When you're in a group doing something terribly aggressive and you are not--you don't feel like you're at risk you might laugh and laughter is a signal to other members of your group as a signal of solidarity and what you're doing together.

当你在群体在做具有强烈攻击性的行为时,你不会觉得自己有危险,你会笑,你笑是向其他群体成员传达一个信号,你们团结一致，共同进退。

耶鲁公开课 - 心理学导论课程节选