An approach to test for individual differences in the effects of situations without using moderator variables.

The effects of situations may vary importantly across people. If the relevant individual difference variables are known, moderation analyses can test for this possibility. But what if the moderators are not measured or are unknown? We demonstrated how a Highly-Repeated Within-Person (HRWP) design can be used to answer this question, by examining the effect of support seekers' expressions of distress separately for each participant. Although on average, participants' willingness to provide social support increased as a function of support seekers' levels of distress, the opposite was true for some participants; their willingness to provide support significantly decreased as support seekers' expressed distress increased. These findings underscore the importance of examining reliable individual differences in the effects of situations, and show that this is possible without first identifying and measuring individual difference variables that moderate the situation effects. Furthermore, the HRWP design prevents stimulus sampling problems and substantially increases statistical power.