Defining exactly what constitutes a psychological disorder can be tricky and, definitions have changed over time.
"...a syndrome characterized by a clinically significant disturbance in an individual's cognitive, emotion regulation, or behavior that reflects a dysfunction in the psychological, biological, or developmental process underlying mental functioning. Mental disorders are usually associated with significant distress in social, occupational, or other important activities."
To identify the extends beyond normal behavior to enter the territory of "psychopathology", mental disorders can be conceptualized as referring to problems in four areas: deviance, distress, dysfunction, and danger.
Deviance: This term refers to thoughts, emotions, or behaviors that deviate from what is common or at odds with what is deemed acceptable in society. In the case of depression, you might report thoughts of guilt or worthlessness that are not common among other people.
However, if you were to define disorder just only as something that lies outside of the statistical norm, then people who are considered exceptionally talented or gifted in a particular area would be regarded as abnormal. So rather than focus on actions that are considered outside of the normal statistically speaking, psychologists tend to concentrate on the results of those behaviors.
Distress: This symptom refers to negative feelings either felt within a person or that result in discomfort in others around that person. In the case of depression, you might report extreme feelings of distress over sadness or guilt, irritation, and aggression.
Dysfunction: With this symptom, professionals are looking for the inability to achieve daily functions like going to work. In the case of depression, you might report that you can't get out of bed in the morning or that daily tasks take you much longer than they should.
Therefore, behaviors that are considered maladaptive and cause significant personal distress, and interrupt daily functioning are more likely to be labeled as disorders.
Danger: This term refers to behavior that might put you or someone else at some type of detrimental risk. In the case of depression, this could include reporting that you are having thoughts of suicide or harming yourself.
In this way, you can see that the distinction between normal versus psychopathological behavior comes down to how issues are affecting you or the people around you.
In conclusion, knowing these signs may help us to identify our mental issues and seek professional support as soon as possible. Please be noted that diagnosis should only be made by professionals, rather than self-made assumptions.
American Psychiatric Association. Diagnostic and Statistical Manual of Mental Disorders (DSM–5).
Davis TO. Conceptualizing psychiatric disorders using "Four D's" of diagnoses. The Internet Journal of Psychiatry. 2009;1(1):1-5.