Your question somewhat surprised me, because it illustrates how fully our perception of disease has changed. While in large part heredity has always been recognized as contributing to disease susceptibility, the focus through most of the 20th century was on the influence of external factors in causing disease. (Of course, this was due in large part to the desire to develop effective treatments-- while we are able to manipulate environmental variables, there is still little we can do about a person's genetic composition). Now, with the recent sequencing of the human genome, the pendulum has swung almost entirely the other way. Much excitement over the possibility of finally describing and treating hereditary diseases by direct understanding of the genetic deficiencies involved has been generated, to the point that environmental factors are often neglected.

It is important to consider that a great deal of data have been accumulated over the years which indicate that environmental factors do contribute to disease, oftentimes even with genetic diseases, and that the new emphasis on genetic origins of disease does not invalidate these observations. These environmental factors are listed among known "risk factors" for a given disease. For example, although the major risk factor for schizophrenia is a person's heredity, other contributing factors such as prenatal conditions or personal social and psychological experiences have been identified. The influence of environment on the expression of schizophrenia is illustrated by the fact that schizophrenia may occur in one identical twin but not in the other, despite the fact that the twins share identical genetic information (and often nearly identical environments, indicating how even slight differences may be profound).

The reason that environmental factors may influence even hereditary disease expression lies in the way genes function in our bodies. Our genes are carefully regulated by complicated mechanisms that ensure a gene expresses itself, or becomes active, only at the right time (many genetic diseases are actually due to failures in these regulatory mechanisms, rather than to problems with the identified gene itself). Thus, our bodies respond in complex ways to specific environmental inputs. A genetic defect may be present but not a problem until some environmental trigger causes it to exert a specific response. For example, many people have a genetic predisposition to skin cancer due to a poor copy of the gene that is responsible for producing a DNA repair protein. This protein repairs damage to DNA caused by UV light, but because the repair protein produced by the gene is a bit defective, it doesn't do a good job, resulting in improperly repaired DNA that may ultimately lead to cancer. However, this genetic defect is never a problem to the individual unless he or she is exposed to UV light. Thus, despite a flawed gene which heightens the likelihood of contracting skin cancer, the environmental trigger of high exposure to sunlight is necessary for the problem to emerge.

Schizophrenia is a much more complex disease that is currently only poorly understood but that almost certainly derives from the interaction of many genes. The patterns of expression for multiple interacting genes are extraordinarily complex, and accordingly schizophrenia comes in many forms and may affect different people in different ways. The specific environmental factors that may trigger the expression patterns that result in the development of schizophrenia are likewise complex and difficult to clearly identify. I believe that as we gain a better understanding of the genetic basis for schizophrenia, we will begin to better understand the environmental factors that increase the likelihood of contracting this disease and how the factors affect gene expression. This understanding, of course, will help us to identify more effective preventive measures and treatments.

I should also mention that a further debate on the role of environmental factors in such diseases as schizophrenia is beginning to grow. Some scientists, such as Paul Ewald at Amherst College, are beginning to push the idea that nearly every disease is ultimately caused by the influence of viruses or bacteria. These scientists would suggest that our current emphasis on genetic information is somewhat misplaced, and that we should return to a closer investigation of the role of microorganisms in effecting disease. While currently on the fringe of science and for the most part denounced by the medical establishment, there is compelling evidence for many of their arguments.

I hope that this discussion helps you to better understand why some people believe schizophrenia has a largely environmental component. Unfortunately, disorders such as schizophrenia are complex enough that simple answers aren't forthcoming. I think that ultimately for these diseases the answer will

This is a very good question and one I have wondered about in the past since I have a family member who is debilitated with this disease.
From what I have read, the latest studies believe that the occurrence of schizophrenia are caused by a combination of factors that include genetic as well as environmental. Poor nutrition as a child, a mother having a virus while pregnant or any event that could effect the normal development of a childs brain could do it. The research that has led to these ideas was a result of examining the brains of schizophrenia patients and seeing that there were areas of their brains that were not "wired correctly" or not developed properly. That is not to say that every one who has some developmental problems will get schizophrenia, there is still a genetic link and only those with the unfortunate combination of the gene and the environmental or developmental problems will get the disease. There is still a lot to be studied with this disease, and all the new information is really only the tip of the iceberg. Hope this helps.