Board Thread:Speculation House/@comment-14138255-20141031002615/@comment-25555436-20141122194510

As far as we know. But what do we actually know about the Grimm? That they are attracted to bad emotions, can live hundreds of years, and can put experience to good use. And possibly have no soul. But other than that, we know pretty much nothing about them.

And the WoR 3 said how studying the Grimm has been pretty much impossible as they tend to die when captured, for some reason, and even if captured they are very "dangerous". So it's possible most of what people in Remnant know about them it's pure speculation.