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.