed another day!