Something that Robert Provine has pointed out, as you can see here, and the reason why we were laughing when we heard those funny laughs at the start, and why I was laughing when I found my parents laughing, is that it's an enormously behaviorally contagious effect.
Robert Provine on osoittanut, että syy naurullemme, kun kuulimme hassuja nauruja, ja minäkin nauroin vanhempieni naurulle on se, että nauraminen tarttuu.