It has been noted that obsessional patients appear to be equally sensitive to ideas regarding the possibility
that they may cause harm by both their actions and by their failure to act (i.e., omission). This observation is
discrepant with findings in non-clinical populations. The cognitive theory of obsessive-compulsive disorder
suggests that it is the very occurrence of intrusive thoughts about potential harm that mediates this effect. In this
study, 22 obsessional patients and 30 non-clinical participants were provided with details of ambiguous situations
and either a negative or neutral intrusive thought pertaining to this situation. Behavioural and emotional responses
to these situations were rated using self-reported measures. It was found that situations including an intrusive
thought about harm were associated with higher intensity behavioural and emotional responses compared with the
same situation when the intrusion was neutral. Obsessional participants scored higher overall; only on the rating of
perceived responsibility was there an interaction between group and item type. These results are consistent with
the idea that the occurrence of an intrusion about harm modifies both obsessional and non-clinical participants'
reactions in ways that suggest obsessionality, and support cognitive theories that emphasize that obsessional
experiences arise from normal processes.