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

A group of psychiatric conditions, also known as mood disorders, that are characterized by disturbances of affect, emotion, thinking, and behavior. Affective or mood disorders are strong mental disturbances that interfere with daily life (Fig. 1). Depression is the most common of these disorders. About 10–20% of those affected by depression also experience manic episodes; hence, this condition is known as manic depression or bipolar affective disorder. The affective disorders are not distinct diseases; instead, they are psychiatric syndromes that probably have multiple or complex etiologies. See also: Bipolar disorder; Depression; Emotion; Mental disorders; Stress (psychology); Stress and depression

The repeated and/or excessive use of alcohol (ethanol) that continues despite negative physical, psychiatric, or social consequences. Alcoholism is characterized by consuming alcohol more often, in larger amounts, or over longer periods than intended (Fig. 1); persistent desire or craving of alcohol; and physical consequences of chronic alcohol consumption, especially alcohol tolerance and alcohol withdrawal symptoms during abstinence. Although the initial motivation to consume alcohol results mainly from the expected euphoric effects of alcohol, cue-induced implicit responses, habit, and relief from withdrawal drive continued consumption in the long term. The World Health Organization (WHO) ranks alcohol as one of the primary causes of the global burden of disease in industrialized countries. It is suggested that alcoholism results from an interaction of chronic alcohol exposure, individual genetic makeup, and environmental perturbations over time. This complex gene–environment interaction results in a larger heterogeneity among subjects suffering from alcoholism. See also: Addiction and addictive disorders; Genetics

A disease of the nervous system characterized by a progressive dementia that leads to profound impairment in cognition and behavior. Dementia occurs in a number of brain diseases where the impairment in cognitive abilities represents a decline from prior levels of function and interferes with the ability to perform routine daily activities (for example, balancing a checkbook or remembering appointments). Alzheimer's disease is the most common form of dementia and affects the structure of the brain (Fig. 1 and Fig. 2), particularly areas of the brain that are used for memory and other cognitive functions. In general, approximately 5% of individuals over age 65 are affected by Alzheimer's disease. The onset of the dementia typically occurs in middle to late life, and the prevalence of the illness increases with advancing age to include 25–35% of individuals over age 85. See also: Brain; Cognition; Dementia; Memory

A psychiatric disorder in which a dramatic reduction in caloric intake consequent to excessive dieting leads to significant bodily, physiological, biochemical, emotional, psychological, and behavioral disturbances. Anorexia nervosa is one of three formally recognized major eating disorders (the other two are bulimia nervosa and binge eating disorder). Other recognized eating disorders most prevalent in children include pica, rumination disorder, and avoidant/restrictive food intake disorder. Anorexia nervosa is a serious illness that can drastically affect the health of those individuals suffering from the disorder (Fig. 1). The phrase "anorexia nervosa" was originally coined by the eminent English physician William Gull in 1874 in his description of a series of four case studies of young women with deliberate weight loss. Around the same time, the French physician Charles Lasègue postulated that the deliberate starvation was driven by a neurosis originating from a desire to avoid pain. In the latter half of the twentieth century, environmental and cultural explanations became central to etiological explanations of anorexia nervosa, in line with the Western cultural ideal of thinness. More recently, much research has centered on biological and genetic factors underpinning the illness. Despite these developments, there are still many unknowns concerning anorexia nervosa, and a definitive etiology and treatment strategies are yet to be established. See also: Eating disorder; Mental disorder; Psychology

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 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

A neurodevelopmental condition that impairs the way that a person relates to and communicates with other people. Persons with autism have difficulties with social communication and interaction. They also can have unusual behaviors, such as insistence on sameness, obsessions, or stereotypic behaviors (for example, hand flapping, spinning, and toe walking). The condition varies greatly in the presenting symptoms, the timing of presentation, the range and severity of symptoms, and its association with other conditions. Recognition of autism is increasingly more common (Fig. 1), but the cause of autism is still under investigation, although genetics and parental age seem to be contributing factors. With intensive early intervention, significant improvements in a large percentage of children with autism can be achieved. See also: Autism and eye gaze abnormalities; Autism and the social brain; Developmental psychology; Factors related to risk of autism; Genetics; Nervous system (vertebrate); Psychotherapy; Sociobiology

A major mental disorder in which there are lifelong episodes of both mania and depression; also known as manic-depressive illness. Bipolar disorder, also termed manic-depressive illness or manic depression, is characterized by sudden and often unexplained mood swings, ranging from delirious mania to severe depression (see illustration). These mood changes are regularly accompanied by other mental and behavioral symptoms, including fluctuations of volition, activity level, and cognitive functioning. The first recognizable descriptions of mania and depression date back to the writings of Aretaeus of Cappadocia (a Greek physician who lived around 150–200 CE). The modern history of bipolar disorder begins in the mid-nineteenth century with the concept of folie circulaire (“circular insanity”), proposed by the French psychiatrist Jean-Pierre Falret. Later, around the beginning of the twentieth century, it was defined by the work of the German psychiatrist Emil Kraepelin. See also: Affective disorders; Depression; Mental disorders

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