Taken from another source:
ASPCA | Breed Specific Legislation Following a thorough study of human fatalities resulting from dog bites, the United States Centers for Disease Control (CDC) decided not to support Breed Specific Laws. The CDC cited, among other problems, the inaccuracy of dog bite data and the difficulty in identifying dog breeds (especially true of mixed-breed dogs). The CDC also noted the likelihood that as certain breeds are regulated, those who exploit dogs by making them aggressive will replace them with other, unregulated breeds.
In the aforementioned study,
the CDC noted that many other factors beyond breed may affect a dog’s tendency toward aggression—things such as heredity, sex, early experience, reproductive status, socialization and training. These last two concerns are well-founded, given that:
More than 70 percent of all dog bite cases involve unneutered male dogs.
An unneutered male dog is 2.6 times more likely to bite than is a neutered dog.
A chained or tethered dog is 2.8 times more likely to bite than a dog who is not chained or tethered.
97 percent of dogs involved in fatal dog attacks in 2006 were not spayed/neutered:
78 percent were maintained not as pets, but rather for guarding, image enhancement, fighting or breeding.
84 percent were maintained by reckless owners—these dogs were abused or neglected, not humanely controlled or contained, or allowed to interact with children unsupervised.