Here is a great example of a sincere smile. Notice how Isabelle Giordano's upper and lower eye lids are partially closed. This is the most essential component of true, felt happiness/joy. So often we concentrate on just the mouth, both when we make a smile and when we measure the sincerity of others. Crow's feet may appear or be accentuated (if pre-existing / age-related) as well, but the partial eye closure is the key to a true smile.
Notice also how Isabelle's mouth only shows her upper teeth. The "squared-off" false smile which people will make, pulls their mouths more outward as well as pulling the lower lip down. You should see no more than a fraction of the lower teeth - any more and it's not a true, felt smile.
Also very important is the relaxed appearance of Ms. Giordano's forehead. Smiles where you see wrinkling of the forehead muscles are examples of another very common sign of a false smile. Interestingly, this also tends to be used more in insincere personality types. Negotiators beware!
See also:
Nonverbal Communication Analysis No. 2474: Mayor Bob Filner's False Smile - Insincerity, Body Language and San Diego (Video, Photo)
Nonverbal Communication Analysis No. 2448: Sabine Lisicki Post Match Interview After Defeating Serena Williams Top Teeth Biting Lower Lip & Sincere Smiles (Video, Photos)
Nonverbal Communication Analysis No. 2451: Jennifer Psaki, Egypt and Smiling Out of Context (Video, Photo)
Nonverbal Communication Secret # 2091: Sincere Smiling (a la' Blake Lively) and Laughing - Reduces Anxiety and Heart Rate, Boosts Immune System and Relieves Pain
Nonverbal Communication Analysis # 2312: Bruce Willis' Body Language - Contempt vs. Suppressed Smile vs. Pseudo Smile vs. Sincere Smile
Nonverbal Communication Analysis # 2345: Tiger Woods & Lindsey Vonn - Body Language of True Affection?
Nonverbal Communication Analysis # 2191: Anne Hathaway's Impersonation of Katie Holmes on SNL - "Norming" Katie and Variations on a Theme
Negotiation Body Language Secret # 353: Brad Pitt's Smiling Eyes - Sincere and Insincere Smiles
Dating & Romance Nonverbal Communication Secret # 1010: What Makes the Twinkle in Her Eye?
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