There has been a study1. This says to me that time travellers are good at cleaning up after themselves.
Via Hacker News2 where there is lots of fun discussion3.
Paul Clarke's weblog - I live in A small town, Kent. Wed and dad to two, I am a full stack web engineer, + I do js / Node, some ruby, other languages ect ect. I like pubs, running, eating, home-automation + other diy jiggery-pokery, history, genealogy, TV, squirrels, pirates, lego, + TIME TRAVEL.