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In a classic 1952 article, researcher Hans Eysenck challenged the assumed effectiveness of psychotherapy by evaluating the results of existing pretest-posttest designs. Although the original clinical studies showed patient improvement after receiving therapy, Eysenck compared these results to archival data from state hospitals and insurance companies for untreated patients. He found that both the treated and untreated groups recovered at similar rates. This highlighted that the improvements observed in the treated group were likely due to spontaneous remission rather than the psychotherapy itself, underscoring the absolute necessity of including a proper control group to accurately measure treatment efficacy.

Eysenck's 1952 Psychotherapy Effectiveness Study

In 1980, Mary Lee Smith, Gene Glass, and Thomas Miller published a comprehensive summary of hundreds of experiments that utilized random assignment to compare treatment and control conditions in psychotherapy. Their findings demonstrated that psychotherapy is generally highly effective, revealing that approximately $$80\%$$ of participants receiving treatment showed greater improvement than the average participant in a control group.

Smith, Glass, and Miller's 1980 Psychotherapy Effectiveness Study

What did Hans Eysenck's 1952 study conclude was the likely reason for the observed improvements in both psychotherapy patients and untreated individuals?

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