In decisions involving risk, a tendency for involvement or participation in a group to cause the individual opinions of the group members to change in the direction of greater risk, causing group decisions to be generally riskier than the average of the individual decisions of the participating group members. It was discovered independently in 1957 by the US psychologist Robert Charles Ziller (born 1924) and in 1961 by a US graduate student James (Arthur Finch) Stoner (born 1935), who used quite different methods and who both expected the opposite effect. The finding has been replicated many times and holds true for most decisions involving risk, although dilemmas have been found that reliably produce cautious shifts. It is explained partly by a social comparison effect, based on a cultural tendency for people to admire riskiness rather than caution in most circumstances, as a result of which most people like to consider themselves at least as willing to take risks as their peers, but during group discussion some group members discover that others are riskier than themselves, and they therefore alter their opinions in a risky direction to restore their self-images, causing a risky shift in the group decision. A second explanatory mechanism is the persuasive argumentation effect, which arises from the same cultural tendency to admire riskiness rather than caution, causing group members to be more willing to voice pro-risk than procaution persuasive arguments during the group discussion, resulting in more persuasive arguments in a risky than a cautious direction. Social comparison and persuasive argumentation effects can also explain cautious shifts, and some authorities believe that they can also explain the more general group polarization phenomenon. Also called a choice shift.