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Instead, most of our stress today comes from mental processes: from worrying about things. And the HPA axis isn’t designed to handle that kind of stress. We “activate a physiological system that has evolved for responding to acute physical emergencies,” Sapolsky writes, “but we turn it on for months on end, worrying about mortgages, relationships, and promotions.” And over the past fifty years, scientists have discovered that this phenomenon is not merely inefficient but also highly destructive. Overloading the HPA axis, especially in infancy and childhood, produces all kinds of serious and long-lasting negative effects—physical, psychological, and neurological.
    The tricky thing about this process, though, is that it’s not actually the stress itself that messes us up. It is the body’s reaction to the stress. In the early 1990s, Bruce McEwen, a neuroendocrinologistat Rockefeller University, proposed a theory of how this works, one that is now broadly accepted in the field. According to McEwen, the process of managing stress, which he labeled allostasis, is what creates wear and tear on the body. If the body’s stress-management systems are overworked, they eventually break down under the strain. McEwen called this gradual process allostatic load, and he says that you can observe its destructive effects throughout the body. For example, acute stress raises blood pressure to provide adequate blood flow to the muscles and organs that need to respond to a dangerous situation. That’s good. But repeatedly elevated blood pressure leads to atherosclerotic plaque, which causes heart attacks. That’s not so good.
    Although the human stress-response system is highly complex in design, in practice it has all the subtlety of a croquet mallet. Depending on what kind of stress you experience, the ideal response might come from one of any number of defense mechanisms. If you’re about to receive a flesh wound, for instance, then it would be a good idea for your immune system to start producing copious antibodies. If you need to run away from an attacker, you want your heart rate and blood pressure to elevate. But the HPA axis can’t distinguish between different types of threat, so it activates every defense, all at once, in response to any threat. Unfortunately, this means you often experience stress responses that are not at all helpful—like when you need to speak before an audience, and suddenly your mouth goes dry. Your HPA axis, sensing danger, is conserving fluids, preparing to ward off an attack. And you’re standing there looking for a glass of water and swallowing hard.
    Think of the HPA axis as a superdeluxe firehouse with a fleet of fancy, high-tech trucks, each with its own set of highly specialized tools and its own team of expertly trained firefighters. When the alarm bell rings, the firefighters don’t take the time to analyze exactly what the problem is and figure out which truck might be most appropriate. Instead, all the trucks rush off to the fire together at top speed, sirens blaring. Like the HPA axis, they simply respond quickly with every tool they might need. This may be the right strategy for saving lives in fires, but it can also result in a dozen trucks pulling up to put out a single smoldering trash can—or worse, responding to a false alarm.

5. Scared to Death
    Nadine Burke Harris saw the results of this firehouse effect in her patients all the time. One day at the Bayview clinic, she introduced me to one of them, a teenager named Monisha Sullivan who had first come to the clinic when she was sixteen and a new mother. Monisha’s childhood was about as stressful as they come: She was abandoned just a few days after she was born by her mother, who was a heavy user of crack cocaine and other drugs. As a child, Monisha lived with her father and her older brother in a section of Hunters Point with a lot of gang violence until her father, too, got lost in a drug habit; when Monisha was ten,