Introduction

In the previous chapters, the role of genetic and nongenetic factors (e.g. life events, personality traits and chronic difficulties) in relation to the onset of depression was discussed. This leads to the question, which was already touched upon earlier: to what degree do genetic and nongenetic factors interact, that is, to what degree do they influence each other? In the two following chapters this question will be dealt with in more detail. First, the genetics of depression will be discussed. In the next chapter, we will specifically deal with the gene–environment interactions that are thought to underlie depressive illness.

Genetics of depression

Genetic epidemiology of depression

Mental health practitioners are used to thinking in terms of ‘visible’ environmental risks in relation to onset and persistence of psychiatric disorders. Stressful life events, chronic difficulties and dysfunctional parental interactions are but a few examples. Traditional psychiatric epidemiology was concerned mainly with such environmental risks. Conversely, clinical genetics was until recently almost exclusively concerned with Mendelian syndromes, for which single-gene defects could be mapped by positional cloning. Over the past decades, however, there has been increasing awareness that, for common psychiatric disorders, ‘hidden’ genetic factors can have a substantial influence on the effect of environmental exposures or even pose as risk factors.