Sociological studies of the early 20th century can be regarded as predecessors of today's psychiatric epidemiology.[1]:6 These studies investigated for instance how suicide rates differ between Protestant and Catholic countries or how the risk of suffering from schizophrenia is increased in neighborhood characterized with high levels of social isolation. After World War 2 researchers began using community surveys to assess psychological problems.[1]:7 By the 1980s the development of new diagnostic assessment instruments and reliable criteria for mental disorders given by the DSM-3 began a trend to estimating the prevalence of mental disorders next to symptoms.Today, epidemiological studies focus on the etiology of mental disorders, i.e. the identification and quantification of causes underlying psychiatric problems and their mechanisms, rather than mere estimation of prevalence.[1]:11 It is not ethically possible to experimentally expose study participants to stressors suspected to cause psychiatric disorders, thus epidemiological techniques are required to study the etiology. For this purpose longitudinal studies, which follow children and adults for a long period of time, often for many years, are particularly useful. These allow the study of naturally occurring exposures and how they affect changes in psychiatric symptoms. Two notable historical studies focusing on etiology are the Dunedin Multidisciplinary Health and Development Study and the Christchurch Health and Development Study.[2] These studies began in the 70's and studied the impact of perinatal problems, genetic variants, sexual abuse and other adverse exposures on psychological problems in childhood and later in adulthood.

Psychiatric disorders show substantial heritability according to twin studies. A meta-analysis of most twin-studies conducted found a combined heritability of 46% for psychiatric disorders.[3] Given the large contribution of genetic variants on psychiatric disorders, one major focus of psychiatric epidemiology is psychiatric genetics. A combination of family and molecular studies are used within psychiatric epidemiology to uncover the effects of genetics on mental health. Twin studies estimate the influences of all genetic variants and effects, but, due to relying purely on relatedness information, are limited in explaining the specific genetic mechanisms and architecture underlying psychiatric traits. Molecular studies confirm findings from family studies that genetic variants can partly explain the occurrence of psychological problems, e.g. by quantifying the total contribution of common genetic variants.[4][5] Furthermore an increasing number of specific genetic loci are being associated with psychiatric disorders in large genome-wide association studies.

Next to genetic exposures, a wide variety of environmental exposures are being studied as well, such as nutrition,[citation needed], urbanicity,[8] stressful life events,[9] and bullying.[citation needed] In contrast to genetic studies, the investigation of environmental exposures of psychiatric problems face the question of bidirectional causality. For example, both directions of causation are possible: experiencing social stress might cause depression, or being depressed might worsen relationships with others and thus cause social stress (or it may even be the case that both interact, possibly as a self-reinforcing feedback loop). Finding an association between the occurrence of an environmental exposure and a disorder could be the result of either or both situations. Multiple strategies exist in psychiatric epidemiology to assess the direction of causality. One possibility is measuring the exposure and outcome multiple times. Researchers can then analyze how much a change in psychiatric symptoms can be attributed to the exposure at a previous time point, but also whether changes in the exposure can be predicted by previous symptom levels (cross-lagged model). Such a model was e.g. applied to the study of internalizing and externalizing psychological problems and stressful life events.[10] Both, psychiatric problems as well as life events, were measured multiple times during grade 7,8,9,10 and 12. The researchers observed that stressful life events precede both internalizing and externalizing psychological problems, but appear to be also the result of experiencing such symptoms. An alternative approach is the use of twin studies, because discordance between monozygotic twins suggests environmental effects.