Patient's pre-treatment personality characteristics and interpersonal predispositions are known to predict outcome of psychotherapy. In order to choose an optimal treatment it is essential to know which of these psychological suitability factors predict different outcome in short and long-term therapy. In the Helsinki Psychotherapy Study the role of suitability factors between short-term and long-term therapies was studied.

A total of 326 outpatients aged 20-46 years and suffering from depressive or anxiety disorders were randomly assigned to short-term therapy (short-term psychodynamic psychotherapy or solution-focused therapy combined) or long-term psychodynamic psychotherapy and were followed for 3 years. Psychiatric symptoms were assessed with the Symptom Check List, Global Severity Index (SCL-90-GSI) and psychological suitability factors with a 7-item suitability assessment scale.

Patients with good or moderate values of psychological suitability factors gained more from short-term than from long-term therapy during the first year of follow-up. Among patients with poor values of certain suitability factors, long-term therapy appeared more effective than short-term therapy during the third year of follow-up. When combining two individual suitability factors four patient groups could be found: patients who gained faster from short-term therapies, patients who gained equally from both short and long-term therapies, patients who gained only from long-term therapies, and those who gained neither from short nor long-term therapy.

Patients with good or moderate values of suitability factors can be successfully treated with short-term therapy, whereas patients with poor values need long-term therapy or some other treatment to recover. More research is needed to verify these findings.