Habituation to a specific practice, such as drinking alcoholic beverages or using drugs (for example, heroin, cocaine, or amphetamines), and disorders characterized by the chronic use of a drug, alcohol, or similar substance. Addiction and addictive disorders usually result in (1) the development of tolerance for the substance, with the need for increasing amounts to achieve the desired effect; (2) physical dependence, characterized by a sequence of well-defined signs and physiological symptoms, including the withdrawal or abstinence syndrome on cessation of use of the substance; and (3) compulsive drug-seeking behavior, with chronic, regular, or intermittent use, despite possible harm to self or others (Fig. 1). Since the early 1960s, research has been increasing in the biology of addictive diseases, and emphasis has shifted from psychological, sociological, and epidemiological studies to investigations of the metabolic, neurobiological, and molecular bases of addiction. See also: Incentive-sensitization disease model of addiction; Neurobiology; Pharmacology; Psychology; Psychopharmacology; Public health

Behavior that is intended to threaten or inflict physical injury or emotional harm on another person or organism. Aggression is a type of behavior (physical or verbal, or both) displayed by an individual who aims or is motivated to harm another individual (or living creature) who does not wish to be harmed (Fig. 1). A broader definition of aggression may include such categories as verbal attack, discriminatory behavior, and economic exploitation, and can often take the form of bullying (that is, unwanted, aggressive behavior that involves a real or perceived power inequity). The key inclusion of intention in defining aggression makes it difficult to apply the term unequivocally to animals in which there is no clear means of determining the presence or absence of intention. As a result, animal violence is usually equated with aggression. See also: Bullying; Emotion; Motivation; Psychology; Sociobiology

A group of distinct psychiatric disorders including generalized anxiety disorder, panic disorder, obsessive-compulsive disorder, social anxiety disorder, specific phobia, and posttraumatic stress disorder. Whereas fear is an adaptive emotion that is seen in many animal species, anxiety disorders are characterized by anxiety that is excessive insofar as it leads to marked distress and impairment of functioning. Anxiety disorders (Fig. 1) are among the most prevalent, disabling, and costly of psychiatric disorders, partly because they are frequently underrecognized and undertreated. Fortunately, there is growing understanding of the mechanisms underlying these disorders, and a number of effective treatments are available. The most prevalent anxiety disorders are generalized anxiety disorder, panic disorder, obsessive-compulsive disorder, social anxiety disorder, specific phobia, and posttraumatic stress disorder (see table). See also: Emotion; Phobia; Stress (psychology)

A disorder of language acquired as a result of brain disease. Aphasia is impairment in the use of spoken or written language caused by brain disorders (see illustration) that cannot be explained by paralysis or incoordination of the articulatory organs, impairment of hearing or vision, impaired level of consciousness, or impaired motivation to communicate. Thus, aphasia strikes at a key human attribute—communication with other people. However, aphasia must be distinguished from three other conditions: motor speech disorders, developmental language disorders, and psychosis. Motor speech disorders represent abnormalities of speech articulation, caused by weakness or incoordination of the muscles of speech. Aphasia is an acquired loss of previously intact language ability, not a developmental problem, and is often called dysphasia. Psychotic disorders usually involve thought, that is, the content of language, rather than language itself. See also: Brain; Linguistics; Psycholinguistics; Psychosis; Speech; Speech disorders; Speech perception

A common psychiatric disorder (especially in children) characterized by attentional difficulties, impulsivity, and hyperactivity. Attention deficit hyperactivity disorder (ADHD; also punctuated as attention-deficit/hyperactivity disorder) is a mental disorder marked by an individual's inability to pay attention or to focus on tasks, as well as acting impulsively or being extremely active (hyperactive) (see figure). The condition typically takes shape in early childhood, and the signs and symptoms of ADHD often lead to functioning or developmental deficiencies. In Europe, ADHD is known as hyperkinetic disorder. Earlier names for ADHD include attention deficit disorder, minimal brain dysfunction, minimal brain damage, hyperactivity, hyperkinesis, and hyperactive child syndrome. Over time, these names were modified as a result of their implications about etiology and core symptoms: Minimal brain dysfunction seemed to imply that children with this disorder were brain-damaged, whereas hyperactivity and its synonyms named a feature seen in many (but not all) of these children. See also: Developmental psychology; Mental disorders

The study of the hereditary factors that shape behavior. Behavior genetics, that is, the scientific investigation of the hereditary factors that influence behavior, may be studied in animals and humans. Charles Darwin, who originated the theory that natural selection is the basis of biological evolution, was persuaded by Francis Galton that the principles of natural selection applied to behavior as well as to physical characteristics. Specifically, members of a species vary in the expression of certain behaviors because of variations in their genes (Fig. 1), and these behaviors have survival value in some environments. One example of such a behavior is curiosity: some organisms are more curious than others, and curiosity is advantageous for survival in some settings. Therefore, the science of behavior genetics is an extension of the aforementioned ideas and seeks (1) to determine to what extent the variation of a trait in a population (the extent of individual differences) is due to genetic processes, to what extent it is due to environmental variation, and to what extent it is due to joint functions of these factors (heredity–environment interactions and correlations); and (2) to identify the genetic architecture (genotypes) that underlies behavior. See also: Behavioral ecology; Genetics; Human genetics; Population genetics; Psychology

A mental health illness characterized by a compulsive or obsessive focus on a perceived flaw in appearance. Body dysmorphic disorder (often abbreviated BDD) is a psychiatric illness in which a person has a severe, excessive preoccupation with a perceived defect or shortcoming in their physical appearance (see illustration), and that concern is not attributable to any other psychiatric problem (such as anorexia nervosa or another eating disorder). The defect may be imaginary or the product of distorted perception. Individuals afflicted with this obsession with their appearance can suffer clinically significant distress or impairment in social, occupational, and other important areas of functioning. In extreme cases, individuals choose to undergo multiple plastic surgery procedures, with no or temporary relief after each surgery. Patients affected by body dysmorphic disorder often become severely depressed and suicidal, requiring treatment in psychiatric facilities or hospitals. See also: Anorexia nervosa; Depression; Eating disorders; Mental disorders; Psychology; Psychosomatic disorders; Stress (psychology); Suicide

Unwanted, aggressive behavior that involves a real or perceived power inequity. The behavior of bullying is often chronic and includes actions such as threats, spreading rumors, attacking someone physically or verbally, and purposefully excluding or isolating someone from a group. This aggressive behavior can have severe and long-lasting negative effects on the health and well-being of all participants, regardless of their role in the bullying context (see illustration). In other words, for both bullies and their victims, the consequences of bullying can be dire. See also: Aggression; Developmental psychology; Motivation; Personality theory; Psychology; Sociobiology

The internal structures and processes that are involved in the acquisition and use of knowledge, including sensation, perception, attention, learning, memory, language, thinking, and reasoning. The mental processes and behaviors that constitute cognition are important areas of research (Fig. 1). In particular, cognitive scientists investigate these processes and behaviors through analyses involving psychology, philosophy, linguistics, computer science, brain research, and neuroscience. In addition, cognitive scientists propose and test theories about the functional components of cognition based on observations of an organism's external behavior in specific situations. See also: Brain; Consciousness; Developmental psychology; Information processing (psychology); Intelligence; Learning; Linguistics; Memory; Perception; Problem solving (psychology); Sensation

A psychotherapy with roots in both cognitive science and behavioral science. Cognitive-behavioral therapy (CBT) is the most widely used evidence-based psychotherapeutic practice for a variety of mental health disorders, including anxiety, depression and mood disorders. It is considered an action-oriented approach to psychotherapy, focusing on understanding the relationship among emotions (feelings), thoughts and behaviors that occur daily (Fig. 1). The therapist assists the client (patient) in recognizing this relationship and practicing effective CBT strategies to address symptoms, as well as working toward stated goals and behavior change, eventually to decrease and manage symptoms of a particular disorder more effectively. Early theories of CBT assumed that identifying and changing maladaptive thinking would lead to changes in behavior and affect (emotions); however, advances in psychological science suggest that the corrective mechanism is likely changing one's relationship to the maladaptive thoughts rather than attempting to change the actual thoughts. See also: Affective disorders; Anxiety disorders; Cognition; Depression; Emotion; Mental disorders; Psychology; Psychotherapy