Up until recently, we thought that one year of human life equaled seven dog years. The origins of that myth are unknown. What we do know is that people have been trying to figure out a way to calculate dog years in human years since the 1200s.

The history of dog years

An early example of this is an inscription at Westminster Abbey that dates back to the year 1268. Its inscription explains that one human year is equivalent to nine dog years. About 50 years ago, scientists lowered that number to seven. William Fortney, a Kansas State University veterinarian, told The Wall Street Journal, “My guess is it was a marketing ploy. It was a way to encourage owners to bring in their pets at least once a year.”

While scientists, veterinarians, and dog lovers have been trying to debunk the dog-years myth, it persists in books, news articles, and the popular imagination. “You can’t really kill the seven-year rule,” says Kelly M. Cassidy, curator of a biology museum at Washington State University, who in her spare time maintains an online compilation of dog-longevity studies.

This one-size-fits-all rule makes a compelling way for people to track their dog’s development. That rule, however, is what precisely changed the “seven-year glitch.”

Flaws with the 7-year-rule

Scientists preferred more nuanced conversions. The typical lifespan of hundreds of canine breeds can range from 8 to 16 years. We have also observed that dogs grow quickly in the first couple of years, with bigger breeds aging faster. We know that two years in human years equals about 20 years of age for large dogs. “Eight years in one breed is not equivalent to eight years in another,” explained David J. Waters, associate director of Purdue University’s Center on Aging and the Life Course.

Scientists and veterinarians aren’t sure as to why big dogs don’t live as long as smaller dogs. Scientists found the same results with other species; with rats, for instance, smaller ones outlived much larger rats.

The seven-year rule persists in many countries, but veterinarians who put it to the test found many problems. The new formula is also based on uncertain numbers. Three main sources that scientist point to include pet insurance records, breed-club surveys, and veterinary hospitals.

Scientists believe the first two may be biased toward longer-living dogs, because owners who belong to clubs and buy insurance may spend more to prolong their dogs’ lives.

Veterinary hospitals may also be biased toward a shorter lifespan because they tend to admit the toughest cases, not the healthiest dogs.

“True numbers are moving targets,” says Jeff Sampson, canine genetics consultant to the Kennel Club in the U.K. “As veterinary medicine improves and more dogs are immunized, fewer die young of distemper and parvovirus today than 30 years ago.”