I only barely skimmed the thread so my comments might have already been mentioned...

Seems like a knife is much like a gun when used for self defense. Every situation is different, and you have to make the decision whether it is necessary to pull it. Is your life really in danger or is the guy just looking for some quick cash? And of course, why are you in that situation to begin with?

I think the point of the study was that a knife makes people more cocky and willing to get into bad situations. However, if you are a responsible, intelligent person, I certainly don't think you're safer without a means of defending yourself.