In this report, we illustrate the considerable impact of researcher degrees of freedom with respect to exclusion of participants in paradigms with a learning element. We illustrate this empirically through case examples from human fear conditioning research, in which the exclusion of ‘non-learners’ and ‘non-responders’ is common – despite a lack of consensus on how to define these groups. We illustrate the substantial heterogeneity in exclusion criteria identified in a systematic literature search and highlight the potential problems and pitfalls of different definitions through case examples based on re-analyses of existing data sets. On the basis of these studies, we propose a consensus on evidence-based rather than idiosyncratic criteria, including clear guidelines on reporting details. Taken together, we illustrate how flexibility in data collection and analysis can be avoided, which will benefit the robustness and replicability of research findings and can be expected to be applicable to other fields of research that involve a learning element.

Our findings suggest that a smaller (left) hippocampus is not the result of stress/trauma exposure. Furthermore, hippocampal volume does not increase with successful treatment. Instead, we demonstrate for the first time that a smaller (left) hippocampus constitutes a risk factor for the persistence of PTSD.

Negative emotionality is a well-established and stable risk factor for affective disorders. Individual differences in negative emotionality have been linked to associative learning processes which can be captured experimentally by computing CS-discrimination values in fear conditioning paradigms. Literature suffers from underpowered samples, suboptimal methods, and an isolated focus on single questionnaires and single outcome measures. First, the specific and shared variance across three commonly employed questionnaires [STAI-T, NEO-FFI-Neuroticism, Intolerance of Uncertainty (IU) Scale] in relation to CS-discrimination during fear-acquisition in multiple analysis units (ratings, skin conductance, startle) is addressed (NStudy1 = 356). A specific significant negative association between STAI-T and CS-discrimination in SCRs and between IU and CS-discrimination in startle responding was identified in multimodal and dimensional analyses, but also between latent factors negative emotionality and fear learning, which capture shared variance across questionnaires/scales and across outcome measures. Second, STAI-T was positively associated with CS-discrimination in a number of brain areas linked to conditioned fear (amygdala, putamen, thalamus), but not to SCRs or ratings (NStudy2 = 113). Importantly, we replicate potential sampling biases between fMRI and behavioral studies regarding anxiety levels. Future studies are needed to target wide sampling distributions for STAI-T and verify whether current findings are generalizable to other samples.