Abstract Complex trauma is a clinical concept developed to address the nature and effects of interpersonal traumatic experiences over the development of children and adolescents. To contribute to the development of ecologically valid diagnostic tools and a better understanding of the effects of complex trauma, an ethogram of emotional regulation and stress response behaviours was proposed and used as the main instrument in the Crossed-Puzzles Game. In total 84 children and adolescents aged 6–14 (50% of children with complex trauma in state protective care) were selected from institutions and the community. Both groups were matched by age, sex, and socioeconomic status. PCA, PERMANOVA, Mann-Whitney’s U , Pearson correlations, and logistic regression analyses were performed. The ethogram’s category ‘Externalising of frustration’ was identified as one of the strongest predictors for the complex trauma group. The ethogram’s validity as a research and assessment instrument from a clinical ethology perspective of complex trauma in psychology was discussed.