Groupthink groups work in secret. They isolate themselves from outsiders, and refuse to modify their beliefs to bring them into line with society's beliefs. They avoid leaks by maintaining strict confidentiality and working only with people who are members of their group. A biased leader who exerts too much authority over the group members can increase conformity pressures and railroad decisions. In groupthink groups the leader determines the agenda for each meeting, sets limits on discussion, and can even decide who will be heard.
Groupthink becomes more likely when the group is stressed, particularly by time pressures.
The Heaven's Gate group experienced such stress, as the arrival of the comment Hale-Bopp and the Christian holy days forced them to come to a decision regarding their assumed transportation. When groups are stressed they minimize their discomfort by quickly choosing a plan of action, with little argument or dissension. Then, through collective discussion, the group members can rationalize their choice by exaggerating the positive consequences, minimizing the possibility of negative outcomes, concentrating on minor details, and overlooking larger issues. The group, then, was ripe for groupthink. Overestimation of the group.