The term pit bull is often used loosely to describe dogs with similar physical characteristics, and the morphological (physical) variation amongst "bully breed" dogs makes it difficult for anyone, even experts, to visually identify them as distinct from "non-pit bulls". While mixed breed dogs are often labeled as "pit bulls" if they have certain physical characteristics such as a square shaped head or bulky body type, visual identification of mixed breed dogs is not recommended by the scholarly community.
Pit bulls were created by breeding bulldogs and terriers together to produce a dog that combined the gameness and agility of the terrier with the strength of the bulldog. In the United Kingdom, these dogs were used in blood sports such as bull-baiting and bear-baiting. These blood sports were officially eliminated in 1835 as Britain began to introduce animal welfare laws. Since dogfights were cheaper to organize and far easier to conceal from the law than bull or bear baits, blood sport proponents turned to pitting their dogs against each other instead. Dog fighting was used as both a blood sport (often involving gambling) and a way to continue to test the quality of their stock. For decades afterwards, dog fighting clandestinely took place in small areas of Britain and America. In the early 20th century pit bulls were used as catch dogs in America for semi-wild cattle and hogs, to hunt, and drive livestock, and as family companions. Some have been selectively bred for their fighting prowess.
Pit bulls also constitute the majority of dogs used for illegal dog fighting in America. In addition, law enforcement organisations report these dogs are used for other nefarious purposes, such as guarding illegal narcotics operations, use against police, and as attack dogs. On the other side of the law, pit bulls have been used as police dogs.
In an effort to counter the fighting reputation of pit bull-type dogs, in 1996 the San Francisco Society for the Prevention of Cruelty to Animals renamed pit bull terriers to "St. Francis Terriers", so that people might be more likely to adopt them. 60 temperament-screened dogs were adopted until the program was halted, after several of the newly adopted pit bulls killed cats. The New York City Center for Animal Care and Control tried a similar approach in 2004, relabeling their pit bulls as "New Yorkies", but dropped the idea in the face of overwhelming public opposition.
Violent interactions between humans and canines have been studied by the US government, notably the Center for Disease Control and Prevention, as well as academic veterinary researchers. The interpretation of these studies, breed identification and relevance issues, and variable circumstances have given rise to intense controversy.