The problem of explaining why one individual contracts cancer and another does not has led researchers to theorize that there could be a genetic predisposition that either causes some people to produce a greater number of abnormal cells or inclines them to have a weak immune response to abnormal cells. The observation that the incidence of cancer is substantially higher in some families than in others has been an impetus to a great deal of research in this area.

Indeed, special strains of experimental mice have been bred for use in cancer research precisely because of their increased susceptibility to cancer. Yet a major study conducted with these cancer-prone mice casts considerable doubt on any "it's genetics alone" theory. In the study, Dr. Vernon Riley, of the University of Washington, subjected a group of these mice to high levels of stress, while keeping a control group of the cancer-prone mice in a stress-free environment. At the time of the study, 80 percent of the mice would have been expected to develop cancer. As it turned out, however, 92 percent of the mice that had been placed under stress developed cancer, while only 7 percent of the stress-free mice did so. Thus, although all the mice had a genetic predisposition to cancer, the amount of stress in the environment had a very significant impact on the development of cancer.

Other efforts to explain cancer in terms of genetic predisposition have involved comparing cancer rates in different countries. For instance, the Japanese have one of the lowest rates of breast cancer in the world. Until a few years ago, it was thought this might be due to an inherited racial resistance, a genetic predisposition against breast cancer for all Japanese. But then the discovery was made that Japanese women living in the United States were four times more susceptible to breast cancer than were those living in Japan. Apparently, the differences in these cases are not racial, or genetic, but have something to do with living in Japan rather than in America.

Other cross-cultural studies have produced similarly inconclusive results. In addition, since genetic predisposition must be passed on from generation to generation, changes in predisposition in a whole society would occur very slowly. Therefore, the sharply increased incidence of cancer in industrialized society over the past twenty-five to fifty years is not readily explainable with the genetic argument.

Although genetic factors may play some role, we do not believe that by themselves they can explain the different patterns of cancer incidence throughout the world. It is important to consider the stressful changes that occur along with industrialization and integrate that information into our current thinking about cancer incidence.

*13\347\2*

Cancer