Facial expressions of emotion—such as the joyful smile you might display when encountering a friend or your angry frown when being cut off in traffic—are powerful social signals that are able to evoke emotional reactions in others. Emotional expressions, however, can also profoundly influence the expresser, changing the way they feel and perceive the world.

This bidirectional relationship between emotional expression and experience was first hypothesized by Charles Darwin. The facial feedback hypothesis (FFH) was formalized in the 20th century. It proposes that activating one’s facial muscles can generate new feelings or modulate one’s ongoing emotions, as well as change the way others’ emotions are perceived. Indeed, hindering people from spontaneously mimicking others’ emotional expressions (called “facial mimicry,” something we all tend to do, believe it or not) also diminishes their ability to recognize them. The reason is that information about the state of muscle activation or relaxation is constantly fed back from the face to the brain (Wood et al., 2016).

To test these ideas scientifically, various methods have been used to either activate facial muscles—and thus generate the corresponding emotion—or instead inhibit them and, therefore, prevent or reduce spontaneous expression, including facial mimicry. For example, participants report feeling happier when posing a smile and less happy when posing a frown (Coles et al., 2022; Laird, 1974). Expressing an emotion in the face can also change how images or videos are perceived. For example, cartoons are perceived as more humorous when smiling than when frowning (Laird, 1974).

Another often-used method consists of asking participants to hold a pen or chopstick either sideways between the teeth, without touching it with the lips (which creates a smile-like expression), or lengthwise between the lips without using the teeth (which makes smiling nearly impossible). Using the pen-in-mouth technique, it was found, for example, that smiling, compared to frowning, makes cartoons appear funnier (Strack et al., 1988) and lowers the threshold of perceiving happiness in faces and bodies (Marmolejo-Ramos et al., 2020).

Further important methods involve studying individuals who have some form of facial muscle paralysis, be this desired and short-lived, as after Botox injections (Davis et al., 2010), unwanted and longer-lasting, e.g., due to Bell’s palsy (Korb et al., 2016), or even congenital due to Moebius syndrome or other forms of inborn facial paralysis (Schiano Lomoriello et al., 2023).

Using the techniques mentioned above, as well as other similar ones, scholars have amassed considerable evidence supporting the FFH’s claim of an interaction between one’s facial muscle activity, on the one hand, and one’s felt emotion and perception of the world on the other hand (Wood et al., 2016). This work is relevant not just from a scientific point of view but also because it can help improve well-being. Indeed, “laughter yoga” has been inspired by it, and research in this domain may help improve emotion recognition in groups with social interaction difficulties, such as autism.

Nevertheless, the FFH has remained highly controversial, as its related effects tend to be weak and variable (Coles........

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