Researchers believe that experiencing strong negative emotions such as fear or anxiety will increase people's need to affiliate. To test this idea, the researchers randomly assigned participants to one of two small groups. Members of Group 1 were left for 10 minutes in a room with no windows. While waiting, the lights went out for a brief period of time and the participants could hear screams from another room. Members of Group 2 were likewise left for 10 minutes in a room with no windows, but the lights were left on and no screams were heard. Researchers observed how frequently members of each group started conversations with other members of their group on non-experiment related topics. In this experiment, the independent variable is