Brain Rules (Updated and Expanded). John Medina

of much help in providing a definition, either. There is no unique grouping of physiological responses capable of telling a scientist whether you are experiencing stress. That’s because many of the mechanisms that cause you to shrink in horror from a predator are the same mechanisms used when you are having sex—or even while you are consuming your Thanksgiving dinner. To your body, saber-toothed tigers and orgasms and turkey gravy look remarkably similar. An aroused physiological state is characteristic of both stress and pleasure.

      So what’s a scientist to do? A few years ago, gifted researchers Jeansok Kim and David Diamond came up with a three-part definition that covers many of the bases. In their view, if all three are happening simultaneously, a person is stressed.

      A measurable physiological response: There must be an aroused physiological response to the stress, and it must be measurable by an outside party. I saw this the first time my then 18-month-old son encountered a carrot on his plate at dinner. He promptly went ballistic: He screamed and cried and peed in his diaper. His aroused physiological state was immediately measurable by his dad, and probably by anyone else within a half mile of our kitchen table.

      A desire to avoid the situation: The stressor must be perceived as aversive—something that, given the choice, you’d rather not experience. It was obvious where my son stood on the matter. Within seconds, he snatched the carrot off his plate and threw it on the floor. Then he deftly got down off his chair and tried to stomp on the predatory vegetable.

      A loss of control: The person must not feel in control of the stressor. Like a volume knob on some emotional radio, the more the loss of control, the more severe the stress is perceived to be. This element of control and its closely related twin, predictability, lie at the heart of learned helplessness. My son reacted as strongly as he did in part because he knew I wanted him to eat the carrot, and he was used to doing what I told him to do. Control was the issue. Despite my picking up the carrot, washing it, then rubbing my tummy while enthusiastically saying “yum, yum,” he was having none of it. Or, more important, he wanted to have none of it, and he thought I was going to make him have all of it. Feeling out of control over the carrot equaled out-of-control behavior.

      When you find this trinity of components working together, you have the type of stress easily measurable in a laboratory setting. When I talk about stress, I am usually referring to situations like these.

      We’re built for stress that lasts only seconds

      You can feel your body responding to stress: Your pulse races, your blood pressure rises, and you feel a massive release of energy. That’s the famous hormone adrenaline at work. This fight-or-flight response is spurred into action by your brain’s hypothalamus, that pea-size organ sitting almost in the middle of your head. When your sensory systems detect stress, the hypothalamus signals your adrenal glands to dump buckets of adrenaline into your bloodstream. There’s a less famous hormone at work, too—also released by the adrenals, and just as powerful as adrenalin. It’s called cortisol. It’s the second wave of our defensive reaction to stressors. In small doses, it wipes out most unpleasant aspects of stress, returning us to normalcy.

      Why do our bodies need to go through all this trouble? The answer is very simple. Without a flexible, immediately available, highly regulated stress response, we would die. Remember, the brain is the world’s most sophisticated survival organ. All of its many complexities are built toward a mildly erotic, singularly selfish goal: to live long enough to thrust our genes on to the next generation. Our reactions to stress help us manage the threats that could keep us from procreating.

      And what kinds of survival threats did we experience in our evolutionary toddlerhood? Predators would make the top 10 list. So would physical injury. In modern times, a broken leg means a trip to the doctor. In our distant past, a broken leg often meant a death sentence. The day’s weather would have been a concern, the day’s offering of food another. A lot of very immediate needs rise to the surface. Most of the survival issues we faced in our first few million years did not take long to settle. The saber-toothed tiger either ate us or we ran away from it—or a lucky few might stab it, but the whole thing was usually over in moments. Consequently, our stress responses were shaped to solve problems that lasted not for years, but for seconds. They were primarily designed to get our muscles moving us as quickly as possible out of harm’s way.

      These days, our stresses are measured not in moments with mountain lions, but in hours, days, and sometimes months with hectic workplaces, screaming toddlers, and money problems. Our system isn’t built for that. And when moderate amounts of stress hormones build up to large amounts, or hang around too long, they become quite harmful. That’s how an exquisitely tuned system can become deregulated enough to affect a report card or a performance review—or a dog in a metal crate.

       Cardiovascular system

      Stress affects both our bodies and our brains, in both good and bad ways. Acute stress can boost cardiovascular performance—the probable source of those urban legends about grandmothers lifting one end of a car to rescue their grandchildren stuck under the wheels. Over the long term, however, too much adrenaline produces scarring on the insides of your blood vessels. These scars become magnets for molecules to accumulate, creating lumps called plaques. These can grow large enough to block the blood vessels. If it happens in the blood vessels of your heart, you get a heart attack; in your brain, you get a stroke. Not surprisingly, people who experience chronic stress have an elevated risk of heart attacks and strokes.

       Immune system

      Stress also affects our immune response. At first, the stress response helps equip your white blood cells, sending them off to fight on your body’s most vulnerable fronts, such as the skin. Acute stress can even make you respond better to a flu shot. But chronic stress reverses these effects, decreasing your number of heroic white-blood-cell soldiers, stripping them of their weapons, even killing them outright. Over the long term, stress ravages parts of the immune system involved in producing antibodies. Together, these can cripple your ability to fight infection. Chronic stress also can coax your immune system to fire indiscriminately, even at targets that aren’t shooting back—like your own body. Not surprisingly, people who experience chronic stress are sick more often. A lot more often. One study showed that stressed individuals were three times more likely to suffer from the common cold, especially if the stress was social in nature and lasted more than a month. They also are more likely to suffer from autoimmune disorders, such as asthma and diabetes.

      To show how sensitive the immune system can be to stress, you need look no further than an experiment done at UCLA. Trained actors practiced Method acting, in which if a scene calls for you to be scared, you think of something frightening, then recite your lines while plumbing those memories. On one day, the actors performed using only happy memories. On another day, they performed using only sad memories. The researchers took blood samples, continually assessing their immune systems. On the “happy days,” the actors had healthy immune systems. Their immune cells were plentiful, happy, readily available for work. On the “sad days,” the actors showed something unexpected: a marked decrease in immune responsiveness. Their immune cells were not plentiful, not as robust, not as available to protect against infection.

       Memory and problem solving

      Stress affects memory. The hippocampus, that fortress of human memory, is studded with cortisol receptors like cloves in a ham. This makes it very responsive to stress signals. If the stress is not too severe, your brain performs better when it is stressed than when it is not stressed. You can solve problems more effectively and you are more likely to retain information. There’s an evolutionary reason for this. Our survival on the savannah depended upon remembering what was life-threatening and what was not. Ancestors who could commit those experiences to memory the fastest (and recall them accurately with equal speed) were more apt to survive than those who couldn’t. Indeed, research shows that memories of stressful experiences are formed almost instantaneously in the human brain, and they can be recalled very quickly during times of crises.

      If the stress is too severe or too prolonged, however, stress begins to harm learning. Stressed people don’t do math very well. They don’t process language very efficiently. They have poorer