Comment 62 for bug 120672

(In reply to comment #59)
> Please include URLs to those usability studies if you have them.

Maybe not a study per say, but I guess the famous Nielsen got his views from somewhere.
"7. Use positive and active wording for checkbox labels, so that it's clear what will happen if the user turns on the checkbox. In other words, avoid negations such as "Don't send me more email," which would mean that the user would have to check the box in order for something not to happen."

I'm sure I could dig up more...