Personality can be defined as a stable way in which an individual perceives, reacts, and interacts with his or her environment. The Big Five theory of personality is a dimensional and reliable approach to classifying personality across five factors: neuroticism, extroversion, openness to experience, agreeableness, and conscientiousness. Individuals high in neuroticism tend to be very anxious, while those considered low in neuroticism tend to be calm. "High extroversion" characterizes individuals who are very sociable, while low extroversion identifies people who are very reserved. "Openness to experience" ranges from curious to conventional. "High agreeableness" is associated with helpfulness, low agreeableness with cynicism. Finally, "high conscientiousness" describes individuals who are very organized, while individuals low on the conscientiousness scale can be described as aimless.

One major criticism of the Big Five model is that it characterizes personality essentially in stereotypes; it ascribes personality constructs to individuals in order to predict future actions. However, personality, like stereotypic views, is not necessarily a good predictor of future behavior. The big five model is based on attributional biases: If a person commits an act, such as dishonesty, we immediately attribute it to the person's disposition and their personality, while that individual may attribute his or her behavior to situational circumstances. On the other hand, people do tend to possess a personality structure that they use to predict future behavior; such as I may have lied, but I am an honest person.

Personality disorders are inflexible and maladaptive patterns of interacting with the world. They are characterized by three essential elements: they are pervasive, persistent and pathological. "Pervasive" refers to the fact that the disorders are true across situations. "Persistent" refers to their stability over time, meaning that they usually begin by early adulthood and exist throughout the individual's lifetime. Finally, these disorders are characterized as "pathological" in that they result in either social or occupational problems and personal distress to the individual. They are on axis II of the DSM-IV criteria, meaning that they are defined as attributes that are part of the person and to a certain degree make up their identity. The problem with this definition, however, is that it serves as a type of self-fulfilling prophecy and seems to blame the individual. In addition, these disorders are usually ego-syntonic, which makes treatment intervention very difficult.

There are three clusters of personality disorders. Cluster A disorders are characterized by odd and eccentric behavior and consist of paranoid, schizoid, and schizotypal subtypes. Individuals in this category tend to have poor peer relationships, attract a lot of teasing as children, and tend not to do well in school. All three of these disorders tend to first become apparent in adolescence and childhood and are diagnosed more often in men. Cluster B disorders are experienced usually by impulsive and emotional individuals who behave erratically and have difficulty maintaining interpersonal relationships. This cluster consists of antisocial, borderline, histrionic, and narcissistic personality disorders. Cluster C disorders are defined by fear and avoidant elements and includes, avoidant, dependent, and obsessive-compulsive personality disorders. There are no gender differences in the overall prevalence of personality disorders, though antisocial personality disorder is consistently more common amongst men than women. Yet, some critics contend that the definitions for certain categories, such as dependent personality disorder, may exhibit a certain gender bias based on stereotypes and are therefore inherently sexist.