Again, from "Thinking, Fast and Slow." Again, emphasis mine.
Reciprocal links are common in the associative network. For example, being amused tends to make you smile, and smiling tends to make you feel amused. Go ahead and take a pencil, and hold it between your teeth for a few seconds with the eraser pointing to your right and the point to the left. Now hold the pencil so the point is aimed straight in front of you, by pursing your lips around the eraser end. You were probably unaware that one of these actions forced your face into a frown and the other into a smile. College students were asked to rate the humor of cartoons from Gary Larson's The Far Side while holding a pencil in their mouth. Those who were "smiling" (without any awareness of doing so) found the cartoons funnier than did those who were "frowning." In another experiment, people whose face was shaped into a frown (by squeezing their eyebrows together) reported an enhanced emotional response to upsetting pictures - starving children, people arguing, maimed accident victims.
Simple, common gestures can also unconsciously influence our thoughts and feelings. In one demonstration, people were asked to listen to messages through new headphones. They were told that the purpose of the experiment was to test the quality of the audio equipment and were instructed to move their heads repeatedly to check for any distortions of sound. Half the participants were told to nod their head up and down while others were told to shake it side to side. The messages they heard were radio editorials. Those who nodded (a yes gesture) tended to accept the message they heard, but those who shook their head tended to reject it. Again, there was no awareness, just a habitual connection between attitude of rejection or acceptance and its common physical expression. You can see why the common admonition "act calm and kind regardless of how you feel" is very good advice: you are likely to be rewarded by actually feeling calm and kind.
These are lessons that I should fully integrate into my everyday life. All the time I find myself scrunching my eyebrows and pursing my lips, and I'm sure it effects my overall mood. I imagine that I seem to see my work as a curse to be endured, whereas I truly enjoy and relish it... Kahneman leaves me somewhat unsure, though at least he gives a clear remedy.