单选题
When the heart breaks down, it beats irregularly or not at all. A bone can chip or snap. But when the complex network of neurons in our brain malfunctions, the result can be a near-endless variety and combinations of mental illnesses.
It"s normal to sometimes be sad, happy, anxious, confused, forgetful or fearful, but when a person"s emotions, thoughts or behavior frequently trouble them, or disrupt their lives, they may be suffering from mental illness. According to the World Health Organization (WHO), about 450 million people worldwide are affected by mental, neurological or behavioral problems at any time.
Among the best known and most common mental illnesses is depression—a prolonged, debilitating sadness, sometimes accompanied by a feeling of hopelessness and thoughts of suicide. Seasonal affective disorder is a type of depression that affects some people in the autumn and winter and is triggered by the shrinking hours of daylight and colder temperatures. Personality disorders are behaviour patterns that are destructive to the person themselves or those around them. In dissociative disorders, someone experiences a sudden change in consciousness or their concept of self. In dissociative amnesia (遗忘症), for example, the result is a loss of part or all of their memories. Anxiety disorders are characterized by powerful feelings of stress and physical signs of fear—sweating, a racing heart—due to some cue in the environment, or for no obvious reason at all.
Madness has long been linked with genius. Many famous artists, writers and scientists have suffered from mental disorders, leading some to wonder if there is a link between these illnesses and creativity. The mathematician John Nash struggled with schizophrenia while he developed the theory that earned him a Nobel Prize. The artist Vincent Van Gogh, the composer Robert Schumann and the writer Fyodor Dostoevsky are said to have suffered from a range of mental disorders including hypergraphia, a compulsion to write—a sign perhaps their art emerged from an unrelenting urge to communicate.
One possibility is that genes that predispose people to such devastating illnesses persist because when the syndromes are present in a milder form, this heightened creativity gives people an evolutionary advantage.
单选题
According to the passage, mental diseases may be caused when ______.