Not sure if there is any scientific basis to extend this study to productivity, but people eat less when they can see how much they've already eaten.  When there were visual cues (an un-bussed table) of how much food study participants ate, they continued to eat less.

This makes me wonder:  If we can see how much work we've already done (a long timesheet, for example), are we less likely to do more?  The same goes for a messy desk.  If we are surrounded by cues of work we've done, do we work less?