Group size may have confounding effects on some dependent variables in experiments conducted in a collective setting. Three experiments revealed unexpected effects of group size on self-paced study and test time, although this was not the main purpose of the studies. Experiments 1 and 2 suggest that the study time of female subjects increases with the group size up to five subjects and then decreases as the group size increases to nine subjects (as used in Experiment 2). When we considered the first subject to finish studying within the group, we noted the same relationship. The phenomena were not found with the male subjects. The withingroups variability of the study time was much larger for the female than for the male subjects. Some of the findings from Experiments 1 and 2 were confirmed in Experiment 8, notwithstanding the much smaller number of subjects and a very different experimental setting. The test times were examined and, again, effects of group size were often obtained. We report these findings in order to make experimenters sensitive to possibly confounding effects from group size when carrying out experiments in a collective setting.Running experiments with a number of subjects together in the same room saves considerable time for the researcher. In contemporary research on learning and memory, it is implicitly accepted that the data are not affected by the collective setting. Because of the convenience of computers, it has become common practice to run several subjects together, the actual number depending on how many show up for the session. However, a survey of recent literature revealed a virtual absence of publications in which the effect of group size as a potentially noisy variable on the dependent measures is taken into account. In view of the older studies on audience effects in social learning (for review, see Geen & Gange, 1977), this lack of methodological care and completeness in reporting is rather surprising.The present paper describes three experiments that were not designed for investigating the effect of group size, but in which group size did emerge as a significant factor. In each experiment, it was expected that the main variable (either test expectancy in Experiments 1 and 2 or the experimenter's characteristics in Experiment 3) was not powerful enough to reveal major effects with a small number of observations. In addition, it was not possible to have many observations with the same subjects. Therefore, we decided to collect data from a fairly large number of subjects. To save time, a number of subjects (always from the same sex) were invited at the same time. In all the experiments, the study time was self-paced_ Since there were several groups of subjects in each condition, we had to decide to Requests for reprints should be sent to Gery d'Ydewalle,