When Does Your Brain Stop Making New Neurons?

A. Infant; B. 42 Years Old; C. 53 Years Old.

The scientists are not so naive as to think they have discovered a magic wand that can turn animosity into compassion and hatred into benevolence, but the tarantula definitely raised their hopes. Over the years psychologists Phillip Shaver and Mario Mikulincer had uncovered more and more evidence that people's sense of emotional security shapes whether they become altruistic or selfish, tolerant or xenophobic, open or defensive. Once upon a time, that would have been that, for whatever their roots such traits were thought to be, by adulthood, as hard-wired as a computer's motherboard.

But with the new millennium scientists were finding that brain wiring can change, even in adults. That got Shaver, a professor at the University of California, Davis, and Mikulincer, at Israel's Bar-Ilan University, thinking: could they activate unused or dormant circuits to trigger the sense of emotional security that underlies compassion and benevolence? To find out, they gave volunteers overt or subliminal cues to activate brain circuitry encoding thoughts of someone who offered unconditional love and protection—a parent, a lover, God. The goal was to induce the feeling of security that makes it more likely someone will display, say, altruism and not selfishness. It worked. People became more willing to give blood and do volunteer work, and less hostile to ethnic groups different from their own. Offered a chance to inflict pain on an Israeli Arab with whom they were paired in an experiment (serving him painfully spicy hot sauce), Israeli Jews did not dole it out as they did without the security-circuit activation. They held back. And when they saw a young woman distraught over having to pick up a tarantula as part of an experiment, they volunteered to take her place.

OK, so they didn't all sign up to work in Darfur. But as recently as a decade ago, proposing that an adult brain could be rewired for compassion—or anything else, for that matter—without experiencing a life-altering epiphany would have been career suicide for a neuroscientist. Not anymore. Experts are overthrowing the old dogma that, by the ripe old age of 3, the human brain is relatively fixed in form and function. Yes, new memories could form, new skills could be mastered and wisdom could (in some) be gained. But the basic cartography of the adult brain was thought to be as immutable as the color of your eyes. This "neurological nihilism," as psychiatrist Norman Doidge calls it in his recent book, "The Brain That Changes Itself," "spread through our culture, even stunting our overall view of human nature. Since the brain could not change, human nature, which emerges from it, seemed necessarily fixed and unalterable as well."

But the dogma is wrong, the nihilism groundless. In the last few years neuroscientists have dismantled it pillar by pillar, with profound implications for our view of what it means to be human. "These discoveries change everything about how we should think of ourselves, who we are and how we get to be that way," says neuroscientist Michael Merzenich of the University of California, San Francisco. "We now know that the qualities that define us at one moment in time come from experiences that shape the physical and functional brain, and that continue to shape it as long as we live."

The brain remains a work in progress even on so basic a parameter as its allotment of neurons. For decades, scientists assumed that adding new neurons to this intricate machine could only spell trouble, like throwing a few extra wires into the guts of your iPod. But in 1998 Peter Eriksson of Sweden's Sahlgrenska University Hospital and colleagues discovered that brains well into their 60s and 70s undergo "neurogenesis." The new neurons appear in the hippocampus, the structure deep in the brain that takes thoughts and perceptions and turns them into durable memories. And studies in lab animals show that the new neurons slip into existing brain circuits as smoothly as the Beckhams onto the Hollywood A list.

Brain structure is also malleable, recording the footprints of our lives and thoughts. The amount of neural real estate devoted to a task, such as playing the violin, expands with use. And when an area of the brain is injured, as in a stroke, a different region—often on the mirror-image side—can take over its function. That overthrew the long-held view called "localizationism," which dates back to 1861, when French surgeon Paul Broca linked the ability to speak to a spot in the left frontal lobe. But contrary to the belief that particular regions are unalterably wired for specific functions, even one as basic as the visual cortex can undergo a career switch. In people who lose their sight at a young age, the visual cortex processes touch or sound or language. Receiving no signals from the eyes, the visual cortex snaps out of its "waiting for Godot" funk and reactivates dormant wires, enabling it to perform a different job.

If something as fundamental as the visual cortex can shrug off its genetic destiny, it should come as little surprise that other brain circuits can, too. A circuit whose hyperactivity causes obsessive-compulsive disorder can be quieted with psychotherapy. Patterns of activity that underlie depression can be shifted when patients learn to think about their sad thoughts differently. Circuits too sluggish to perceive some speech sounds (staccato ones such as the sound of "d" or "p") can be trained to do so, helping kids overcome dyslexia. For these and other brain changes, change is always easier in youth, but the window of opportunity never slams shut.

From these successes, neuroscientists have extracted a powerful lesson. If they can identify what has gone wrong in the brain to cause, say, dyslexia, they might be able to straighten out aberrant wiring, quiet an overactive circuit or juice up a sluggish one. It won't happen overnight. But UCSF's Merzenich believes we have glimpsed only the surface of the ability of the brain to change. "The qualities that define a person have a neurological residence and are malleable," he says. "We know that in a psychopath, there is no activation of brain areas associated with empathy when he sees someone suffering. Can we change that? I don't know exactly how, but I believe we can. I believe that just as you can take a 17-year-old and put him through basic training, inuring him to violence, we can take a person who is insensitive and make him sensitive to others' pain. These things that define us, I'm convinced, can be altered." Only more research—and it's coming—will reveal how easily, and how much.

But what of the genes that shape our disposition and temperament? Here, too, malleability rules. As is often the case, this effect is easiest to detect in lab animals. Rats develop starkly different personalities depending on how they are reared. Specifically, if Mom is attentive and regularly licks and grooms them, they become well-adjusted little rodents, mellow and curious and non-neurotic mouse or rat. If Mom is neglectful, her pups grow up to be timid, jumpy and stressed out. Once, this was attributed to the powerful social effects of maternal care. But it turns out that Mom's ministrations can reach into the pups' very DNA. Maternal neglect silences genes for receptors in the pups' brains, with the result that they have few receptors and hence a hair-trigger stress response. Maternal care keeps these genes on, so the pups' brains have lots of receptors and a muted stress response. Inattentive moms also silence the genes for estrogen receptors in their daughters' brains; the females grow up to be less attentive mothers themselves. "It's almost Lamarckian," says Francis Champagne of Columbia University, referring to the discredited idea that offspring can pass along traits they acquire during life. "But experiences during a lifetime are passed on to the next generation."

Scientists are now beginning to see the first glimmerings of this in people, too. Very young children born with the form of a gene called 5-HTT associated with shyness usually are quiet and introverted. But by age 7, scientists led by Nathan Fox of the University of Maryland find, many are not. Only if the children have certain experiences—best guess: being raised by a stressed mother unable to provide emotional and physical protection—does the "shyness gene" live up to its billing. The molecular mechanism by which experiences reach down into the double helix and inhibit or elicit the expression of a gene is not as clear in people as it is in lab rats. At least, not yet. But it's an early sign that we are not necessarily slaves to the genes we inherit.

Few laypeople understand that neurological nihilism and genetic determinism have been so discredited. Most still embrace the idea that our fate is written in our DNA, through the intermediary of the brain wiring that DNA specifies. "It's puzzling that determinism is so attractive to so many people," says UCSF's Merzenich. "Maybe it's appealing to view yourself as a defined entity and your fate as determined. Maybe it's in our nature to accept our condition."

There is an irony to that. When people believe that their abilities and traits are fixed, interventions meant to improve academic performance or qualities such as resilience and openness to new experiences have little effect. "But if you tell people that their brain can change, it galvanizes them," says psychologist Carol Dweck of Stanford University, whose 2006 book "Mindsets" explores the power of belief to alter personality and other traits. "You see a rapid improvement in things like motivation and grades, or in resilience in the face of setbacks." None of that happens, or at least not as readily, in people who believe they are stuck with the brain they have.

This is not to say that everything will yield to the new brain science. There may turn out to be aspects of ourselves that resist every effort at change, for which we may be glad. But for too many decades, science sold the brain short. It is way too early in the battle against neuro-nihilism to declare anything beyond the reach of the brain's potential to transform itself.