Harry decides to do a self-experiment using what he knows about the facial feedback hypothesis: he is going to smile more to see if smiling causes him to feel happier. Which would MOST likely support the hypothesis that smiling makes people feel happier?

The James-Lange theory

Explanation: The James-Lange theory argues that emotional reactions felt in human stirs from physical interpretation attached to the physiological (relates to the way or manner in which the bodily parts of an individual functions) response which emanates due to an external or environmental stimulus.

In essence, James-Lange theory proposed by William James and Carl Lange opines that physical interpretation of physiological responses determines emotional reaction.

In the scenario above, Harry wants to use physiological response and physical interpretation attached to facial smile will translate into an emotional state of being happier.