The American Staffy Bullmastiff is not a purebred dog. It is a cross between the Bullmastiff and the American Staffordshire Terrier. The best way to determine the temperament of a mixed breed is to look up all breeds in the cross and know you can get any combination of any of the characteristics found in any of the breeds in the hybrid. Not all of these designer hybrid dogs being bred are 50% purebred to 50% purebred. It is very common for breeders to breed multi-generation crosses.