Personality, deeply ingrained and relatively enduring patterns of thought, feeling, and behavior. Personality usually refers to that which is unique about a person, the characteristics that distinguish him or her from other people. Thought, emotion, and behavior as such do not constitute a personality, which is, rather, the dispositions that underlie these elements. Personality implies predictability about how a person will act or react under different circumstances.
Formation and Development
Heredity and environment interact to form personality. From the earliest age, infants differ widely because of variables that either are inherited or result from conditions of pregnancy and birth. Some infants are more attentive than others, for example, whereas some are more active. These differences can influence how parents respond to the infant—one illustration of how hereditary conditions affect environmental ones. Among the personality characteristics that are known to be at least partly determined by heredity are intelligence and temperament; some forms of psychopathology are also in part hereditary.
In addition to the influences of heredity, what happens to a developing child has a greater or lesser effect depending on when it happens. Many psychologists believe that critical periods exist in personality development. These are periods when an individual is more sensitive to a particular type of environmental event. During one period, for example, language ability changes most rapidly; during another, the capacity for guilt is most likely to be developing.
Most experts believe that a child's experiences in the family are crucial for personality development. How well basic needs are met in infancy, along with later patterns of child rearing, can leave a permanent mark on personality. Children whose toilet training is started too early or carried out too rigidly, for example, may become defiant. Children learn behavior appropriate to their sex by identifying with their same-sex parent; a warm relationship with that parent facilitates such learning. Children are also influenced by their siblings.
Some authorities emphasize the role of social and cultural traditions in personality development. In describing the behavior of members of two New Guinea tribes, for example, the American anthropologist Margaret Mead demonstrated this cultural relationship. Although the tribes are of the same racial stock and live in the same area, one group is peaceful, friendly, and cooperative, whereas the other group is assertive, hostile, and competitive.
Traditionally, psychologists hold that the traits of an individual combine to form a personality, and that this personality shows great consistency over time. Recently, however, many psychologists have argued that traits exist only in the eye of the beholder, and that a person's personality varies with the situation.

Tests
The interview, a widely used method of personality assessment, is a means of eliciting from the subject a report of past, present, and anticipated future responses. Most interviews are unstructured, but some use set questions asked in a given sequence. Skilled interviewers pay attention to what is said and notice how responses relate to nonverbal cues such as posture and facial expressions.
Direct observations are made either in a natural setting or in a laboratory. In naturalistic observations, the assessor notes reactions to everyday situations, typical responses to people, and expressive behavior. In the laboratory, the investigator experimentally manipulates situations and observes the subject's behavior under these controlled conditions. The personality assessor might also rely on the reports of others who have observed the subject in the past.
Personality tests are of two general types—self-report inventories and projective tests. Self-report inventories, such as the Minnesota Multiphasic Personality Inventory, pose questions about personal habits, attitudes, beliefs, and fantasies. In projective testing, the subject's responses to ambiguous or unstructured situations are assumed to reflect inner reality. The Rorschach test, for example, is a projective test consisting of a series of inkblots, about which the subject reports his or her perceptions; the assessor subsequently interprets these responses.

Disorders
Personality disorders are lifelong conditions in which personality traits are so inflexible and maladaptive that they cause social and occupational impairments and considerable distress, to others if not to the people themselves. Many different types of personality disorders are recognized. The paranoid personality, for example, is unduly suspicious and mistrustful. Histrionic personalities are characterized by overly dramatic behavior and expression. People with narcissistic personalities tend to be self-important and need constant attention and admiration. Those with antisocial personality disorders have a history of violating the rights of others and of failing to observe social norms.