The 2000 version of the American Psychiatric Association’s diagnostic manual, the Diagnostic and Statistical Manual of Mental Disorders, 4th edition (termed DSM-IV for short), contains nearly 400 mental disorders, distributed across seventeen broad categories. These categories include childhood disorders, schizophrenia and psychotic disorders, mood disorders, substance-related disorders, anxiety disorders, eating disorders, sleep disorders, personality disorders, sexual- and gender-identity disorders, and impulse-control disorders. The number of mental disorders has ballooned from the first DSMedition in 1952 to the present, reflecting an increased "splitting" of broad mental illness categories into narrower ones.


DSM-IV describes individuals along five axes, or dimensions of functioning, the first two of which focus on what is conventionally regarded as mental illness. By providing users with a multiaxial formulation, DSM-IV aims to paint a reasonably comprehensive picture of each individual.

Axis I lists most major mental disorders, such as schizophrenia, mood disorders, and anxiety disorders. In general, researchers view these disorders as acute problems that are superimposed on individuals’ preexisting functioning.

Axis II lists personality disorders and mental retardation, which ostensibly differ from Axis I disorders in their greater persistence over time. Personality disorders are extremes of personality traits, such as impulsivity, dependency, and anxiety, that are inflexible, maladaptive, or both. Mental retardation is characterized by an IQ (intelligence quotient) of approximately 70 or below, severe deficits in adaptive functioning (e.g., inability to cook or dress oneself), and onset prior to adulthood.

Axis III lists medical conditions that can be relevant to individuals’ psychological functioning. Medical conditions can adversely affect the prognosis of many mental disorders and mimic the symptoms of many others. For example, hypothyroidism and stroke can produce the full clinical picture of major depression, a condition characterized by extreme sadness and loss of pleasure, along with such features as extreme guilt, sleeping and eating difficulties, and suicidal thinking.

Axis IV lists recent life stressors, such as death of a relationship partner or friend, and environmental problems, such as housing difficulties, extreme poverty, or inadequate access to mental health services. These psychological factors can precipitate or maintain mental illnesses. For example, research indicates that certain negative life events, especially those involving losses of loved ones, can trigger major depression in predisposed individuals. Evidence also suggests that schizophrenia, a condition marked by unusual thinking, delusions (fixed false beliefs), and hallucinations (perceptions occurring in the absence of any stimulus), can be triggered in some predis posed individuals by the stresses associated with poverty. Nevertheless, evidence suggests that causality also operates in the reverse direction because, for example, the deteriorating job skills associated with schizophrenia can lead to poverty.

Axis V describes the individual’s overall level of daily functioning. The inclusion of this axis acknowledges that people with the same psychiatric disorder differ markedly in their levels of adaptation to the environment. For example, some people with major depression are almost constantly bedridden, suicidal, or both, whereas others manage to perform surprisingly well in their occupational and family lives despite intense psychological pain.


Theoreticians have long struggled with the question of what, if anything, all disorders of psychological functioning have in common. Although they have proposed numerous definitions of psychiatric disorder, all of these definitions have their shortcomings.

A subjective distress model posits that all mental disorders are marked by inner turmoil. Although many mental disorders, such as panic disorder (a condition marked by sudden surges of intense anxiety), are associated with subjective distress, some others, such as antisocial personality disorder (a condition marked by a long-standing history of illegal and irresponsible behavior) are associated with minimal subjective distress.

A statistical model posits that mental disorders are marked by statistical rarity. Although many mental disorders, such as infantile autism (a condition originating in infancy that is associated with serious deficits in language, social bonding, and imagination), are relatively rare in the population, others, such as major depression, are fairly common.

An evolutionary model posits that all mental disorders generate biological disadvantage, such as reduced life span or reduced ability to reproduce. Although some mental disorders, such as major depression, are associated with increased rates of suicide and therefore clear-cut biological disadvantage, others, such as specific phobia (an intense and irrational fear of an object, place, or situation), are not associated with any apparent reduction in lifespan or reproductive capacity.

In the 1990s Jerome Wakefield attempted to remedy these shortcomings by proposing that all mental disorders are harmful dysfunctions. According to Wakefield, all mental disorders (1) produce undesirable consequences for the affected individual, society, or both (harm); and (2) are characterized by the failure of a psychological system to perform its evolved function (dysfunction). For example, according to Wakefield, paranoia ("delusional disorder" in DSM-IV) is a mental disorder because (1) people with paranoia frequently experience marked distress, and (2) paranoia reflects a failure of the human threat system to operate properly. Specifically, in paranoia the threat detection system either reacts to nonexistent dangers or overreacts to mild ones. Yet the harmful dysfunction formulation has its limitations. In particular, some conditions, such as anxiety disorders, do not appear to stem from dysfunctions. Instead, in most cases these conditions seem to result from evolved systems reacting in adaptive ways to subjectively perceived threats.

In light of the problems with previous attempts to define disorder, some authors have proposed that psychiatric disorders are best conceptualized in terms of a family resemblance model. Just as brothers and sisters within a large family tend to look similar but do not share any single facial characteristic, mental disorders typically share a loosely covarying set of features. These features include subjective distress, statistical rarity, biological dysfunction, impairment, societal disapproval, irrationality, and loss of control over one’s behavior. From this perspective, there is no "bright line" demarcating abnormality from normality, but rather a constellation of partly overlapping attributes that set most psychiatric disorders apart from healthy functioning.


The mental illness concept has long had its harsh critics. Advocates of labeling theory, such as Thomas Szasz (b. 1920), have argued that diagnoses of mental illness are merely pejorative names that society attaches to behavior that it finds objectionable. In a 1961 book Szasz referred famously to mental illness as a myth, contending that what the mental health profession calls psychological disorders are actually "problems in living," that is, difficulties in adjusting one’s behavior to societal demands. Many labeling theorists further contend that psychiatric diagnoses are culturally relative, because the behaviors that societies deem abnormal vary markedly across place and time.

Labeling theory has served a valuable function by reminding psychologists and psychiatrists that diagnoses are readily abused. Many nonscientific "diagnoses" in the popular psychology literature, such as sexual addiction, codependency, and road-rage disorder, are scant more than descriptive labels for undesirable behaviors. These labels yield little new information. For example, labeling an aggressive driver with road-rage disorder informs us only that this person frequently loses his or her temper while driving, a fact of which we were already aware.

Nevertheless, labeling theory’s critique of psychiatric diagnostic systems and the concept of mental illness falls short on at least three grounds. First, many devalued behaviors, such as laziness, slovenliness, rudeness, and racism, are not mental disorders. Therefore, there is more to psychiatric disorder than social undesirability.

Second, many labeling theorists have overstated the cultural relativity of mental disorders. Admittedly, some mental disorders are specific to certain cultures. For example, koro, a condition characterized by a pathological fear that one’s genitals are disappearing, is localized largely to parts of Southeast Asia. Nevertheless, many major psychological disorders appear to be present across most, if not all, cultures. In 1976 Jane Murphy conducted a significant study of two societies that had experienced essentially no contact with Euro-American culture—a group of Yorubas in Nigeria and a group of Eskimos near the Bering Strait. These cultures had terms for disorders that were strikingly similar to several Euro-American disorders, including alcoholism, schizophrenia, and psychopathic personality, a condition marked by dishonesty, callousness, guiltlessness, and lack of empathy.

Third, many psychological diagnoses do more than describe already known behaviors. As Eli Robins and Samuel Guze observed in a classic 1970 article, valid psychological diagnoses provide us with novel information. For example, if we accurately diagnose an individual with bipolar disorder, formerly known as manic depression, we will learn several things about that individual that we did not know previously. Among other things, we will learn that this individual (1) experiences relatively sudden episodes of both mania (a condition marked by dramatically elated mood, energy, and self-esteem, along with poor judgment and impulsivity) and depression, typically punctuated by periods of essentially normal functioning; (2) is more likely than nonaffected individuals to have one or more biological relatives with mood disorders; (3) is at heightened risk for other psychological difficulties, including substance abuse and suicide; and (4) will probably respond positively to certain medications, such as lithium carbonate and antiseizure agents.


Several large survey studies conducted in the 1980s, 1990s, and early twenty-first century have yielded valuable knowledge regarding the prevalence of major mental disorders in the general population. A 2005 study by Ronald Kessler and his colleagues revealed that between 25 percent and 30 percent of Americans suffer from anxiety disorders such as phobias, about 20 percent suffer from mood disorders such as depression and bipolar disorder, and about 15 percent suffer from substance-abuse disorders such as alcoholism. A surprisingly large proportion of Americans, perhaps 25 percent, also suffer from impulse-control disorders such as kleptomania (marked by recurrent stealing) and pathological gambling. On a lifetime basis, the most prevalent single disorder appears to be major depression, which afflicts about 17 percent of Americans at some point in their lives.

Survey studies also reveal important gender differences in the prevalence of some mental disorders. For example, in most populations major depression is about twice as common in women than in men. Antisocial personality disorder, in contrast, is about three times as common in men as in women. The reasons for these gender differences are unknown, but remain an active area of research investigation.

Race differences in the prevalence of psychopathology tend to be less pronounced than gender differences, although there are notable exceptions. For example, post-traumatic stress disorder, a condition marked by severe anxiety and avoidance reactions following a traumatic event (e.g., a rape, shooting, or motor vehicle accident), appears to be more prevalent in African Americans than in whites, perhaps because individuals in poor, inner-city areas, including many African Americans, often witness and experience traumatic events. Alcoholism is more prevalent in Native Americans than in other individuals, probably for a mixture of genetic and environmental reasons, the latter including poverty and alienation from the broader American society.


The past several centuries have witnessed a variety of approaches to the etiology, or causation, of psychiatric illnesses. Etiological models of psychological disorder have shifted over time in accord with prevailing societal conceptions.

During the Middle Ages many people embraced a demonic model, which viewed mental illnesses largely as the product of evil spirits infecting the mind. Not surprisingly, exorcism was often the preferred "treatment" of the day. In succeeding centuries, conceptions of the causes of mental disorder became progressively more rooted in naturalistic as opposed to spiritual explanations.

Psychodynamic approaches, originated by Sigmund Freud (1856-1939) and his followers, place substantial emphasis on the role of early life experiences, unconscious influences, and psychological conflict in the genesis of mental disorder. For example, psychodynamic theorists might view obsessive-compulsive disorder, an anxiety disorder characterized by repeated intrusive thoughts (e.g., fears of contamination) and by unsuccessful efforts to neutralize them (e.g., frantic cleaning), as an unconscious psychological defense against deep-seated fears of loss of emotional control. These fears, in turn, may trace their roots to aversive childhood experiences, such as physical or sexual trauma. Despite their value in generating hypotheses concerning the causes of mental illness, psy-chodynamic theories have proven difficult to test.

Behavioral approaches, influenced by the work of John B. Watson (1878-1958), B. F. Skinner (19041990), Joseph Wolpe (1915-1997), and others, conceive of mental disorder as maladaptive learned habits. For behaviorists, atypical and disturbed behaviors are governed by the same learning processes as other behaviors. For example, behaviorists might attempt to explain a phobia of dogs in terms of an early unpleasant experience with a dog in conjunction with subsequent avoidance behavior. By avoiding dogs whenever they are within sight, the victim of a dog bite experiences short-term relief. Yet this person also forfeits the opportunity to learn that most dogs are not as dangerous as he or she fears.

Cognitive approaches, pioneered by Albert Ellis (b. 1913), Aaron Beck (b. 1921), and others, posit that psychological disorders derive from irrational patterns of thinking. Cognitive theorists emphasize that an individual’s interpretations of events, rather than events themselves, are the principal determinants of behavior. They regard unfounded beliefs about oneself, the world, and the future—such as the belief that "I must be perfect" or that "I must be liked by everyone to be a worthwhile person"—to be risk factors for depression and other disorders.

Biological approaches focus on physiological factors, such as genetic influences, early damage to the central nervous system, and hormonal abnormalities, as predisposing factors in mental illness. There is compelling evidence from twin and adoption studies—which permit researchers to disentangle the roles of genes and environ-ment—that genetic factors play a substantial role in a wide array of psychiatric disorders, including schizophrenia, mood disorders, and anxiety disorders. There is also preliminary but growing evidence that viral infections prior to birth may set the stage for subsequent schizophrenia.

The advent of sophisticated neuroimaging techniques such as PET (positive emission tomography) and fMRI (functional magnetic resonance imaging) has been a substantial boon to biological approaches, as these techniques have allowed researchers to discover which brain areas are underactive or overactive during certain tasks. For example, many individuals with schizophrenia exhibit decreased activity in their frontal lobes, which is consistent with the poor judgment, inadequate planning, and memory deficits often observed in this condition. The human genome project also promises to add to our understanding of the biological underpinnings of mental disorder, because it is permitting researchers to identify genes linked to specific psychological disorders.

Proponents of differing etiological models have often sorted themselves into highly partisan camps, separated as much by ideology as evidence. Yet, few if any of these eti-ological approaches are mutually exclusive. Moreover, most researchers and theorists agree that the causes of mental disorders are likely to be multifactorial, that is, produced by many variables rather than one. In some cases, these causal variables may interact. For example, several studies indicate that a genetic abnormality that affects serotonin, a chemical messenger in the brain, may combine with life stressors to trigger depression. As a consequence, the most fruitful approaches to understanding mental disorder will probably involve multidisciplinary collaborations among researchers from diverse theoretical perspectives.

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