ABSTRACTThis research investigated the association of two individual difference moderators of psychological phenomena with the time of semester students choose to participate in experiments. The Personal Need for Structure (PNS) scale and the Stein Self‐Description Questionnaire were administered to the students in an introductory psychology class at the beginning of a university semester. Students' responses on these measures were related to their chosen dates of required participation in psychological research. Higher levels of PNS predicted earlier experiment participation and quicker completion of the experiment requirement once started. Personality types distinguished by the Stein Self‐Description Questionnaire revealed that those indifferent or hostile to authority participate in experiments later in the semester than do the curious or more conforming types. Implications of these findings for external validity of social psychological research at different points in the semester are discu