From an article discussing the internals of Apple, Inc.’s design process:
Norman cites research in cognitive science suggesting that people’s emotions affect the way their minds process information. In his 2004 book Emotional Design, he writes, “Positive emotions are critical to learning, curiosity, and creative thought. … The psychologist Alice Isen and her colleagues have shown that being happy broadens the thought processes and facilitates creative thinking.”
In multiple studies, Isen, a professor of psychology and S. C. Johnson Professor of Marketing at Cornell University, made subjects feel happy through a number of means, including gifts of candy and words or pictures with pleasant associations. The subjects were then asked to perform tasks that measure creativity; over the course of 20 years, Isen and her colleagues regularly found that subjects exhibited much more creativity when they were in a good mood.
And conversely, Norman says, when you’re in a bad mood, when you’re tense, you tend to be less creative–and less patient with the tools you’re using. “Someone in a positive mood,” Norman says, “faced with something that doesn’t work, might say, ‘Oh, I’ll get around it.’ But someone in a negative mood will get frustrated and have a ‘Damn it’ moment.” That’s where design comes in. “Studies tie attractive design to positive attitude,” he says.”