While it has become generally accepted that our state of mind plays some role in health and disease, most people believe that the ultimate determinant of longevity resides in their genes. Whenever I have asked a patient how long he or she expects to live, their first instinct is to say, ?Well, my mother died at age X, and my father?? This is another way of saying that my genes are my fate.

The twenty-first century has been heralded as the age of the genome, and medical genetics is widely touted as the future of health care. Diseases will soon be diagnosed by identifying the faulty gene; therapy will either fix or replace it. While genes are undeniably important factors in causing disease, their roles have been vastly overemphasized. For the most common diseases such as cancer and atherosclerosis, genes are predispositions, not inevitabilities.

Identical twins have the same genetic risk for disease, yet several studies have shown substantial differences in their health histories. This is because another crucial factor in determining one's propensity for disease: an individual's environment. According to Craig Venter, former CEO of the company that first decoded the human genome, ?The wonderful diversity of the human species is not hard-wired in our genetic code. Our environments are critical.?

A recent report in the New England Journal studied 44,788 twins to evaluate the comparative importance of genes and the environment in causing the most common types of cancer. The study concluded: ?Inherited genetic factors make a minor contribution to susceptibility in most types of cancer. The overwhelming contributor to the causation of cancer is the environment.? It is now widely accepted that 80 to 90 percent of human cancer is due to nongenetic factors.

The overriding importance of environmental elements is also clear in the development of atherosclerosis, the number one killer in the United States. Atherosclerosis is a multifactoral disorder resulting from an interaction of several predisposing abnormalities such as high cholesterol, hypertension and diabetes. Research has conclusively shown that reducing these risks substantially decreases the probability of having a heart attack or stroke. For example, a loss of just 7 percent of body weight in obese people reduces the incidence of diabetes by 58 percent, while shedding 10 pounds will normalize the blood pressure of those with borderline hypertension, no matter what their genetic propensity.

The human genome project has given us a more nuanced understanding of how genes work. It is now clear that they are not static blueprints that dictate our biological behavior. Most genes have switches, called promoters that control how, when, and even if they become active, a phenomenon known as gene expression. Other regulatory elements, called gene enhancers, also play a role. Even slight alterations in promoters or enhancers can lead to dramatic changes in gene expression. But the factors that determine whether or not genes are turned on or off, and for how long, remain largely unknown.

Animal studies have begun to show that social, behavioral and environmental elements can determine whether or not genes are expressed. For example, stress has been demonstrated to cause diabetes in genetically prone animals, while those with the same genetic susceptibilities not exposed to stress are less likely to develop the disease.

Recent insights into Pavlov's famous conditioning experiments in dogs provide another striking example. A century ago, the Russian scientist showed that the brain of dogs could be trained to anticipate the arrival of food. We now know that this type of training changes the brain through the expression of seventeen genes that have been given the name CREB genes.

These findings demonstrate that a change in mental conditioning not only affects gene expression, but also can actually change the way our genes operate. They prove that our genes no longer should be thought of as immutable determinants of our fate, but as dynamic entities, switching on and off in response to outside influences, as much the result as the cause of our mental, emotional, and biological processes.

People with a strong will to live understand that when they take life-enhancing measures, their health and longevity will be favorable affected, regardless of their genes. They take responsibility for their lives instead of being victims of events. Each and every one of us has the resources within to affect our health and longevity.