Yeah actually I had an ex boyfriend that had three little brothers all adopted and they were under the age of four. Two of them had mental problems, were mentally slow, and I don't remember all the details you get it. The third one was totally normal and had no real problems but he acted just like the other ones just because he learned from them. He wouldn't talk right for his age just because they wouldn't, and he basically was waaaay behind on everything because he just somehow got stuck on their level since they were so close in age and spent so much time together I guess.
And you know what I just remembered? I can't remember if this was something someone told me that really happened, or a movie lol but I guess the story I'm thinking of is basically this - Some mother forced her normal little girl to go into an insane assylum for some reason because she didn't want to take care of her or something and she grew up there and even though there was never anything actually originally wrong with her because she grew up around crazy people she acted like a crazy person. I bet I'm way off on this story lol it's soooo old, maybe it was a movie. I wish I could remember, but you get it.