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Being altogether human, which means in part understanding who and what we are, we are curious about the answer to the question of which force plays a stronger developmental role: genes or environment. The debate over "nature or nurture" has raged for two thousand years. At opposite extremes are euthenists, who cite bad parenting and the evils of society as the cause of all mental problems, and the proponents of eugenics, who blame faulty genes for all of society's ills and want to prevent all "bad" people from reproducing.

In reality there is no debate. Most of who we are is a result of the interaction of our genes and our experiences. In some cases, the genes are more important, while in others the environment is more crucial. We tend to oversimplify because we want to identify a single cause of a particular problem, so we can pour our efforts into one "cure." Some people hope that programs such as Head Start, which are designed to change a child's environment, will improve intellectual development. Others hope that one drug or one gene alteration will cure all aggressive behavior. Such simplistic approaches will rarely work. The real question is how genes and the environment influence each other, brain structure, and behavior. Untangling each factor's contribution is difficult because we can never fully understand an individual's genes outside an environment, and we can never study the effects of environment on a person while "isolating out" his genes.

Since the 1990s the pendulum has swung toward nature. It seems as though we hear almost daily of a new discovery; genes are now linked to Alzheimer's disease, bedwetting, obesity, and even to overall happiness. Many aspects of development that were previously attributed to learning, bad habits, or environment are now thought to be determined by genes. Many of us are fascinated by the international Genome Project, which is mapping the function of the 100,000 genes in the human genome, some 30,000 to 50,000 of which are designated for the brain. But we must remember that genetics is not destiny. A mere 50,000 genes for the brain are not nearly enough to account for the 100 trillion synaptic connections that are made there. Genes set boundaries for human behavior, but within these boundaries there is immense room for variation determined by experience, personal choice, and even chance.

The point to remember is that genes can be active or inactive and that everything we do affects the activity of our genes. We tend to think of genes as tiny entities that are isolated from the rest of the body, but they reside in every cell and are affected by anything that affects that cell, whether the cell is in the thigh or the cortex. For example, genes activate the exploratory network in a child's brain, and the more enriched the child's environment, the more these genes turn on, and the more the child explores. Adults experience many similar effects: learning increases the activation of genes that turn on the production of proteins in the brain needed to solidify memory.

In a few cases, one gene has complete control over whether you will develop a particular trait; if a man has the gene for color blindness or Huntington's disease, he will suffer these ills. Otherwise, it's rare that a single gene controls anything. In a few other cases, such as heart disease, genes predispose you to possible trouble, but your lifestyle can be the more important determining factor.

Most of our traits are caused by the interaction of many genes as influenced by the environment. That is why it is highly unlikely that specific patterns of behavior, such as stealing or brilliance in math, can be wholly inherited. If sons act like fathers in these cases it is primarily because the son is raised in a criminal environment or is praised for solving math problems and encouraged to play games such as chess that promote spatial thinking.

Environment can even negate strong genetic predispositions. For example, Type II diabetes is highly genetic, but if the susceptible person can avoid becoming overweight in midlife there is a good chance that the genes for the disease will not be activated. In another example, it has been found that even though twins carry an identical set of genes, it is not uncommon for one in a pair to exhibit severe Tourette syndrome-a neurological disorder characterized by the presence of tics and streams of nasty language-while the other twin's case is hardly observable. Interaction with the environment accounts for the great difference in severity.

Studies of identical twins separated at birth are often used to test the debate between nature and nurture. While these can be valuable, this test is hamstrung from the start for several reasons, one of which is that differences in the position of each twin in relation to the placenta may bring with them differences in blood supply, hormonal levels, and other factors that are not intrinsic to the genes of the twins. Whether a twin is a "front child" or a "back child," a "spleen child" or a "liver child," makes a difference.

We have much more to learn before we will be able to draw conclusions about which one, nature or nurture, is more important, and for what areas of development. If environment is all-important, it is hard to explain child prodigies-children who seem simply to sit down and play the piano or chess at a very early age, and are able to learn very fast, very well, with little or no instruction. It seems that prodigies must possess an inborn "talent," which means a gene or genes for the intellectual and physical capabilities needed to play the piano or chess.

The remarkable twinning effect directly contradicts the notion that environment is more important than genes. In these cases, twins who are raised apart (with no contact) and are reunited years later find that their lives are very similar. This was the case for a pair of twin brothers separated five weeks after birth and raised eighty miles apart in Ohio. When Jim Lewis and Jim Springer were reunited at the age of thirty-nine, they found they had both married women named Linda, divorced, and remarried women named Betty. Both chain-smoked Salem cigarettes, drank Miller Lite, loved stock-car racing, hated baseball, and vacationed on the same stretch of beach in Florida. Studies of 7,000 sets of twins by the Minnesota Center for Twin and Adoption Research show that a number of traits may be driven by genes, including alienation, leadership, vulnerability to stress, and even religious conviction and career choice.

But then it turns out that for some twins separated at birth, one twin ends up as a schizophrenic adult and the other does not. How is this possible if they have the same genes? Environment may be the answer. Other twin studies show that environment can mitigate or exaggerate the effect of genes; twin halves raised by parents living in a tough inner city demonstrate more aggressive and violent behavior than the other twin halves raised in suburbs.

The point to remember is that the issue is not nature versus nurture. It is the balance between nature and nurture. Genes do not make a man gay, or violent, or fat, or a leader. Genes merely make proteins. The chemical effect of these proteins may make the man's brain and body more receptive to certain environmental influences. But the extent of those influences will have as much to do with the outcome as the genes themselves. Furthermore, we humans are not prisoners of our genes or our environment. We have free will. Genes are overruled every time an angry man restrains his temper, a fat man diets, and an alcoholic refuses to take a drink. On the other hand, the environment is overruled every time a genetic effect wins out, as when Lou Gehrig's athletic ability was overruled by his ALS. Genes and the environment work together to shape our brains, and we can manage them both if we want to. It may be harder for people with certain genes or surroundings, but "harder" is a long way from predetermination.

Excerpted from A User's Guide to the Brain by John J. Ratey, M.D. Copyright © 2002 by John J. Ratey, M.D.. Excerpted by permission of Vintage, a division of Random House, Inc. All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.

Tags: Neuropsychology

About the Author

John J. Ratey, M.D., is an associate professor of psychiatry at Harvard Medical School. He has lectured extensively and published many articles on the topic of treating adults with ADD. Dr. Ratey is the author of A User's Guide to the Brain and the co-author of Driven to Distraction. He lives in Cambridge, Massachusetts, where he has a private practice.