I adopted Josie at about 5 weeks. She wasn't weened yet. And we've had a lot of behavioral problems with her. I've read up on it . . . it's called "feline aggression", from what i can tell. They say that when kittens are with their siblings and their momma, they learn things like, how to "play bite vs. kill bite". If they get too rough, the sibling or momma cat makes a sound, signaling that they're getting too rough. Evidently, there is a lot of learning going on during those first few months. We have problems with Josie being too rough. You could probably search the web for "feline aggression" and find some articles about it.

I may be completely wrong about this, just from my experience researching it.