Anecdotal evidence would support the statement that laughter is contagious. Now there is mounting scientific evidence.
According to Sophie Scott, a neuroscientist at the University College London, “We’ve known for some time that when we are talking to someone, we often mirror their behavior, copying the words they use and mimicking their gestures. Now we’ve shown that the same appears to apply to laughter, too — at least at the level of the brain.”
Researcher Robert Provine has found that laughter is 30 times more frequent in groups compared to private settings
So chances are, if you overhear two people laughing, you may find yourself smiling.
Want to test? Though not scientific, odds are you will find yourself laughing along with this video.