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. This biopsychology textbook focuses on the biological underpinnings of psychological disorders, emphasizing brain function and genetics. While treatment approaches are introduced, more comprehensive coverage is found in clinical and abnormal psychology courses.

Psychological disorders emerge from complex interactions among biological, social, and environmental factors. The precise nature of these interactions in shaping disorders for individuals remains a challenging question. Interacting factors and varied symptom profiles (within and across individuals) complicate diagnosis. To promote consensus in diagnoses, researchers and clinicians have developed classification systems that traditionally focus on observable symptoms. Although biological factors influence psychological functioning (Wyatt & Midkiff, 2006), biological evidence was rarely used to inform diagnosis. Historically, technological limitations (e.g., in neuroimaging and genetic testing) hampered scientists from adequately examining the 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 contribute 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:

• 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. For example, if an individual is uncontrollably preoccupied with germs and bathes for hours each day, 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 reflect a psychological disorder if they cause the person considerable distress, or greatly impair their ability to function as a normal individual. As an illustration, a person’s fear of social situations might be so distressing that they 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 culturally expected, they wouldn’t signify a mental disorder.^[2]

Understanding the Classification Systems of Psychological Disorders

Studying psychological disorders begins with systematically discerning significant signs and symptoms. Identifying and labeling a set of symptoms is crucial for a proper diagnosis. This process enables professionals to use a common language and communicate 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.

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 that are interesting to students: schizophrenia, mood disorders including depression and bipolar disorder, anxiety, obsessive-compulsive disorder, and post-traumatic stress disorder.^[3]