While lots of research on smiles triggering happiness was performed in the last half-century, that spurt of interest was actually a renewed interest in the topic. The theory goes back to Darwin, who proposed in the 19th century that facial expressions didn't only reflect emotions, but also caused them.
A lack of good evidence for how that happened put the theory on the back burner for many years. But in the 1980s, some interesting studies on the physiology of smiling brought it back into the consciousness of the psychology field. One study found that when subjects contorted their faces to indicate fear, their body temperatures increased and their pulses sped up. Dr. Zajonc's research took this observation further, into a full-fledged proposal for why a smile might trigger happiness. It basically goes like this:
When the temperature of any body part changes, the chemical activities connected with that area also change. Therefore, when facial muscles are activated in an expression, the biochemical processes associated with those areas of the face are altered according to their temperature change. And research suggests that a cooler brain creates good emotions, while a warmer brain produces negative emotions [source: Goleman].
Zajonc points to the part of the body called the internal carotid artery, which is the "pipe" that delivers the majority of blood to the brain. This artery flows through an opening called the cavernous sinus, which contains lots of facial veins. When someone smiles, causing certain facial muscles to stretch and tighten, veins are constricted. This would cut down on the blood flowing to the cavernous sinus, which in turn would reduce the amount of blood flowing through the carotid artery to the brain. Less blood volume means the temperature of that blood drops.
When that cooler blood gets to the brain, brain temperature would drop, too, triggering a happy feeling. The theory works in reverse, too: Zajonc says that when the muscles involved in a frown tighten, the result is increased blood flow to the cavernous sinus and, by extension, a warmer brain.
So, if Zajonc is right -- and not everybody thinks he is, but it's an interesting possibility -- does that mean you could avoid sadness for the rest of your life by faking a smile?
Definitely not. Even proponents of the theory don't suggest that smiling can make unhappiness go away. The theory basically states that in a state of emotional neutrality, putting a smile on your face can tip you in the direction of a positive feeling.
So don't walk into a funeral and make everybody smile as big as they can. You'll look insensitive, and it probably won't make anyone feel any better.
For more information on smiling and emotion, look over the links on the next page.