Most psychological researchers and clinicians agree that mental health is best understood through a “biopsychosocial” perspective that considers how biological, psychological, and sociocultural factors contribute to psychopathology. In this biopsych book, we focus more on biological factors, like brain function and genetics, that underlie psychological disorders and less on treatment-related aspects that may be covered in courses on clinical or abnormal psychology.

Psychological disorders emerge from a complex interaction of biological, social, and environmental factors. How exactly these factors interact to shape psychological disorders for different individuals remains a challenging question. The complexity of interacting factors and variability in symptom profiles across individuals complicates the diagnosis of psychological disorders. In order to promote consensus in diagnosing psychological disorders, psychological researchers and clinicians have developed classification systems. These classification systems traditionally focus on observable symptoms to diagnose a disorder. While it’s widely recognized that biological factors contribute to psychological functioning (Wyatt & Midkiff, 2006), biological evidence has been rarely used to inform diagnosis. In earlier years, this was partly due to technological limitations (e.g., in neuroimaging and genetic testing) that prevented scientists from adequately examining biological bases of psychological disorders. Now, given technological advances, scientists have begun exploring how biological factors, such as brain structure, brain function, brain chemistry, and genetics, may lead to psychological disorders and may eventually inform diagnosis.

What are Psychological Disorders?

Perhaps the simplest approach to conceptualizing psychopathology is to label behaviors, thoughts, and inner experiences that are atypical, distressful, dysfunctional, and sometimes even dangerous, as signs of a psychological disorder. For example, if you ask a classmate for a date and you are rejected, you probably would feel a little dejected. Such feelings would be normal. If you felt extremely depressed—so much so that you lost interest in activities, had difficulty eating or sleeping, felt utterly worthless, or contemplated suicide—your feelings would be atypical, would deviate from the norm, and could signify the presence of a psychological disorder. However, just because something is atypical, does not necessarily mean it is disordered.^[1]

The American Psychiatric Association (APA) Definition of a Psychological Disorder

A formal definition developed by the American Psychiatric Association (APA, 2022) characterizes a psychological disorder as a condition that consists of the following:

• There are significant disturbances in thoughts, feelings, and behaviors. A person must experience inner states (e.g., thoughts and/or feelings) and exhibit behaviors that are clearly disturbed—that is, unusual, but in a negative, self-defeating way. Often, such disturbances are troubling to those around the individual who experiences them. For example, if an individual is uncontrollably preoccupied by thoughts of germs and spends hours each day bathing, their inner experiences and behaviors would be considered atypical and negative (disturbed) and would likely trouble family members.
• The disturbances reflect some kind of biological, psychological, or developmental dysfunction. Disturbed patterns of inner experiences and behaviors should reflect some flaw (dysfunction) in the internal biological, psychological, and developmental mechanisms that lead to normal, healthy psychological functioning. For example, the hallucinations observed in schizophrenia could be a sign of brain abnormalities.
• The disturbances lead to significant distress or disability in one’s life. A person’s inner experiences and behaviors are considered to reflect a psychological disorder if they cause the person considerable distress, or greatly impair their ability to function as a normal individual (often referred to as functional impairment, or occupational and social impairment). As an illustration, a person’s fear of social situations might be so distressing that it causes the person to avoid all social situations (e.g., preventing that person from being able to attend class or apply for a job).
• The disturbances do not reflect expected or culturally approved responses to certain events. Disturbances in thoughts, feelings, and behaviors must be socially unacceptable responses to certain events that often happen in life. For example, it is perfectly natural (and expected) that a person would experience great sadness and might wish to be left alone following the death of a close family member. Because such reactions are in some ways culturally expected, the individual would not be assumed to signify a mental disorder.^[2]

Understanding the Classification Systems of Psychological Disorders

A first step in the study of psychological disorders is systematically discerning significant signs and symptoms. Arriving at a proper diagnosis—that is, appropriately identifying and labeling a set of defined symptoms—is crucial. This process enables professionals to use a common language and aids in communication about the disorder with the patient, colleagues, and the public. For these reasons, classification systems that systematically organize psychological disorders are necessary. The current main classification system is the Diagnostic and Statistical Manual of Mental Disorders, Fifth Edition, Text Revision (DSM-5-TR) published by the American Psychiatric Association (2022). Another classification system used for mental and behavioral disorders is the International Classification of Diseases (ICD-11) and is primarily used by the World Health Organization to support the global comparison of morbidity statistics. While the ICD-11 is also used in medical settings around the globe, the DSM-5 remains the main instrument for diagnosis. Despite its dominance in the field, using the DSM-5 for clinical diagnosis has some limitations. For example, individuals displaying very different symptoms may be diagnosed with the same psychological disorder, or conversely, individuals displaying very similar symptoms may be diagnosed with different disorders. Given this variation in symptom display and the DSM-5’s focus on mapping symptoms onto psychological disorders, it makes sense that scientists are examining how biological factors may inform understanding and diagnosing psychological disorders.

Characterizing psychological disorders based on only symptoms is common for clinical purposes, but is limited for research purposes. For example, the variability of symptom profiles within and across psychological disorders, as well as comorbidity (the simultaneous presence of multiple disorders), makes it challenging for researchers to investigate the neurobiological mechanisms underlying a specific disorder. Accordingly, the Research Domain Criteria (RDoC) is a research framework used to investigate mental disorders and is favored by granting agencies like the National Institute of Mental Health (NIMH) (Insel et al., 2010). Rather than using diagnostic categories (like “bipolar” or “schizophrenia”), the RDoC considers psychopathology in the context of six major domains of neurobehavioral functioning: arousal/regulatory systems, positive valence systems, negative valence systems, social processes, cognitive systems, and sensorimotor systems. Each domain is studied across the full spectrum of functioning in terms of molecular, genetic, behavioral, physiological, and self-report measures. The RDoC framework also prioritizes studying how biological and behavioral development and environmental and cultural factors may contribute to psychopathology. Ultimately, the RDoC framework advances our understanding about how constellations of different factors, both in isolation and together, may contribute to the development of psychopathology.

In the rest of this chapter, we give an overview of some common psychological disorders and their underlying biological factors. Overall, there are hundreds of psychological disorders (characterized by the DSM-5 in over 1000 pages). Here we cover a few broad classes that are prevalent and interesting to students: schizophrenia, mood disorders including depression and bipolar disorder, anxiety, obsessive-compulsive disorder, and post-traumatic stress disorder.^[3]