Zajonc, R. B., Emotion and Facial Efference: A Theory Reclaimed, Scince, 1985, 288, 15-21.

Basically, the article is about a previous theory by Israel Waynbaum that facial expressions precede (and cause) emotional feelings. Zajonc compares this unknown theory to Darwin's theories and propose it as a promising basis for a comprehensive theory on emotions.

Previous thoughts (from Aristotle to Darwin) saw facial expressions as the result of internal emotional states. Facial expressions were seen as pre-warning of emotional responses by others. However, why do humans need such non-verbal communcation and complex facial muscles when we have language?

Waynbaum felt that the the face muscles can constrict facial arteries and veins and thus somehow control cerebral blood flow, which is know to be important in emotions.

Darwin's Theories on Emotions
Darwin tried to extend his theories on evolutions of structures to behavior. He felt that behavior also evolves, and concluded from the universality of many facial expressions (sadness, happiness, etc.) that such behaviors also evolved from lower life forms.

His theory had three principles: 1. Facial expressions are "serviceable habits" that helped the organism react to sensations and internal states. 2. For many actions there are anti-thetical reactions caused by opposite stimulus conditions. 3. That acts like tears, trembling, and rage are caused by the nervous system.

These principles don't give us much clue as to how facial expressions are created.

Waynbaum's Theory on Emotional Expression
He noticed that 1. The blood supply of face and brain come from the same source (carotid artery). The cerebral blood supply must be stable. 3. The face has many, many muscles. especially around arteries. Why?

His theory is that emotional states cause changes in cerebral flow which cause disequilibrium. To bring the system back to equilibrium, the facial muscles constrict or increase facial blood flow by relaxing or contracting.

He also asserted that elation follows the smile, not the opposite. The blood flow changes caused by contracting the facial muscles in the smile alter cerebral blood flow and cause an emotional change. He extends this reasoning to account for all kinds of other bizarre facial habits associated with emotions -- wrinkled forheads, rubbing one's eyes, hand on forehead, pulling earlobes, licking lips, etc.

Had Waynbaum worked today

If he worked today he would have known that facial and brain blood flow interdependency is limited, and the brain gets blood from more than just the carotid artery, and cerebral blood flow is also regulated by non-musculatory methods.

However, blood recirculation is more readily controlled by muscles, and so Waynbaum's theories may work if we consider rate of blood draining (rather than inflow) as a controlling mechinsm for emotions.
Waynbaum (if working today) may have also drawn a connection between blood flow and brain temperature (regulated by temperature difference between arterial and venous blood to and from the vein. We known that different emotional states show different brain temperature distributions, which affect neurotransmitters.

Interestingly, migraine sufferers make an unusual number of facial movements, and also carotid flow increases during a headache.

Tests of the Vascular Theory
We know now that drugs that change cerebral blood flow can cause emotional changes, and that profound emotional changes are found with cerebral vascular disease and stroke. Other studies have proven the stability of cerebral blood flow.

But which comes first, the emotion or the facial expression? Possibly a brief initial cognitive state produces a facial expression which then modifies cerebral blood flow causing changes to the brain that produce the full emotional response. Zajonc states that:

"The entire emotional process can therefore be conceptualized as being treiggered by an internal sensory or cognitive event that leads to peripheral muscular, glandular, or vascular action that in turn results in a change of the subjective hedonic tone."

In this light, facial movements associated with emotions are no different than sneezing or yawning. They all have biological bases.

Empathic responses may be explained that seeing someone else make an expression and then mimicking it instinctively will give one the similar (though probably more limited) response.

Theoretical Consequences of Vascular Theory
Blushing

Waynbaum felt that blushing is caused by massive blood flow in the caotid artery (ostensibly for reaction to a negative stimulus) but that have no apparent output (fleeing, fighting, etc.) . So the blood ends up in the face.

Pallor could also be explained by the act of using facial muscles to restrict facial flow making more blood available for the brain.

One study where subjects made facial expressions without any emotional stimulus. The researcher found that for example expressions of disgust lowered heart rate, but expressions of sadness increased heart rate. It appeared that facial changes did affect physiology.

It's also possible that facial shape and resulting abilities at blood flow control may affect peoples general dispositions (failure to make a genuine smile may affect their personality disposition).

If Waynbaum's theories are true, than biofeedback on facial muscles should help regulate cerebral blood flow and thus regulate emotional states.