It’s absolutely true that knowing other people are using the same car makes one (okay me) less likely to trash it. I also try to remember to push the driver seat all the way back in case the next person using the car is taller than me, which is highly likely.
And an unintended consequence of using I-Go (and I would assume Zipcar)–I don’t have to drive the same car every time I need one. This makes certain car-owning friends of mine green with envy. They’re stuck with the car they bought unless they want to spend even more money and rent something now and then. I spend maybe 1/10th of what they do in a year on car-related expenses AND I have a what amounts to a fleet to choose from.
Link to Straight Dope Chicago column: http://chicago.straightdope.com/sdc20091112.php