 Some researchers have surmised that the disorder may have evolutionary roots, in the same way that others suggest evolutionary contributions to schizophrenia, sickle cell anemia, psychopathy and other disorders.Psychology and psychiatry have not generally embraced evolutionary explanations for behaviors, and the proposed explanations for the evolution of depression remain controversial.Major depression (also called "major depressive disorder", "clinical depression" or often simply "depression") is a leading cause of disability worldwide, and in 2000 was the fourth leading contributor to the global burden of disease (measured in DALYs); it is also an important risk factor for suicide.