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The phenomenon that decisions of groups of individuals differ in a systematic way from decisions of individuals in isolation has been docu'mented in a wide variety of experiments. This paper revisits this question with a design soliciting both individual decisions of group members and the groupls joint decision in second'mover contributions in gift'exchange games, and in lottery choices. We examine which group members are influ'ential in the group decision. In gift'exchange games, if group decisions are obtained without deliberation by public voting, there is no shift relative to the median individual decisions, indicating that the social context itself does not change behavior. When deliberation is allowed and no decision rule is imposed on the group, besides the median member, the individ'ual one position away in the selfish direction also becomes influential in the group decision. The findings contradict the social comparison theory and are consistent with the persuasive argument theory from social psy'chology. We demonstrate that a researcher incorrectly assuming that the group decision is a function of the mean individual decision would conclude that there is a selfish level shift. In lottery choices, the median individual decision becomes the group decision with very high probability. Never'theless, if the distribution of individual choices in a given choice problem is asymmetric enough in some direction, since median group members are likely to be in that direction, group decisions are even more tilted in that direction. Our results highlight that the question whether people behave differently in group settings can only be addressed relative to a …