The Majority of "Pitbulls" Are Mixed Breed Dogs
A recent comprehensive study on the DNA of shelter dogs determined that the vast majority of dogs labeled as "pitbulls" are mixed breed dogs. The study tested the DNA of 919 dogs and found that of the 244 dogs with DNA from pitbull-type ancestry, 98% of the dogs were mixed breed dogs while only 2% were purebred. Moreover, the study also found that the majority (62%) of the dogs with pitbull-type ancestry had less than a 50% DNA concentration from any of the four unique breeds commonly included in the modern pitbull-type category such as the American Pit Bull Terrier, American Staffordshire Terrier, Staffordshire Bull Terrier, and the American Bully. As a general rule, dogs must have more than a 50% DNA concentration of a specific breed to be labeled as a mix of their dominant breed - for example, "Boxer mix" - and if they don't, then they are by definition considered to be mixed breed dogs (without any breed identifiers). Therefore, the study determined that the majority of dogs labeled as "pitbulls" are in fact mixed breed dogs (and not "pitbull mixes") as they have less than a 50% DNA concentration from pitbull-type ancestry or have another non-pitbull type breed as the dominant breed (over 50%) in their DNA. These scientific findings about the DNA of pitbull-type dogs have significant implications for dog bite statistics, medical studies on dog bites, media reports on dog bites, and for breed-specific dog bans (aka breed-specific legislation or BSL) as we discuss below.
DNA Test Results of 244 Pitbull-Type Dogs
239 (98%) of the pitbull-type dogs in the study were identified to be mixed breed dogs by DNA and only 5 dogs (2%) were determined to be purebred.
152 (62%) of the dogs with DNA from any of the pitbull-type breeds had less than a 50% DNA concentration from pitbull-type ancestry and are therefore by definition mixed breed dogs — and not "pitbulls" or "pitbull mixes".
The dogs with DNA from pitbull-type breeds had an average DNA concentration of 43.5% from pitbull-type ancestry (38.5% average in shelter 1 and 48.4% average in shelter 2).
Impact On Dog Bite Statistics And BSL
The conclusions of this study have significant implications for dog bite-related breed data used in dog bite statistics, in medical studies on dog bites, in media reports on dog bites, and especially for breed-specific legislation (BSL). For example, cities with BSL typically impact pitbull-type dogs with more than a 50% DNA concentration from pitbull-type ancestry; therefore, the results of this study indicate that the majority (almost two-thirds) of pitbull-type dogs would not be impacted by BSL if challenged by the owner with DNA evidence. Furthermore, another study determined that 60% of dogs visually identified as "pitbulls" do not have DNA signatures from any of the pitbull-type breeds. The results of these studies again confirm that the breed information in media reports on dog bite-related incidents attributed to pitbull-type dogs cannot be used as a reliable source for breed data as the majority of the dogs reported as "pitbulls" or "pitbull mixes" are more likely to be mixed breed dogs by DNA or have a non-pitbull type breed dominant in their DNA. Unfortunately, unreliable and unscientific breed information in media reports is commonly used as a source for breed data in misleading dog bite statistics and in disputed medical studies on dog bites. Because of the unreliable breed information in media reports and based on the DNA evidence in this study and other studies, we'd fully support mandatory DNA testing for dogs involved in serious dog bite-related incidents as this would finally validate that the majority of these incidents do not involve pitbull-type dogs but instead, mostly mixed breed dogs or other breeds and mixes altogether. The complexities of accurately identifying a dog's breed are well known; however, the data shows that accurately identifying a dog's breed by DNA (for dogs that are impacted by BSL due to their appearance, for dog bite-related incidents, and to support data in dog bite statistics) might be BSL's greatest weakness after all.
The genetics of behavior is so complex ... breed-typing is worse than stereotyping members of our own species. Breed labels would be better dropped altogether.
- Dr. Clive Wynne, ASU Canine Science Collaboratory
Updated: January 20, 2023