Mentalizing in the moment – An investigation of intrapersonal fluctuations in real-time mentalizing capacity in the context of stress and arousal in an ecological momentary assessment study

• Mentalization significantly intraindividually fluctuates. • Stress is associated and predicts mentalization over time. • Arousal only predicts mentalization to oneself not to others. Although it is assumed that mentalization fluctuates over time, the ability to reflect on and interpret one's own and others' mental states has so far mainly been studied cross-sectionally. The present study tested the usage of a new mentalization short scale in a longitudinal Ecological Momentary Assessment (EMA) design to depict intrapersonal fluctuations in mentalization in real time. In addition, stress and arousal were investigated as possible influencing factors. Eighty-six participants (86 % female) between 18 and 51 years old ( M = 22.93 SD = 5.36) completed a baseline questionnaire and took part in a seven-day EMA phase with six daily assessments on mentalization, stress, and arousal. Mentalization related to oneself (self-certainty) and related to others (other-certainty) correlated moderately positively between baseline assessment and EMA. According to intraclass correlations (ICC), about half of the variance of self-certainty (ICC = 0.46) and other-certainty (ICC = 0.43) occurred within persons. In addition, stress was significantly associated with at the same measurement point and significantly predicted self- and other-certainty to the next measurement point. Arousal was associated with both mentalization dimensions at the same measurement point but was only able to significantly predict self-certainty. Mentalization of self and others intraindividually fluctuates in everyday life and is related to stress and arousal. However, only stress can longitudinally predict both dimensions of mentalizing and is thus the more robust predictor. The prediction of fluctuations and mentalizing deficits by stress offers implications for monitoring and interventions in mental disorders.