Chaos is contagious

http://scienceblogs.com/notrocketscience/2008/11/the_spread_of_disorder_can_graffiti_promote_littering_and_th.php

This blog entry is about an experiment conducted by Kees Keizer and his colleagues from the University of Groningen. They set up various pairs of situations - one where rules were not being broken, and one where rules were obviously being broken. The first example is of an alley where bikes were parked. The wall had a prominent No Grafitti sign. Flyers for a fictional sportswear shop were attached to each bike. With no grafitti on the wall, 33% littered. When they repeated the situation but with grafitti on the wall, 69% dumped their flyers on the street.

Anecdotally, I'm sure we can recall situations where we've seen this to be true in our daily lives. If either me or Lawrence starts to leave things lying around the apartment, the other has much less incentive to keep things clean.