If you aren't already paralyzed with stress from reading the financial news, here's a sure way to achieve that grim state: read a medical-journal article that examines what stress can do to your brain. Stress, you'll learn, is crippling your neurons so that, a few years or decades from now, Alzheimer's or Parkinson's disease will have an easy time destroying what's left. That's assuming you haven't already died by then of some other stress-related ailment such as heart disease. As we enter what is sure to be a long period of uncertainty—a gantlet of lost jobs, dwindling assets, home foreclosures and two continuing wars—the downside of stress is certainly worth exploring. But what about the upside? It's not something we hear much about.

In the past several years, a lot of us have convinced ourselves that stress is unequivocally negative for everyone, all the time. We've blamed stress for a wide variety of problems, from slight memory lapses to full-on dementia—and that's just in the brain. We've even come up with a derisive nickname for people who voluntarily plunge into stressful situations: they're "adrenaline junkies."

Sure, stress can be bad for you, especially if you react to it with anger or depression or by downing five glasses of Scotch. But what's often overlooked is a common-sense counterpoint: in some circumstances, it can be good for you, too. It's right there in basic-psychology textbooks. As Spencer Rathus puts it in "Psychology: Concepts and Connections," "some stress is healthy and necessary to keep us alert and occupied." Yet that's not the theme that's been coming out of science for the past few years. "The public has gotten such a uniform message that stress is always harmful," says Janet DiPietro, a developmental psychologist at Johns Hopkins University. "And that's too bad, because most people do their best under mild to moderate stress."

The stress response—the body's hormonal reaction to danger, uncertainty or change—evolved to help us survive, and if we learn how to keep it from overrunning our lives, it still can. In the short term, it can energize us, "revving up our systems to handle what we have to handle," says Judith Orloff, a psychiatrist at UCLA. In the long term, stress can motivate us to do better at jobs we care about. A little of it can prepare us for a lot later on, making us more resilient. Even when it's extreme, stress may have some positive effects—which is why, in addition to posttraumatic stress disorder, some psychologists are starting to define a phenomenon called posttraumatic growth. "There's really a biochemical and scientific bias that stress is bad, but anecdotally and clinically, it's quite evident that it can work for some people," says Orloff. "We need a new wave of research with a more balanced approach to how stress can serve us." Otherwise, we're all going to spend far more time than we should stressing ourselves out about the fact that we're stressed out.

When I started asking researchers about "good stress," many of them said it essentially didn't exist. "We never tell people stress is good for them," one said. Another allowed that it might be, but only in small ways, in the short term, in rats. What about people who thrive on stress, I asked—people who become policemen or ER docs or air-traffic controllers because they like seeking out chaos and putting things back in order? Aren't they using stress to their advantage? No, the researchers said, those people are unhealthy. "This business of people saying they 'thrive on stress'? It's nuts," Bruce Rabin, a distinguished psychoneuroimmunologist, pathologist and psychiatrist at the University of Pittsburgh School of Medicine, told me. Some adults who seek out stress and believe they flourish under it may have been abused as children or permanently affected in the womb after exposure to high levels of adrenaline and cortisol, he said. Even if they weren't, he added, they're "trying to satisfy" some psychological need. Was he calling this a pathological state, I asked—saying that people who feel they perform best under pressure actually have a disease? He thought for a minute, and then: "You can absolutely say that. Yes, you can say that."

This kind of statement might well have the father of stress research lying awake worried in his grave. Hans Selye, who laid the foundations of stress science in the 1930s, believed so strongly in good stress that he coined a word, "eustress," for it. He saw stress as "the salt of life." Change was inevitable, and worrying about it was the flip side of thinking creatively and carefully about it, something that only a brain with a lot of prefrontal cortex can do well. Stress, then, was what made us human—a conclusion that Selye managed to reach by examining rats.

Selye had virtually no lab technique, and, as it turned out, that was fortunate. As a young researcher, he set out to study what happened when he injected rats with endocrine extracts. He was a klutz, dropping his animals and chasing them around the lab with a broom. Almost all his rats—even the ones he shot up with presumably harmless saline—developed ulcers, overgrown adrenal glands and immune dysfunction. To his credit, Selye didn't regard this finding as evidence he had failed.Instead, he decided he was onto something.

Selye's rats weren't responding to the chemicals he was injecting. They were responding to his clumsiness with the needle. They didn't like being dropped and poked and bothered. He was stressing them out. Selye called the rats' condition "general adaptation syndrome," a telling term that reflected the reason the stress response had evolved in the first place: in life-or-death situations, it was helpful.

For a rat, there's no bigger stressor than an encounter with a lean and hungry cat. As soon as the rat's brain registers danger, it pumps itself up on hormones—first adrenaline, then cortisol. The surge helps mobilize energy to the muscles, and it also primes several parts of the brain, temporarily improving some types of memory and fine-tuning the senses. Thus armed, the rat makes its escape—assuming the cat, whose brain has also been flooded with stress hormones by the sight of a long-awaited potential meal, doesn't outrun or outwit it.

This cascade of chemicals is what we refer to as "stress." For rats, the triggers are largely limited to physical threats from the likes of cats and scientists. But in humans, almost anything can start the stress response. Battling traffic, planning a party, losing a job, even gaining a job—all may get the stress hormones flowing as freely as being attacked by a predator does. Even the prospect of future change can set off our alarms. We think, therefore we worry.

Herein lies a problem. A lot of us tend to flip the stress-hormone switch to "on" and leave it there. At some point, the neurons get tired of being primed, and positive effects become negative ones. The result is the same decline in health that Selye's rats suffered. Neurons shrivel and stop communicating with each other, and brain tissue shrinks in the hippocampus and prefrontal cortex, which play roles in learning, memory and rational thought. "Acutely, stress helps us remember some things better," says neuroendocrinologist Bruce McEwen of Rockefeller University. "Chronically, it makes us worse at remembering other things, and it impairs our mental flexibility."

These chronic effects may disappear when the stressor does. In medical students studying for exams, the medial prefrontal cortex shrinks during cram sessions but grows back after a month off. The bad news is that after a stressful event, we don't always get a month off. Even when we do, we may spend it worrying ("Sure, the test is over, but how did I do?"), and that's just as biochemically bad as the original stressor. This is why stress is linked to depression and Alzheimer's; neurons weakened by years of exposure to stress hormones are more susceptible to killers. It also suggests that those of us with constant stress in our lives should be reduced to depressed, forgetful wrecks. But most of us aren't. Why?

Step away from the lab, and you'll find the beginnings of an answer. In the 1970s and '80s, Salvatore Maddi, a psychologist at the University of California, Irvine, followed 430 employees at Illinois Bell during a companywide crisis. While most of the workers suffered as their company fell apart—performing poorly on the job, getting divorced and developing high rates of heart attacks, obesity and strokes— a third of them fared well. They stayed healthy, kept their jobs or found others quickly. It would be easy to assume these were the workers who'd grown up in peaceful, privileged circumstances. It would also be wrong. Many of those who did best as adults had had fairly tough childhoods. They had suffered no abuse or trauma but "maybe had fathers in the military and moved around a lot, or had parents who were alcoholics," says Maddi. "There was a lot of stress in their early lives, but their parents had convinced them that they were the hope of the family—that they would make everyone proud of them—and they had accepted that role. That led to their being very hardy people." Childhood stress, then, had been good for them—it had given them something to transcend.

More recently, Robert Sapolsky of Stanford University has studied a similar phenomenon in alpha males. He's seen plenty of "totally insane son of a bitch" types who respond to stress by lashing out, but he's also interested in another type that gets less press: the nice guy who finishes first. These alphas don't often get into fights; when they do, they pick battles they know they can win. They're just as dominant as their angry counterparts, and they're subject to the same stressors—power struggles, unsuccessful sexual overtures, the occasional need to slap down a subordinate—but their hormone levels never get out of whack for long, and they probably don't suffer much stress-induced brain dysfunction.Sapolsky likes to joke that they've all been relaxing in hot tubs in Big Sur, transforming themselves into "minimalist Zen masters." This is a joke because they've clearly come by their attitudes unconsciously: Sapolsky studies wild baboons.

Sapolsky's and Maddi's work points to a flaw with much of the neurobiological research: so far, it has done a poor job of accounting for differences in how individuals process stress. Researchers haven't identified the point at which the effects of stress tip over from positive to negative, and they know little about why that point differs from person to person. (This is why they don't like to tell people that a little stress can be good, says Rabin—because "we don't know how to judge for each individual what a 'little' stress is.") The research thus tends to paint stress as a universal phenomenon, even though we all experience it differently. "If there are rats or mice or cultured neurons in a dish that seem superresilient to stress, far too many lab scientists view this as a pain in the ass, something that just throws off patterns," says Sapolsky. "It's only people who are tuned into animal behavior or humans and the real world who are interested in how amazing the outliers are." Explaining these outliers' healthy attitudes, says Sapolsky, is now "the field's biggest challenge."

As Maddi's work makes clear, a lot of the explanation stems from early experiences. This may be true of Sapolsky's baboons as well. Sapolsky suspects that part of what makes an animal a dominant Zen master instead of an angry alpha lies in what sort of childhood he had. If an adult baboon picks up on conflict around him but keeps his cool, "quelling the anxiety and exercising impulse control," that may be behavior his mom modeled for him years earlier. The key? Factors such as how many steps the baby baboon could take away from his mother before she pulled him back—i.e., how much she allowed him to learn for himself, even if that meant a few bumps and bruises along the way. "I think the males who had mothers who were less anxious, who allowed them to be more exploratory in the absence of agitated maternal worry, are more likely to be the Zen ones who are calm enough to resist provocation," he says. A little properly handled stress, then, may be necessary to turn children into well-adjusted adults.

Part of the explanation will also be found in genes. Scientists have already identified one that helps control how the brain processes serotonin; some variants seem to protect people from depression, depending on whether they've suffered through previous traumas. This gene may not mediate everyday stress, but others are bound to be fingered eventually, and "once people have found scores of genes," says Sapolsky, "I'm willing to bet the farm that that's going to begin to explain who gets depressed after disastrous unrequited love and who just feels lousy for two weeks."

The X and Y chromosomes also play a role in how people respond to stress, though how much of one isn't clear. Men and women both experience stress as a rise in adrenaline and cortisol. What differs is their reaction. Women "are more likely to turn to their social networks, and that prompts the release of oxytocin, which mutes the stress systems," says Shelley Taylor, a psychologist at UCLA. If they're surrounded by loved ones when a stressor arises, she says, "there's some evidence they don't even show as much of the initial hormonal response." Without that response, there's less risk of long-term harm to the brain. It's a critical concept—yet it wasn't on stress physiologists' radar until the mid-'90s, when Taylor pointed out that most stress research in animals and humans had been conducted overwhelmingly on males.

Finally, there's that murky territory where genes and environment interact, with lifelong effects: the womb. It's not hard to find studies suggesting that maternal stress harms later child development. But what the evidence means, no one knows. "Project Ice Storm," a survey of nearly 150 expectant mothers who toughed out a 1998 squall in Quebec—some without power for up to 40 days—is one of the scariest studies. Late last year researchers reported that the women's children had lower-than-average IQs and language skills at age 5; they say the storm and its stress on the mothers had "significant effects [on the children] … in every area of development that we have examined." The study surveys many children in great detail, but it doesn't mean all pregnant women should panic about their stress levels (or panic about the fact that they've just panicked). An ice storm isn't the same kind of stressor that people encounter in everyday life, and the women in the ice-storm study don't necessarily represent all women. Those who were stuck in Quebec during the storm were likely some of the ones with the fewest resources. Their children may have been prone to low scores as 5-year-olds simply because they were poor.

A lot of the research on stress and infant development can be picked apart this way, says DiPietro, of Johns Hopkins. Also, she notes, "nobody ever got funded by saying stress doesn't harm babies." DiPietro herself is a rare exception. Two years ago, she showed that women under moderate stress in mid-to-late pregnancy wound up with toddlers who were developmentally advanced, scoring highly on language and cognitive tests. In an upcoming paper, she confirms the trend: 2-week-old babies whose mothers were under moderate stress show evidence of faster nerve transmission—and possibly more mature brain development—than those whose moms had stress-free pregnancies. It's hard to know what to make of the findings, but DiPietro has an intriguing theory. A stressed-out mother's "internal environment"—her heartbeat, blood pressure and other signals the fetus can perceive—is constantly in flux. Her restlessness may stimulate the fetus's brain, giving it something to think about. In this light, DiPietro thinks, the kind of mild to moderate stress that is pervasive in many women's hectic lives may be beneficial, perhaps even "essential," for fetal development. The idea is controversial—but if it's correct, it certainly complicates the theory that stress can permanently damage a child in utero.

When Stanford's Sapolsky gives lectures on stress, he cites the "depressing" research on failing neurons, some of which he has conducted. But his talks end optimistically, thanks to his observations in the wild. "If some baboons just happen to be good at seeing water holes as half full instead of half empty … we should be able to as well," he once told an audience. Even if we're not born well equipped to deal with stress, he said, "we can change," because as humans, we ought to be "wise enough to keep this stuff in perspective."

So how do we do that? One place to start is with the human equivalent of Zen baboons: Buddhist monks. Their mental stability and calmness isn't mystical; it's biological. The brain can grow new cells and reshape itself, and meditation appears to encourage this process. Monks who have trained for years in meditation have greater brain activity in regions linked to learning and happiness. "The mind is far more malleable than we previously assumed," says Saki Santorelli, executive director of the Center for Mindfulness in Medicine, Health Care, and Society at the University of Massachusetts Medical School. Studies at the center have shown that meditation can help people cope with stress. It may repair or compensate for damage already done to the brain.

Not all of us want to or can become monks; not all of us can spare even eight weeks for a course at the Center for Mindfulness. But there are quicker ways to learn to harness and handle stress. For this article, I tried one: the Williams LifeSkills program, a cognitive mini-makeover based on the research of Duke University psychiatrist Redford Williams. LifeSkills teaches adherents to approach life like a Zen baboon, picking the right battles—and it can be completed in a day and a half. "You won't achieve enlightenment, but it will help you," Williams told me before I embarked on the course, which gave me a formula for assessing conflicts (How important is this to me? Should I be mad? Can I do something about the problem? Would that be worth the trouble?). He was right. I did feel a bit calmer afterward. But then, I had willed myself to. I liked Williams; I was hoping his program would work.