#2 They’re fake smiling
A false smile could be a sign that something is wrong, and you can usually tell when someone is lying to you by looking for one. However, can you identify one? Most people regard a Duchenne grin as a genuine smile since it is connected to genuine joy.
Duchenne smiles are generated by flexing the muscles around the eyes that cause the skin surrounding the eyes to wrinkle. Without the muscles in the corner of the eyes contracting, the person is smiling at you with a fake smile.
And it turns out that, as Cognition & Emotion reported in 2016, people naturally have a more favorable perception of Duchenne smiles. So when it comes to fake grins, trust your own instincts.