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:

According to James-Lange theory physical changes in a person body can happen or occured due to emotions because when an event happened, the person body will often react to it which leads to emotion after the brain interpret it

For someone to have emotion the person body response has to change and the person may have increase in his respiratory system or heart rate.

Which was why Harry decides to do a self-experiment using what he knows about the facial feedback hypothesis.