Many studies use multiexperiment designs where experiments are carried out at different times of semester. When comparing between experiments, the data may be confounded by between-participants effects related to motivation. Research indicates that course-credit participants who engage in research early in semester have different personality and performance characteristics compared to those tested late in semester. This study examined whether the semester effect is caused by internal (inherent motivation of the participant) or external (looming exams, essays) factors. To do this, sustained attention and intrinsic/extrinsic motivation was measured in groups of course-credit (n = 40) and paid (n = 40) participants early and late in semester. While there was no difference in sustained attention between the groups early in semester, the course-credit group performed significantly worse late in semester. The course-credit group also showed a significant decrease in intrinsic motivation with time whereas the paid participants showed no change. Because changes were not seen for both groups, the semester difference cannot be due to external factors. Instead, the data demonstrate that course-credit participants who engage early have high sustained attention and intrinsic motivation compared to their late counterparts, who leave their participation to the last minute. Researchers who use multiexperimental designs across semester need to control for these effects—perhaps by using paid participants who do not vary across semester.