A famous pen-in-mouth study failed a large registered replication; later work suggests a small effect at best.
What the evidence shows
Strack, Martin & Stepper (1988) had participants hold a pen in their teeth (forcing a smile) or lips (blocking one) while rating cartoons. Those 'smiling' rated the cartoons funnier — apparently their faces fed back into their emotions, and they were unaware of it.
A Registered Replication Report across 17 labs (Wagenmakers et al., 2016) failed to reproduce the effect. The debate continued: some argued the presence of a camera in the replication interfered, and a later large multi-lab study (Coles et al., 2022) found a small facial-feedback effect under some conditions. The honest position is that the strong original claim is not reliably reproducible; any real effect on felt emotion is small and context-dependent, not the robust mood-hack of popular retellings.
Sources
Strack, F., Martin, L. L., & Stepper, S. (1988). Inhibiting and facilitating conditions of the human smile: A nonobtrusive test of the facial feedback hypothesis.
Journal of Personality and Social Psychology, 54(5), 768–777
Participants holding a pen in a smile-like grip rated cartoons as funnier than those in a smile-blocking grip.
DOI: 10.1037/0022-3514.54.5.768 →Wagenmakers, E.-J., et al. (2016). Registered Replication Report: Strack, Martin, & Stepper (1988).
Perspectives on Psychological Science, 11(6), 917–928
Across 17 labs the pen-in-mouth facial-feedback effect on humour ratings did not replicate.
DOI: 10.1177/1745691616674458 →