Cheerfulness isn't all bad, though. Forgas says it encourages creativity. Bad moods, however, incite "attentiveness and careful thinking."
How did he figure this out? He asked volunteers to watch films while thinking of either positive or negative events in their lives, in the hopes they'd be put into good or bad moods.
Then, to test their powers of analysis, he had them judge the truth of urban myths and provide eyewitness accounts of events. The results: the grumps made fewer mistakes and were better at communicating.
"Whereas positive mood seems to promote creativity, flexibility, co-operation and reliance on mental shortcuts," said Professor Forgas, "negative moods trigger more attentive, careful thinking, paying greater attention to the external world."
If you see a smile on my usually grumpy face today, it's temporary — I'm just gloating.