This experiment examined the 'time extension' explanation for the social facilitation effect, which is that people eat more as the number of co-eaters increases. Seventy male and 62 female participants ate a lunch consisting of pizza, cookies, and bottled water, alone or in (same-gender) groups of two or four and were given either 12 or 36 min in which to do so. The independent variables were gender, group size, and meal duration. The main dependent variable was amount consumed in the meal. The results showed that male participants ate more than did females, and participants eating the longer meal ate more than did those eating the shorter meal. However, the effect of group size was not significant. It was also the case that the amounts consumed by participants eating in two-person groups resembled one another to a greater extent than did of pairs of participants who ate alone or by participants in four-person groups. It was concluded that the results of the present paper provide strong support for the idea that the effect of group size on intake seen in previous studies is mediated by meal duration.