Does psychotherapy homework lead to improvements in depression in cognitive-behavioral therapy or does improvement lead to increased homework compliance?

The bidirectional causal relationships between psychotherapy homework (HW) compliance and changes in depression were assessed in 2 groups of depressed outpatients treated with cognitive-behavioral therapy using nonrecursive structural equation modeling techniques. The data were consistent with the hypothesis that HW compliance had a causal effect on changes in depression, and the magnitude of this effect was large. Patients who did the most HW improved much more than patients who did little or no HW. Ln contrast, depression severity did not appear to influence HW compliance. HW compliance did not appear to be a proxy for any other, unobserved (3rd) variable, such as motivation. Although the causal effect of HW on depression was large, the correlation between HW and depression was small. Some possible explanations, along with suggestions for future studies, are proposed.