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 disse, como podem ver aqui, — a razão de nos rirmos com aqueles risos engraçados no começo, e de eu ter rido quando encontrei os meus pais a rir — é que isso é um efeito comportamental extremamente contagioso.