With National Mutt Day just around the corner on July 31, it’s the perfect time to explore our favorite mixed-breed companions. A mutt is a dog of mixed or unknown breed heritage, and there’s a common belief that mutts are healthier due to “hybrid vigor.” Hybrid vigor is the improved health of offspring due to the mixing of parental genetic traits. But how true is this claim? I’ll try to answer that question in today’s blogpost.

Do Mixed Breeds Live Longer?

If hybrid vigor makes dogs healthier, then mutts should live longer than their purebred counterparts. I found a number of scientific studies investigating this hypothesis, and the results were mixed!

A U.S. survey of over 20,000 dogs from three private veterinary practices found no difference in lifespan between mixed breed dogs and purebred ones. Meanwhile, research from the United Kingdom’s VetCompass database, which included more than 30,000 dogs, determined mixed breed dogs in England live longer. The same was true for a study of dogs in Korea. Interestingly, the Korean study also showed that mixed breed cats had a shorter lifespan than purebred cats.

So which study is correct? Actually, all three are probably correct. The seemingly contradictory results likely reflect differences in dog populations studied. We know that smaller dogs generally live longer than larger dogs, and certain breeds like the Jack Russell terrier and miniature poodle frequently live to 15 or more years. The longevity differences observed in the studies depends on which breeds were popular in each country and the level of genetic variation in the purebred dog populations.

Overall Health: The Bigger Picture

The Dog Aging Project, a citizen scientist research consortium with over 50,000 dogs enrolled, offers valuable insights into the health differences between mixed-breed and purebred dogs. Veterinary scientists associated with this project compared the medical conditions of the 25 most common breeds with the mutts in the same database.

The results were telling: 22% of purebred dog owners reported their dog did not have any medical conditions, compared to 21% percent of the mutt owners. While this difference was statistically significant due to the large sample size (27,000+ dogs), the practical difference is minimal—hardly noticeable in a veterinarian’s office.

This data doesn’t mean that certain breeds aren’t predisposed to specific medical conditions, but it suggests that overall hybrid vigor may not provide the dramatic health advantages many people assume.

Selecting for Hybrid Vigor

Since hybrid vigor has long been theorized to have health advantages, I thought people might gravitate towards adopting a mutt over a purebred dog in hopes of choosing a healthier pet. But the research suggests otherwise.

Only 1% of Austrian dog owners considered the health of a dog when choosing a dog to add to their family. The top reasons for choosing a particular dog in that study were breed preference or impulsivity, without consideration of health or longevity. An American study confirms this conclusion, finding no correlation between a breed’s popularity and its health or longevity outcomes.

In the end, each family should choose a dog that fits their lifestyle, budget and personality, and any dog can fit that bill if it is the right one.