They may initially might not have been aggressive. But if the humans provides a certain type of environment, is entirely plausible that pitbulls have developed a specific trait that is now being passed down to their offspring. I do not understand why pitbull owners get mad when people say this. Its basic common sense. Parents pass traits to their offspring. If pitbulls are being breeded for violence, then these traits may get passed down to their offspring more than, say, a yorkie. It doesn't mean its the pitbulls faults. It's entirely the humans fault. Hopefully you get enough kind owners to breed this trait out of them. On the other hand, a pitbull can have an aggressive trait, but if provided love from the right environment, that trait may never become manifested (similar in humans).