We did not discuss causality, nor did we raise the issue of investigating causality by using control groups for patients in the Penn Psychotherapy Project (Archives 1980;37:471-481). These are Dr Loranger's main complaints, which I will do my best to answer.
He is correct that we assumed, but did not prove, that the psychotherapy was a causal agent. In fact, we believe that it was the psychotherapy that had the most important impact. We also readily acknowledge that a small part of the benefits might have been attributable to events in the person's life.
But proving the degree of, or even the existence of, causality is a difficult task—we only know that event A (the treatment) was followed by event B (the outcome). As Dr Loranger and many others have suggested, one approach to the issue of causality would be to compare our treated group with an untreated group.
Luborsky L. Outcome of Psychotherapy-Reply. Arch Gen Psychiatry. 1981;38(9):1070. doi:10.1001/archpsyc.1981.01780340122021