A new scientific study shows that people who are in good moods tend to have unreliable memories. According to the press release, researchers at the University of New South Wales "put different subjects in a positive (happy) or negative (sad) mood state." (Creepy!) After the subjects experienced "eyewitness events" such as a staged purse-robbery, their recall was rated. The individuals in negative moods provided more accurate accounts of what happened.
""The finding makes sense in evolutionary terms," says Professor (Josephn) Forgas. "Animals that are wary of their environment are more likely to perceive threats to their survival."
In another experiment, subjects were asked to write an argument supporting a specific proposition. Apparently, grumpy individuals expressed better critical thinking and communication skills. Link