This thread made me remember a short story by, IIRC, Isaac Asimov, where humanity was being studied by aliens just like (real? human?) scientists study bacteria, and whenever humanity were to achieve a certain degree of wisdom/technology/etc, they manipulated things to start a war or let lose a pandemic disease.
Of course, it's far better than I make it sound. ^^U If I ever find it again, I'll either transcribe it or post a link, promise. :P