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used for short-term memory. We now know that several thousand new nerve cells are born in the hippocampus every day. Neuroscientist Fred Gage at the Salk Institute showed that physical exercise and environmental enrichment (stimulating surroundings) stimulate the growth of new neurons in mice. One sees the same principle at work in zoos. Gorillas and other primates languish if they are kept in confined cages with nothing to do, butthey flourish in large enclosures with trees, swings, and toys. If we could learn exactly how to safely induce neurogenesis in the human brain, we could more effectively treat conditions where brain cells have been lost or severely damaged: Alzheimer’s disease, traumatic brain injury, stroke, and epilepsy. We could also reliably maintain the health of our brains as we age.
    Alzheimer’s researcher Sam Sisodia at the University of Chicago showed that physical exercise and mental stimulation protect mice from getting Alzheimer’s disease, even when they have been engineered to carry a human Alzheimer’s mutation in their genome. Other studies in rodents offer encouragement for the normal brain, too. By choosing to exercise every day, you can increase the number of new nerve cells, just as you do when you actively seek to learn new things. At the same time, you promote the survival of these new cells and connections. In contrast, emotional stress and trauma leads to the production of glucocorticoids in the brain, toxins that inhibit neurogenesis in animal models.
    We can safely discard the myth about losing millions of brain cells a day. Even the parental warning that alcohol kills off brain cells has turned out to be a half-truth. Casual alcohol use actually kills only a minimal number of brain cells, even among alcoholics (who, however, incur many real health dangers). The actual loss from drinking occurs in dendrites, but studies seem to indicate that this damage is mostly reversible. The bottom line for now is that as we age, key areas of the brain involved with memory and learning continue to produce new nerve cells, and that this process can be stimulated by physical exercise, mentally stimulating activities (like reading this book), and social connectedness.
    Myth 5. Primitive reactions (fear, anger, jealousy, aggression) overrule the higher brain
    Most people have at least caught some wind that the first four myths are untrue. The fifth myth, however, seems to be gaining ground.The rationale for declaring that human beings are driven by primitive impulses is partly scientific, partly moral, and partly psychological. To put it in a sentence, “We were born bad because God is punishing us, and even science agrees.” Too many people believe some part of this sentence, if not all of it.
    Let’s examine what seems to be the rational position, the scientific argument. All of us are born with genetic memory that provides us with the basic instincts we need to survive. Evolution aims to ensure the propagation of our species. Our instinctive needs work hand in hand with our emotional urges to gather food, find shelter, seek power, and procreate. Our instinctive fear helps us avoid dangerous situations that threaten the lives of ourselves and our kin.
    Thus an evolutionary argument is used to persuade us that our fears and desires, instinctively programmed in us from the womb, are in charge, overruling our higher, more evolved brain, with its reason and logic (glossing over the all-too-obvious irony that the higher brain invented the theory that demoted it). Undoubtedly, instinctive reactions are built into the brain’s structure. Some neuroscientists find convincing the argument that certain people are programmed to become antisocial, criminals, or rage-aholics, much as others are programmed for anxiety, depression, autism, and schizophrenia.
    But emphasizing the lower brain overlooks a powerful truth. The brain is multidimensional, in order to allow any experience to occur. Which experience