pitbull/lab mix aggression

Question
Hello-
We adopted a pittbull/lab mix in fall 2005. She was 14 months back then and she came from one of PA rescue places working with Petco. She has been a wonderful companion and enjoys chasing squirrels in the backyard. a year after we adopted her, she attacked a runner when my wife was walking her. It wasn't serious and the runner being a neighbor wanted to confirm that she was vaccinated, which she was. In the past 18 months she has had two more attacks both on friends. The last one was actually in our house and came out of nowhere, there was no threat or any unusual behavior. I read on the web that dog temperament shows up at age 3-4. I am wondering if her real temperament is starting to show. Of course I am concerned about thissituation let alone the devastation with my daughter after I told her that we have to do something responsible about her including returning her to the shelter. I wanted to get an expert opinion on this matter. thanks much.

Answer
It is unusual for Pits to be aggressive to humans, but if she is, then the description of "came out of nowhere" is more apt to apply to these dogs than to other breeds.  Their warnings are extremely subtle, unlike some dogs that will be more obvious with their muzzles, eyes, or growls.
This is a situation that I would want you to consult a qualified DVM or Ph.D. level behaviorist about, because there may be a medical reason for the behavior that you need to rule out first, or it may require a combination of medication and training.  You can find one at the Animal Behavior Society web page, or at IAABC's page.  For now, I'd suggest that you muzzle the dog when she is going to be in the vicinity of non family members.  But, you need to acclimate her to the muzzle so that she doesn't think of it as a negative thing.  The Morrco web page has some instructions and you can buy a reasonably priced Italian basket muzzle there, too.