1. A method of measuring group behavior, comprising: obtaining a signal comprising a sequence of images of a group having a plurality of members, wherein the group is exposed to a stimulus; detecting a response of each member of the group to the stimulus over a period; generating an aggregate response to the stimulus by aggregating the response of each member of the group; and generating a group behavior measurement by determining an amount of synchrony of the response of each member relative to the aggregate response, wherein the step of generating the group behavior comprises: generating a similarity matrix by determining a pair-wise correlation between two members of the group for a segment of the period; and determining an entropy of the similarity matrix to measure the group behavior.