Like many other comedians, Amy Poehler and Seth Meyers routinely use a nonverbal trick which makes it easier for their audiences' to laugh. They constantly slip in and out of expressions - between those of neutral faces - and back to those of partially suppressed smiles. Seeing this fluctuation automatically elevates the mood of everyone who views them thus increasing the likelihood of them smiling too - and therefor laughing as well. There is a strong tendency for us to mirror the facial expressions and body language of others. This sets our emotional bar higher (or lower, depending on the expression) - making it more likely for us to "catch" the humor (or sadness). Humor is an emotional contagion and smiling (or laughing) facilitates it's transmission.
It's also very true that maintaining (or oscillating in and out of) such comedic expressions sets up a positive emotional feed-back loop in those very people displaying it. We make ourselves more joyful. The best mood elevator we have at our disposal is not any drug - but in how we nuance our face (and body) - steering our emotions to more joyful waters.
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