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 a remarqué, comme vous pouvez le voir ici, ce qui nous fait rire, quand on entend d'autres rires, comme tout à l'heure, ou comme moi et mes parents, c'est l'effet comportemental de contagion du rire.