Objectives: This study investigates the long-term influence of fluctuations around the individual's means of personality functioning (PF) on mental and physical symptoms in a large sample of patients receiving psychotherapy.

Methods: We reanalyzed observational data of 2094 participants (68% female; age M = 39.89 years; SD = 14.20 years) which took part in the Multicenter Effectiveness Study of Inpatient Psychosomatic Psychotherapeutic Treatment (short: MEPP) at three time points. A Random Intercept-Cross Lagged Panel Model (RI-CLPM) was used to examine within-person longitudinal effects between PF (OPD-SQS) and depressive symptoms (PHQ-9), anxiety (GAD-7) and somatic symptom load (PHQ-15). The model was controlled for age, sex, treatment condition and treatment length.

Results: Strong between-person intercorrelations were observed between all investigated constructs. At the longitudinal within-person level PF at hospital discharge predicted somatic symptom load (p < .01), depressive symptoms, anxiety and itself (all p < .05) at follow up, however, PF showed no effect during treatment (p > .05). Furthermore, depressive symptoms at baseline influenced general symptom burden and PF development during treatment (all p < .05) and showed strong effects on anxiety and somatic symptoms at follow up (p < .001).

Conclusions: The results suggest a sustained influence of intraindividual PF deviations on general symptom development after psychotherapy, but also emphasize the effect of depressive symptoms in psychotherapy outcomes.