Maybe you confused with the mentioning teeth by itself, but its normal for cultural anthropology to try taking physiology, language, healthcare e.t.c as a whole if it giving any clues.
Talking about conscious we should consider that it is one of the reflexes "tamed" by human. Reflexes have its own specific muscle tension/relaxing patterns. You can see reflexes in clean form when looking on babies. Than human learning some communication norms and codes just imitating what it see around. Than he can use trained gestures, sounds e.t.c unconsciously, just little cortex fire and we performing it. We can train to suppress reflexes this way.
Imagine, we have salute gesture like scratching a nose. The muscle tension and motional pattern will be a little different from the case we really have an itchy nose.
Or you have a cough or just imitating cough sound. It's easy to separate. But if you spent time training imitate cough naturally it will look and sound naturally.
So we can differ several cases: Human smiling but suppressing it. Human smiling or well trained or self-trained to smile "sincerely". Human imitating smile of request.
And i behold that Russians: imitate smile on request very rough and this mean deficient practice of imitation. have suppressed smile relatively as often as other ones. smile noticeable rarely.
So we really have rare-firing reflex. What the reason? This is separate and interesting question.
This reminds me of stories I've heard of Japanese office workers bowing while talking over the phone to their boss. It's said that your boss will be able to tell if you aren't bowing, and you will come across as insincere.
Very few of humanitarian science publications is translating from Russian, on the other hand things that opinion journalism is revealing often weird and politicized.
If you want some kind of overview of Russian specific, writers done this work better than scientists through generalized characters and situations. Reading cultural researches you'll be surprised by amount of fictional references.
I can recommend few for a not-dry reading.
N.Leskov "The Tale of Cross-eyed Lefty from Tula and the Steel Flea" http://www.auburn.edu/~mitrege/russian-culture/lefty.doc This is short, partially based on actual events story about XIX century engineer. Its perfect crystal describes difference between russian and western mentality and engineering. I think its touching the russia-west problems not yielding C. P. Snow lectures about western intellectual life )
I.Ilf, I.Petrov "Twelve chairs" http://lib.ru/ILFPETROV/ilf_petrov_12_chairs_engl.txt about "entrepreneur" life in early XX. It's full of killing precise WTF persons and situations, that still filling everyday life.
The conclusion about different culture code and semantic is right.
But there is more some interesting things. Russians have really poor toothcare, and this is joined with non-smiling bidirectionally. There is nothing abnormal for Russian to have couple of missing teeth. I've seen a really rich people (for example owner of Rolls Royce fleet) with just a couple of yellow teeth left... man, you should never smile with this.
Also there is a lot of mimic signals except mouth area muscles tension, especially orbicularis oculi tension, that subconsciously telling that we are smiling. I've noticed that this secondary features is also less often for Russians relative to west and east Europeans. This is also true for voice tone and other subconscious traces of smile. So the low-level mental trigger for smile not firing at all.
So i can assert that this is not about smiling, its about more deep communicational norms like, literally, answering "sorely" on "how are u?", that means "i'm complaining so i'm sincere to you and trusting you", the essence of social stroking is different. Yip, assumptions that the Russians is same in everything but the culture codes is wrong, we are pretty depressed nation.
Anyway, i see that norm is changing nowadays along with life quality increasing, European culture codes expansion, and toothcare, heh.