International contributions of his work are a new diagnosis model of
adjustment disorder, the introduction of
complex post-traumatic stress disorder and
prolonged grief disorder to
ICD-11. In PTSD research he developed assessments of socio-interpersonal risk and protective factors: "
Disclosure of trauma questionnaire (Dysfunctional disclosure)", "
Social acknowledgement as victim", and "
Revised Sense of Coherence Scale" that had been translated into Polish, Bahasia Indonesian, and Chinese. Based on these factors he developed the "
Social interpersonal model of PTSD". This model posits that social and interpersonal factors play a more central role than biological factors or memory-related alterations. Currently, the model gets extended into cultural factors relevant to PTSD (e.g. values, scripts). With regard to lifespan consequences of trauma he developed the "Janus-Face-Model of
posttraumatic growth" (together with Tanja Zöllner). This model differentiates previous uniformly positive models of post-traumatic growth. Other models of mental disorder or phenomena development concern, e.g. the "motivational reserve" (parallel to
cognitive reserve) (with S. Forstmeier) of older people which is based on life and learning history resources and is assumed to temporarily compensate for a dementia-induced reduction in intelligence and general abilities. In intervention research, he was one of the early developers of
Internet interventions for posttraumatic stress, prolonged grief disorder, adjustment disorder in German language and other languages. In 2020, he was among the 'Highly Cited Researchers' of the year, recognized annually by
Web of Science for their exceptional research performance. In cultural clinical psychology, two topics were in the foreground: the "cultural scripts of trauma sequelae" and "
historical trauma" according to the concept of
Maria Yellow Horse Brave Heart. For the Global North, these cultural scripts of trauma sequelae are largely congruent with the codified diagnoses of PTSD and Complex PTSD. A parallel study using emic and etic methods, ongoing since 2022, analyzes cultural scripts of trauma sequelae in Switzerland, Rwanda, Georgia, China, and Israel.
Historical trauma is operationalized as individual and especially socio-psychological long-term effects, perpetuated by discrimination or marginalization of victims and their subsequent generations. In this area, he and colleagues are conducting comparative studies of trauma in different regions of the world (e.g.). In 2026, Cambridge University Press published a book by him on "Historical trauma", collecting empirical evidence on existing empirical literature on various contexts (including Native Americans, Holocaust survivors, victims of Stalinism, victims of genocide in the 20th century) following the approach of a multi-perspective culture of remembrance. == Awards ==