The use of cognitive reappraisal (CR) has been linked with improved emotional wellbeing in populations with a history of traumatic stress. Whilst research suggests that the extent to which individuals master CR (CR ability) moderates the relationship between depression and stress, studies have not attempted to improve CR ability in individuals experiencing stress due to everyday events or test for potential health benefits of this intervention. Past experimental studies using CR have largely employed group designs in which the effects of intervention are averaged across participants, leading to potentially valuable information being disguised. To this end, this study employed a single case experimental design to investigate the impact of repeated use of CR on affect, perceived stress, and depression in a female adult sample with high stress. The study also included an aspect of helping behaviour in the intervention to investigate whether there are additive benefits to using CR for self and to help others compared to using CR for oneself only. Twelve adult females were recruited from the community (university staff and students) to take part in the study lasting 21 days. Daily measures were collected over the course of the study and pre-post study measures were taken at baseline, CR intervention, and follow-up stages. At the beginning of the intervention phase, participants were randomly allocated to one of two groups. Group 1 completed 10 days of the daily CR Task whereby daily stressors were described in writing, reappraised and then described again giving the event a newly acquired meaning. Group 2 completed five days of the CR Task followed by five days of using CR to help reappraise written accounts of daily stressful events written by others (CR Helping Task). At the group level, using randomisation tests, no significant improvements in emotional affect and daily stress were found in response to the intervention. At the individual level, using the reliable change index, depressive symptoms decreased reliably in three out of five participants for whom a decrease was possible and for whom CR Ability increased with the intervention. Perceived stress decreased reliably in five out of 10 participants for whom CR Ability increased. Finally, changes in depressive symptoms did not differ between groups, but, contrary to expectations, perceived stress decreased reliably in a larger number of participants in group 1 compared to group 2. The CR intervention showed promise as a feasible short-term stand-alone intervention and demonstrated the utility of targeting specific aspects within psychological care to clarify mechanisms of change and theory. Further research is needed to explore how to optimise the intervention, particularly in terms of length and the design of the CR Helping Task
