Preface

Epidemiology is a relatively new discipline in human science. Its basic aims are to observe and describe human behavior in a group context. The traditional epidemiological method is based on measuring the distribution of a given phenomenon in a given population. Another important tool is to detect and to describe the distribution of phenomena that deviate from the mean. In addition, the epidemiological method may be used to measure changes in behavioral patterns over time. It is not difficult to realize that epidemiology is closely related to such disciplines as sociology, anthropology and the medical sciences. Throughout this century, epidemiology has become an indispensable partner for the medical sciences. The demonstration of an unequal distribution of a given disease in a given environment has inspired medical researchers to develop and test new etiological hypotheses. Psychiatry is no exception. A close and fruitful collaboration between epidemiology and clinical psychiatry has been a basic condition for the present understanding of morbidity rates, incidence, prevalence and, to some extent, the etiology of mental illness. The epidemiological method has also contributed to present knowledge of crosscultural characteristics in psychiatry. The development of disease registers and internationally accepted diagnostic instruments are examples of this positive development. But and there is a but classical epidemiology only provides us with correlations between demographic characteristics and mental disorder. It does not provide final etiological explanations, as it cannot separate antecedent and consequential factors from each other. An example: several epidemiological studies have demonstrated that there is a statistically significant excess of severe mental disorder among homeless people. In itself, it is an important finding that poor, homeless individuals suffer from mental illness to a great extent. It may lead health authorities and politicians to support this special risk population. But this correlation does not contribute much to etiological clarification. Does psychosis lead to homelessness? Does homelessness lead to psychosis? Or does psychosis result in social decline, leading to homelessness? Traditional epidemiology is not able to answer this essential question. Another classical example is the comorbidity between alcoholism and psychopathology. How are they related? Which comes first? Are they etiological, epiphenomena1 or consequential factors? The epidemiologist cannot answer these essential questions about etiology. Are mental disorders based on genetic transmission from generation to generation? Or do they depend on environmental factors? This question has engaged public discussion for thousands of years, and a conclusive consensus has still not been reached. However, new research strategies have developed during the last 50 years as useful contributions to the ongoing nature-nurture dialogue in mental illness. In particular, twin, adoption and high-risk studies have been introduced as useful methods to increase understanding of this essential area, in which sociology, psychology and biology meet. These new research strategies have increased understanding of the interaction between genetics and the environment, in particular the biological vulnerability concept. Several specific etiological factors have been proposed from this type of study. Hypotheses on risk factors promoting mental illness have also developed due to this new branch of research in psychiatric epidemiology. The research in psychiatric epidemiology in Denmark has proved to be among the most important in the world. Several factors constitute the background for this favorable position: