Rarely is there a public figure who projects a sincere smile as often as Kate Middleton. Sure, people can put forth a fair or even a good social smile - but a social smile is no way the equivalent of a sincere one. And the difference is not just on the face - for it reflects the real emotional state of one's psyche.
A sincere (aka Duchenne) smile has the following components:
1. Partially closed eyelids
2. Furrows in the lower eyelids - these are concave-up and dynamic
3. Corners of the mouth directed significantly upwards (not laterally)
4. Upward vectored Cheek muscles
5. Only upper teeth visible (some exceptions, see below)
6. Relaxed, non-contracted forehead muscles
A smile is never sincere unless the eyelids are partially closed - this is an absolute. This characteristic eyelid closure is seen conjunction with the evanescent, concave-up furrows in the lower eyelids and the upward directed cheeks and mouth corners. Yet both plastic surgeons and Photoshoppers will routinely attempt to smooth out this lower lid crease (But such a change is artificial! Don't do it!). Sometimes - with a downward directed head tilt or visual angle (as is seen a bit here), the lower teeth can be seen during a sincere smile (This may also occur during the crescendo into or decrescendo from sincere laughter. That is, with sincere laughter the lower teeth ARE often visible). The vast majority of the time however, when lower teeth are visible - the smile is NOT one of true Joy-Happiness.
There is absolutely nothing in human experience which will ever take the place of sincerity. The Duchess of Cambridge displays a sincere smile a very high percentage of the time (along with other sincerity tells). It is one reason (and I would argue the primary one) why people truly like her - and often ever feel as if they know her.
Nonverbal Communication No. 3546: Carly Fiorina's and Ted Cruz's Awkward Raised Hand Holding
Nonverbal Communication Analysis No. 3544: John Kasich's Honest Anxiety
Nonverbal Communication Analysis No. 3538: Barack Obama, David Cameron, Number 10, Brexit and Body Language
Nonverbal Communication Analysis No. 3526: Neville Chamberlain and Adolf Hitler Meet - the Munich Agreement
Nonverbal Communication Analysis No. 3498: Donald Trump Mocks Chris Christie when criticizing John Kasich
Nonverbal Communication Analysis No. 3476: Is Ted Cruz worried about a Donald Trump Lawsuit?
Nonverbal Communication Analysis No. 3417: Freshman Mistakes and Faux Pas Abound - Debates and Body Language
Nonverbal Communication Analysis No. 3516: Hillary Clinton, Finger Pointing, Fossil Fuels, Bernie Sanders and Lying
Nonverbal Communication Analysis No. 2115: Prince Harry in Afghanistan - Incredulity & Contempt