Psychiatric epidemiology is a field which studies the causes (etiology) of mental disorders in society, as well as conceptualization and prevalence of mental illness. It is a subfield of the more general epidemiology. It has roots in sociological studies of the early 20th century. However, while sociological exposures are still widely studied in psychiatric epidemiology, the field has since expanded to the study of a wide area of environmental risk factors, such as major life events, as well as genetic exposures. Increasingly neuroscientific techniques like MRI are used to explore the mechanisms behind how exposures to risk factors may impact psychological problems and explore the neuroanatomical substrate underlying psychiatric disorders. [1] : 6
Reviews on psychiatric epidemiology as a main subject were published by Tohen et al. in 2006, [2] Kessler in 2007, [3] and Juul & Nemeroff in 2012. [4]
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 having 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.
As an example, in an attempt to measure the prevalence of mental illness in the United States, Lee Robins and Darrel A. Regier conducted a study called the Epidemiological Catchment Area Project which surveyed samples of the general population at five sites across America. In the study, it was found that about a third of all Americans experience mental illness at some point in their lives. This statistic is often referred to as lifetime prevalence.
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. [5] 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.
Many different instruments are used to assess mental disorders in epidemiological studies depending on the age of the participants, available recourses and other considerations. Studies featuring adolescents and adults often use structured interviewing, a technique in which a series of questions is administered by interviewers to determine whether an individual is disordered or nondisordered. Alternatively, questionnaire are used, which can be administered more easily. In epidemiological studies featuring children, psychopathology is often assessed using parent report, however, multi-informant approaches, e.g. the simultaneous use of parent, teachers and self-report, is popular as well. [5]
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. [6] 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. [7] [8] Furthermore, an increasing number of specific genetic loci are being associated with psychiatric disorders in large genome-wide association studies [9] [10]
Next to genetic exposures, a wide variety of environmental exposures are being studied as well, such as nutrition, [11] urbanicity, [12] stressful life events, [13] and bullying. [14] 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. [15] 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. [6]
Brain development is a complex process that starts during the early stage of embryogenesis and continues into adulthood. [16] Studies focusing on risk factors of exposure on early brain development have found differences in brain anatomy in children of mothers that consumed marijuana compared to unexposed control subjects, emphasizing the importance of intrauterine risk factors in early brain development. [17] Premature birth is also associated with an increased risk of childhood depression [18] and psychiatric hospitalization in young adulthood. [19]
Population-based imaging studies attempt to find neurobiological substrates to explain psychiatric symptomatology. These studies have mainly used magnetic resonance imaging (MRI) sequences to study the brain at a population scale: MRI sequences can be used to noninvasively study structural (e.g. volumetric) brain differences between individuals. Other examples of MRI sequences are functional MRI (fMRI), studying activational patterns of the brain and diffusion tensor imaging (DTI), measuring the development and integrity of white matter tacts. [20] These techniques have been used at various developmental stages, examples for each stage are discussed below.
Imaging studies in children showed a significant association between autistic traits and gyrification patterns (winding patterns) of the brain on structural magnetic resonance imaging (MRI). [21] DTI studies, focusing on white matter development, showed that cognitive ability in children is positively associated with white matter integrity, as expressed by fractional anisotropy (FA). [22]
Population-based studies in a large elderly population found a significant association between vascular white matter disease and depressive symptoms. [23]
Bipolar disorder, previously known as manic depression, is a mental disorder characterized by periods of depression and periods of abnormally elevated mood that each last from days to weeks. If the elevated mood is severe or associated with psychosis, it is called mania; if it is less severe, it is called hypomania. During mania, an individual behaves or feels abnormally energetic, happy or irritable, and they often make impulsive decisions with little regard for the consequences. There is usually also a reduced need for sleep during manic phases. During periods of depression, the individual may experience crying and have a negative outlook on life and poor eye contact with others. The risk of suicide is high; over a period of 20 years, 6% of those with bipolar disorder died by suicide, while 30–40% engaged in self-harm. Other mental health issues, such as anxiety disorders and substance use disorders, are commonly associated with bipolar disorder.
A mental disorder is an impairment of the mind disrupting normal thinking, feeling, mood, behavior, or social interactions, and accompanied by significant distress or dysfunction. The causes of mental disorders are very complex and vary depending on the particular disorder and the individual. Although the causes of most mental disorders are not fully understood, researchers have identified a variety of biological, psychological, and environmental factors that can contribute to the development or progression of mental disorders. Most mental disorders result in a combination of several different factors rather than just a single factor.
Specific phobia is an anxiety disorder, characterized by an extreme, unreasonable, and irrational fear associated with a specific object, situation, or concept which poses little or no actual danger. Specific phobia can lead to avoidance of the object or situation, persistence of the fear, and significant distress or problems functioning associated with the fear. A phobia can be the fear of anything.
Adjustment disorder is a maladaptive response to a psychosocial stressor. It is classified as a mental disorder. The maladaptive response usually involves otherwise normal emotional and behavioral reactions that manifest more intensely than usual, causing marked distress, preoccupation with the stressor and its consequences, and functional impairment.
Conversion disorder (CD), or functional neurologic symptom disorder, is a diagnostic category used in some psychiatric classification systems. It is sometimes applied to patients who present with neurological symptoms, such as numbness, blindness, paralysis, or fits, which are not consistent with a well-established organic cause, which cause significant distress, and can be traced back to a psychological trigger. It is thought that these symptoms arise in response to stressful situations affecting a patient's mental health or an ongoing mental health condition such as depression. Conversion disorder was retained in DSM-5, but given the subtitle functional neurological symptom disorder. The new criteria cover the same range of symptoms, but remove the requirements for a psychological stressor to be present and for feigning to be disproved. The ICD-10 classifies conversion disorder as a dissociative disorder, and the ICD-11 as a dissociative disorder with unspecified neurological symptoms. However, the DSM-IV classifies conversion disorder as a somatoform disorder.
Depersonalization-derealization disorder is a mental disorder in which the person has persistent or recurrent feelings of depersonalization and/or derealization. Depersonalization is described as feeling disconnected or detached from one's self. Individuals may report feeling as if they are an outside observer of their own thoughts or body, and often report feeling a loss of control over their thoughts or actions. Derealization is described as detachment from one's surroundings. Individuals experiencing derealization may report perceiving the world around them as foggy, dreamlike/surreal, or visually distorted.
Impulse-control disorder (ICD) is a class of psychiatric disorders characterized by impulsivity – failure to resist a temptation, an urge, or an impulse; or having the inability to not speak on a thought. Many psychiatric disorders feature impulsivity, including substance-related disorders, behavioral addictions, attention deficit hyperactivity disorder, autism spectrum disorder, fetal alcohol spectrum disorders, antisocial personality disorder, borderline personality disorder, conduct disorder and some mood disorders.
Oppositional defiant disorder (ODD) is listed in the DSM-5 under Disruptive, impulse-control, and conduct disorders and defined as "a pattern of angry/irritable mood, argumentative/defiant behavior, or vindictiveness". This behavior is usually targeted toward peers, parents, teachers, and other authority figures. Unlike conduct disorder (CD), those with ODD do not show patterns of aggression towards people or animals, destruction of property, theft, or deceit. One half of children with ODD also fulfill the diagnostic criteria for ADHD.
The epidemiology of autism is the study of the incidence and distribution of autism spectrum disorders (ASD). A 2022 systematic review of global prevalence of autism spectrum disorders found a median prevalence of 1% in children in studies published from 2012 to 2021, with a trend of increasing prevalence over time. However, the study's 1% figure may reflect an underestimate of prevalence in low- and middle-income countries.
Dan Joseph Stein is a South African psychiatrist who is a professor and Chair of the Dept of Psychiatry and Mental Health at the University of Cape Town, and Director of the South African MRC Unit on Risk & Resilience in Mental Disorders. Stein was the Director of UCT's early Brain and Behaviour Initiative, and was the inaugural Scientific Director of UCT's later Neuroscience Institute. He has also been a visiting professor at Mount Sinai School of Medicine in the United States, and at Aarhus University in Denmark.
Risk factors of schizophrenia include many genetic and environmental phenomena. The prevailing model of schizophrenia is that of a special neurodevelopmental disorder with no precise boundary or single cause. Schizophrenia is thought to develop from very complex gene–environment interactions with vulnerability factors. The interactions of these risk factors are intricate, as numerous and diverse medical insults from conception to adulthood can be involved. The combination of genetic and environmental factors leads to deficits in the neural circuits that affect sensory input and cognitive functions. Historically, this theory has been broadly accepted but impossible to prove given ethical limitations. The first definitive proof that schizophrenia arises from multiple biological changes in the brain was recently established in human tissue grown from patient stem cells, where the complexity of disease was found to be "even more complex than currently accepted" due to cell-by-cell encoding of schizophrenia-related neuropathology.
Dual diagnosis is the condition of having a mental illness and a comorbid substance use disorder. There is considerable debate surrounding the appropriateness of using a single category for a heterogeneous group of individuals with complex needs and a varied range of problems. The concept can be used broadly, for example depression and alcohol use disorder, or it can be restricted to specify severe mental illness and substance use disorder, or a person who has a milder mental illness and a drug dependency, such as panic disorder or generalized anxiety disorder and is dependent on opioids. Diagnosing a primary psychiatric illness in people who use substances is challenging as substance use disorder itself often induces psychiatric symptoms, thus making it necessary to differentiate between substance induced and pre-existing mental illness.
The prevalence of mental disorders has been studied around the world, providing estimates on how common mental disorders are. Different criteria or thresholds of severity have sometimes been used.
Obsessive–compulsive disorder (OCD) is a mental and behavioral disorder in which an individual has intrusive thoughts and feels the need to perform certain routines (compulsions) repeatedly to relieve the distress caused by the obsession, to the extent where it impairs general function.
The epidemiology of child psychiatric disorders is the study of the incidence, prevalence, and distribution of conditions in child and adolescent psychiatry. Subfields of pediatric psychiatric epidemiology include developmental epidemiology, which focuses on the genetic and environmental causes of child psychiatric disorders. The field of pediatric psychiatric epidemiology finds widely varying rates of childhood psychiatric disorders, depending on study population, diagnostic method, and cultural setting.
Depression, one of the most commonly diagnosed psychiatric disorders, is being diagnosed in increasing numbers in various segments of the population worldwide. Depression in the United States alone affects 17.6 million Americans each year or 1 in 6 people. Depressed patients are at increased risk of type 2 diabetes, cardiovascular disease and suicide. Within the next twenty years depression is expected to become the second leading cause of disability worldwide and the leading cause in high-income nations, including the United States. In approximately 75% of suicides, the individuals had seen a physician within the prior year before their death, 45–66% within the prior month. About a third of those who died by suicide had contact with mental health services in the prior year, a fifth within the preceding month.
Childhood schizophrenia is similar in characteristics of schizophrenia that develops at a later age, but has an onset before the age of 13 years, and is more difficult to diagnose. Schizophrenia is characterized by positive symptoms that can include hallucinations, delusions, and disorganized speech; negative symptoms, such as blunted affect and avolition and apathy, and a number of cognitive impairments. Differential diagnosis is problematic since several other neurodevelopmental disorders, including autism spectrum disorder, language disorder, and attention deficit hyperactivity disorder, also have signs and symptoms similar to childhood-onset schizophrenia.
The diagnosis of schizophrenia, a psychotic disorder, is based on criteria in either the American Psychiatric Association's Diagnostic and Statistical Manual of Mental Disorders, or the World Health Organization's International Classification of Diseases (ICD). Clinical assessment of schizophrenia is carried out by a mental health professional based on observed behavior, reported experiences, and reports of others familiar with the person. Diagnosis is usually made by a psychiatrist. Associated symptoms occur along a continuum in the population and must reach a certain severity and level of impairment before a diagnosis is made. Schizophrenia has a prevalence rate of 0.3-0.7% in the United States
Generation R is a prospective, population based cohort study from fetal life until young adulthood in a multi-ethnic urban population in Rotterdam, the Netherlands. The study is designed to identify early environmental and genetic causes of normal and abnormal growth, development and health. Eventually, results forthcoming from the Generation R Study have to contribute to the development of strategies for optimizing health and healthcare for pregnant women and children.
Early childhood trauma refers to various types of adversity and traumatic events experienced during the early years of a person's life. This is deemed the most critical developmental period in human life by psychologists. A critical period refers to a sensitive time during the early years of childhood in which children may be more vulnerable to be affected by environmental stimulation. These traumatic events can include serious sickness, natural disasters, family violence, sudden separation from a family member, being the victim of abuse, or suffering the loss of a loved one. Traumatic experiences in early childhood can result in severe consequences throughout adulthood, for instance developing post-traumatic stress disorder, depression, or anxiety. Negative childhood experiences can have a tremendous impact on future violence victimization and perpetration, and lifelong health and opportunity. However, not all children who are exposed to negative stimuli in early childhood will be affected severely in later life; some children come out unscathed after being faced with traumatic events, which is known as resilience. Many factors can account for the invulnerability displayed by certain children in response to adverse social conditions: gender, vulnerability, social support systems, and innate character traits. Much of the research in this area has referred to the Adverse Childhood Experiences Study (ACE) study. The ACE study found several protective factors against developing mental health disorders, including mother-child relations, parental health, and community support. However, having adverse childhood experiences creates long-lasting impacts on psychosocial functioning, such as a heightened awareness of environmental threats, feelings of loneliness, and cognitive deficits. Individuals with ACEs are more prone to developing severe symptoms than individuals in the same diagnostic category.