Final answer:
Criticism from Hans Eysenck in the 1950s and 1960s prompted research into the effectiveness of psychotherapy. This led to evidence-based practices and upheld the significance of Carl Rogers' client-centered therapy.
Step-by-step explanation:
The criticism in the 1950s and 1960s that led to the initiation of research investigating the effectiveness of psychotherapy was made by Hans Eysenck. Eysenck questioned the efficacy of psychotherapy in treating mental health issues, which spurred further research into therapeutic outcomes and the development of evidence-based practices. Notably, Carl Rogers, known for his client-centered therapy, was influential in shaping how clinicians operate, emphasizing unconditional positive regard, genuineness, and empathy as critical to therapeutic success. Research in psychotherapy effectiveness has been vital for validating various approaches like client-centered, cognitive-behavioral therapy, and others influenced by prominent figures such as Rogers.