Final answer:
Torrey, E. Fuller erroneously concluded in the 1950s that psychotherapy was not effective.
Step-by-step explanation:
The researcher who erroneously concluded that psychotherapy was not effective in the 1950s was Torrey, E. Fuller. This conclusion was based on a misunderstanding of the effectiveness of psychotherapy at that time.
It is important to note that more recent research and evidence have proven the efficacy of psychotherapy in treating mental health conditions.