Well yes, you could? That's why science gets peer reviewed. If it's not something that can be reproduced it won't pass. And psychology is difficult since there's so many factors that can change, which brings back my earlier point, facts can change. :)
Plate tectonics wasn't discovered until recently so before the 60's, it was a fact that continents didn't move. Then it was discovered that they do actually move, and now it's a fact that they do.
This is not related. You're giving a list of gang violence, this is about terrorism.