According to article in the NY Times titled A Feel-Good Theory by Daniel Goleman "putting on a sad face or a smile directly produces the feelings that the expressions represent, according to a new theory of how emotions are produced." According to this research, a smile "affected the temperature of blood flowing to the brain, providing a possible, although disputed, mechanism for regulating emotions." And, smiling at others will typically result in them smiling at you too, which results in two people that are now smiling. So even when you aren't feeling it, smile!
