Final answer:
The number of patients in mental hospitals in the U.S. since the 1950s has decreased primarily due to advancements in psychiatric medications, increased community mental health services, and improved understanding and treatment of mental illnesses.
Step-by-step explanation:
The decrease in the number of patients in mental hospitals in the U.S. since the 1950s is primarily a result of all of the above factors: advancements in psychiatric medications, increased availability of community mental health services, and improved understanding and treatment of mental illnesses. These changes were influenced by reforms and technological advancements, as well as a shift towards community-based treatment options which emphasized the rights and dignity of those with mental illness. The deinstitutionalization movement, which moved patients from state hospitals into community-based treatment settings, was further influenced by the socio-political climate of the time and economic factors, as seen in the closure of state-run facilities during the Reagan administration, leading to a rise in homelessness among some of the mentally ill. Moreover, the development of new psychiatric medications made it possible to manage symptoms more effectively outside of hospital settings.