Abnormal psychology is the study of abnormal thoughts, behaviors, or internal experiences in order to describe, predict, explain, and treat these patterns of functioning. This branch of psychology studies the nature of psychopathology and its causes, and the resulting knowledge is applied in clinical psychology to the treatment of clients with psychological disorders. 

The names and classifications of these disorders are listed in the Diagnostic and Statistical Manual of Mental Disorders (DSM). The DSM is currently in its 5th edition (DSM-V) and has been designed for use in a wide variety of contexts and across clinical settings (including inpatient, outpatient, partial hospital, clinic, private practice, and primary care). 

Defining Disorder and Dysfunction

A psychological disorder is a condition characterized by abnormal thoughts, feelings, and behaviors. Psychopathology is the study of psychological disorders, including their symptoms, etiology (i.e., their causes), and treatment. The term "psychopathology" can also refer to the manifestation of a psychological disorder. Although consensus can be difficult, it is important for mental-health professionals to agree on what kinds of thoughts, feelings, and behaviors constitute the presence of a psychological disorder. 

Certain patterns of behavior and inner experience can easily be labeled as abnormal and signify some kind of psychological disturbance. A person who washes their hands 40 times per day and a person who claims to hear the voices of demons exhibit behaviors and inner experiences that most would regard as abnormal. However, consider the nervousness someone feels when talking to a person they are attracted to or the loneliness and longing for home a freshman might experience during their first semester of college—these feelings may not be regularly present, but they fall in a range most would consider normal. So, what kinds of thoughts, feelings, and behaviors represent a true psychological disorder? Psychologists work to distinguish psychological disorders from inner experiences and behaviors that are merely situational, idiosyncratic, or unconventional.

Atypical Behaviors, Thoughts, and Inner Experiences

Behaviors, thoughts, and inner experiences that are atypical, distressful, dysfunctional, and sometimes even dangerous may be signs of a 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, and contemplated suicide—your feelings would be atypical, would deviate from the norm, and could signify the presence of a psychological disorder. However, simply because something is atypical does not necessarily mean it is disordered.

Cultural Norms and Expectations

Similarly, violating cultural expectations is not, in and of itself, a satisfactory means of identifying the presence of a psychological disorder. Since behavior varies from one culture to another, what may be considered appropriate in one culture may not be viewed as such in other cultures. For example, making eye contact with others can signify honesty and attention in one culture while being a sign of aggression in another (Pazain, 2010). Hallucinations (seeing or hearing things that are not physically present) in Western societies are readily labeled as a sign of psychological disorder. In other cultures, however, such visions may be regarded as normal experiences that are respected and valued (Bourguignon, 1970). It is also important to recognize that cultural norms change over time: what might be considered typical in a society at one time may no longer be viewed that way later—similar to how fashion trends from one era may elicit quizzical looks decades later. 

What Causes Harm?

Inner experiences and behaviors that are atypical or violate social norms could signify the presence of a disorder. One of the more influential conceptualizations of psychological disorder was proposed by Wakefield (1992), who defined disorder as a "harmful dysfunction." Under this model, dysfunction occurs when psychological processes (such as cognition and perception) cannot do what they are meant to. Importantly, this dysfunction must be harmful in that it leads to negative consequences for the individual or for others, as judged by the standards of the individual’s culture. The harm may include significant internal anguish (e.g., high levels of anxiety or depression) or problems in day-to-day living (e.g., in one’s social or work life). 

According to the American Psychiatric Association (APA, 2013), a psychological disorder is a condition characterized by the following criteria: 

• There are significant disturbances in thoughts, feelings, and behaviors. 
• The disturbances reflect some kind of biological, psychological, or developmental dysfunction.
• The disturbances lead to significant distress or disability in one’s life.
• The disturbances do not reflect expected or culturally approved responses to certain events.

Diagnosing Disorder 

The diagnosis and classification of psychological disorders is essential in studying and treating psychopathology. The classification system used by most US professionals is the DSM-V. The diagnostic manual includes a total of 237 specific diagnosable disorders, each described in detail, including its symptoms, prevalence, risk factors, and comorbidity. Over time, the number of diagnosable conditions listed in the DSM has grown steadily, prompting criticism from some. Nevertheless, the diagnostic criteria in the DSM are more explicit than those of any other system, which makes the DSM system highly desirable for both clinical diagnosis and research.

DSM disorders and their prevalence 

The DSM is used as a tool by psychologists to diagnose and treat a wide range of psychological disorders. This graph shows the breakdown of psychological disorders from the DSM-IV in 2007, comparing the percentage prevalence among adult males and adult females in the United States.  Major depressive disorder has the highest total prevalence among males and females.