Mental illnesses have been recognized for thousands of years. Today, advances in science are helping researchers better understand the origins of mental illness and find more effective treatments. But much remains to be learned. Here, Daniel Hall-Flavin, M.D., a psychiatrist at Mayo Clinic, Rochester, Minn., offers a modern interpretation of mental illness, including a look at how mental disorders are defined, the use of medications, the role of psychotherapy and how stress affects well-being.

Mental illness is a term that refers to all the different types of mental disorders, including disorders of thought, mood or behavior. To be classified as a mental illness, the condition must cause distress and result in a reduced ability to function psychologically, socially, occupationally or interpersonally.

This means that someone who has a mental illness may have trouble coping with emotions, stress and anger, for instance, and trouble handling such things as daily activities, family responsibilities, relationships, or work and school responsibilities. You can have trouble, to a greater or lesser degree, with one area or all of them. And you can have more than one type of mental illness at the same time.

Today, we classify mental illness based on the symptoms a person experiences and the clinical features of the illness, such as feeling hopeless or having delusions. But as we continue to gain a clearer understanding of how specific genes interact with illnesses or behaviors, we may be able to develop a much more sophisticated classification system that is directly linked to a biological cause of mental illness, rather than just symptoms. This can help us better tailor treatment to individuals. For instance, some disorders have similar symptoms and clinical features but are actually very different in terms of their underlying biology. To treat them similarly simply because they share the same symptoms may not be appropriate. We suspect, for example, that there are different types of schizophrenia, with different causes, and that they possibly respond differently to different treatments.

Our evolving understanding of genetics and how the brain works may eventually change how we classify mental illnesses. For now, we think of several main classes of mental illness:

• Mood disorders. These include disorders that affect how you feel, such as persistent sadness or feelings of euphoria. They include major depression and bipolar disorder.
• Anxiety disorders. Anxiety is an emotion characterized by the anticipation of future danger or misfortune accompanied by a feeling of being ill at ease. Examples include panic disorder, obsessive-compulsive disorder, specific phobias and generalized anxiety disorder.
• Substance-related disorders. These include problems associated with the misuse of alcohol, nicotine, caffeine and illicit drugs.
• Psychotic disorders. These disorders impair your sense of reality. The most notable example of this is schizophrenia, although other classes of disorders can be associated with psychosis at times.
• Cognitive disorders. These disorders affect your ability to think and reason. They include delirium, dementia and memory problems. Perhaps the most well-known of these disorders is Alzheimer's disease.
• Developmental disorders. This category covers a wide range of problems that usually first begin to make themselves known in infancy, childhood or adolescence. They include autism, attention-deficit/hyperactivity disorder and learning disabilities. But just because they're all grouped in this category doesn't necessarily mean they share a common cause or that there's a relationship among the disorders.
• Personality disorders. A personality disorder is an enduring pattern of inner experience and behavior that is dysfunctional and leads to distress or impairment. Examples include borderline personality disorder and antisocial personality disorder.
• Other disorders. These include disorders of impulse control, sleep, sexual functioning and eating. Also included are dissociative disorders, in which a person's sense of self is disrupted, and somatoform disorders, in which there are physical symptoms in the absence of a clear physical cause, such as hypochondriasis.

No, not all of them. We know that many serious mental illnesses do have a strong biological basis. But that's not the entire story.

Some people, for example, might have an inherited, biological tendency to develop depression. They can experience serious depression even though no specific event triggers it. Others, however, have no known inherited tendency for depression. But if something happens, such as the death of a loved one, it can trigger major depression.

We don't yet know if the underlying neurochemical aspects of these two depressive reactions are the same. In other words, one person may have a mental illness because of their nature — their genetic vulnerabilities and, their neurochemical functioning. And another person may have a mental illness because of nurture — an environmental cause that perhaps changes their neurochemistry. Most of the time, however, it's probably a complex interaction of both nature and nurture.

Yes, there is still a major role for psychotherapy in treating mental illness. Among the many forms of psychotherapy are brief therapy, cognitive-behavioral therapy, psychodynamic therapy, and family or couples therapy. Psychotherapy remains a very important part of treatment for many individuals, couples and families and often is the treatment of choice.

Medication might be all that some people need to restore their brain chemistry to a more normal state. But it's psychotherapy and education that can help change coping behaviors and offer strategies to help understand and modify risk factors for illness. In some cases, medication is entirely ineffective and psychotherapy alone is helpful. In other cases, someone may not be able to take psychiatric medications, for health or other reasons. But very often, a combination of medication and psychotherapy is most effective in treating mental illness.

More people seek help today than in the past. But is that because more people have a mental illness? Or is it because now it's more acceptable to seek help, and help is more readily available? Also, the world is more complicated and fast paced than it once was. Does this change in environment contribute to increased mental illness? We don't know the answers to those questions.

Behaviors that are approved of or frowned upon can vary greatly from culture to culture. But research shows a surprising consistency in the prevalence of serious mental illness across all cultures. Schizophrenia, for instance, occurs in about 1 percent of all populations worldwide. Major depression also has a consistent prevalence in countries around the world. This information supports the idea that serious mental illnesses have a biological basis common to all humans.

Going through a stressful event doesn't necessarily mean that you'll develop a mental illness, such as depression or an anxiety disorder. But it can increase your risk of developing a mental illness. Stress runs the gamut, from daily hassles, such as traffic jams and financial worries, to major life events, such as the breakup of a relationship or the death of a loved one. People who have experienced depression in the past are more vulnerable to depression after a major loss. People with an active, problem-solving style are less likely to experience mental illness such as depression than are those with a passive, emotion-focused style.

People may be hesitant to seek treatment for a number of reasons. They may think that their depression, for instance, will go away on its own. Or they may consider it a sign of weakness. In addition, some people fear the stigma that's sometimes attached to mental illness. But it's important to understand that biological factors play a major role in the development of mental illness, just as they do with other medical disorders. Effective treatment is available that can eliminate or minimize symptoms, improve a person's ability to function and ultimately improve the quality of their life.