The "Duchenne smile", which exposes your crow's feet, was once thought to be an authentic and forgery-proof expression of positive emotion, but it is not so.

More What Duchenne Smiles Do, Less What They Express. Eva G. Krumhuber, Arvid Kappas. Perspectives on Psychological Science, June 17, 2022. https://doi.org/10.1177/17456916211071083

Abstract: We comment on an article by Sheldon et al. from a previous issue of Perspectives (May 2021). They argued that the presence of positive emotion (Hypothesis 1), the intensity of positive emotion (Hypothesis 2), and chronic positive mood (Hypothesis 3) are reliably signaled by the Duchenne smile (DS). We reexamined the cited literature in support of each hypothesis and show that the study findings were mostly inconclusive, irrelevant, incomplete, and/or misread. In fact, there is no single (empirical) article that would unanimously support the idea that DSs function solely as indicators of felt positive affect. Additional evidence is reviewed, suggesting that DSs can be—and often are—displayed deliberately and in the absence of positive feelings. Although DSs may lead to favorable interpersonal perceptions and positive emotional responses in the observer, we propose a functional view that focuses on what facial actions—here specifically DSs—do rather than what they express.

Keywords: Duchenne, smile, facial expression, emotion