Oh. Right - I'll go back to "be cautious", then, and interpret explodingbat's comments as relating to well-mannered, well-trained dogs specifically.

-ANY dog can be psychotic if it isn't trained properly, or if it was bred by a careless breeder focusing on appearance rather than temperament. ANY DOG. On the flip side of the coin, a well-trained, well-mannered dog can easily come in -any- size.

I'll believe it - I've met nice dogs of several different breeds. It's best to judge on a case-by-case basis anyway.