Protection Dogs and BSL
Breed specific legislation is a form of local law that prohibits dogs of certain breeds from living in a given area. Usually it is applied at the city, or county level, less frequently to a whole state. Even the community you live in might have rules that prevent ownership of given breed. Therefore, before getting a family guard dog check into laws, and local rules, that may apply because in some places a dog can be confiscated and put to death because of its breed. The Pitbulls, and kin, are subject to the most discrimination and legal persecution, but others like Rotweilers, Bulldogs, etc. also often included as well.
To help determine if you live in an area where you are prohibited from keeping a certain type of dog look at the links below:
http://www.understand-a-bull.com/BSL/Locations/USLocations.htm
http://www.rott-n-chatter.com/rottweilers/laws/breedspecific.html
http://en.wikipedia.org/wiki/Breed-specific_legislation
Also, check out this book for a different take on the "dog bite epidemic".