After Duchenne, several researchers used these ideas to research smiling. For years it was believed that the Duchenne smiles were the ones that were seen as genuine, that it was not possible to “fake” a smile, because up to 80% of people can’t consciously control the muscles around the eyes that make them crinkle. Why all the interest in whether a smile is real or fake? Because people are quicker to trust and like other people who are showing what is believed to be genuine emotions rather than fake or contrived ones.

https://www.blog.theteamw.com/2011/03/24/100-things-you-should-know-about-people-89-you-can-tell-if-a-smile-is-real-or-face-more-accurately-with-video/