Which form of therapy has played the greatest role in contributing to the sharp reduction in the number of residents in U.S.mental hospitals?

A) psychosurgery
B) cognitive therapy
C) electroconvulsive therapy
D) drug therapy

Drug therapy, particularly the development and widespread usage of antipsychotic medications in the 1950s and 1960s, allowed individuals with severe mental illness to manage symptoms and live in the community instead of being institutionalized in mental hospitals. This led to a sharp reduction in the number of residents in U.S. mental hospitals. The other forms of therapy listed (psychosurgery, cognitive therapy, and electroconvulsive therapy) did not have as significant of an impact on reducing the number of residents in mental hospitals.