Every dog owner is familiar with the concept of “dog years.” It’s the idea that for every year of human time, dogs age seven years. If your dog is two human years old, it’s actually 14 in dog years, or so the idea goes. But just how accurate is that long-held belief? Not very, according to science.

A new study by researchers at the University of California, San Diego examined the genetic ages of dogs, specifically focusing on an age-related change to DNA called methylation. In humans, methylation occurs more steadily than it does in dogs, and using this method to judge the genetic age of dogs reveals something very special about man’s best friend.

The study used DNA samples from over 100 Laborador retrievers and matched the age-related DNA changes to data from over 300 humans of all different ages. By drawing links between the methylation of DNA in both species, the researchers plotted a chart to help us understand how dogs actually age.

As it turns out, dogs age much more rapidly than humans immediately after birth, but that aging eventually slows down. By the time a dog is two years old, their DNA is more similar to the DNA of a 40-year-old human. However, this 20-to-1 aging ratio is only true for the first couple of years, and the rate of genetic aging gradually tapers off.

Eventually, by the time a dog reaches the age of 10 human years, its DNA is most similar to that of a human nearing the age of 70. This is similar to the 7-to-1 age ratio that people often think of as “dog years,” but it’s only true if measured over a large dog’s full lifespan.

It’s also important to note that the study only looked at one breed of dog. Dogs of different sizes have different life expectancies, with larger dogs tending to live shorter lives than smaller breeds. The aging rates of different breeds could vary dramatically from Labradors, but determining that would require another round of research.