No one knows where the dog years rule came from, though virtually all dog owners know it. According to that popular myth, every year a dog spends on the planet is equivalent to seven years for a human. So if a dog lives to be 15 years old, she's actually 105 in human years. No one knows where this piece of common knowledge came from, Priceonomics writes, though there are some indications that monks at Westminster Abbey in the 13th century were the first to put forth a similar figure (9:1, in that case).
The problem with this simple ratio is that it's not reflective of reality. As Priceonomics writes, "if this ratio had any truth to it, humans would be capable of reproducing by age seven, and high percentages of us would live to be 150."
Researchers and others who have simply taken the time to think about the ratio have recognized its illogic for decades. In 1953, for example, a French researcher published a more nuanced version of the rule, based on emperical evidence: dogs age 15 to 20 times faster than humans do during their first year of life, but that ratio soon tapers off to about one dog year being the equivalent of five human years, Priceonomics writes.
Things quickly got more complicated than that, however. As most dog owners know, dog life spans are not equivalent. Larger breeds tend to pass away well before smaller ones. As aging researchers calculated, a ten-year-old small dog is about 56 in equivalent human years, for example, whereas a large dog is 66 and a super-big dog is 78. To further complicate things, Preceonomics points out, some breeds, like beagles, demonstrate different aging ratios than their equivalent-sized counterparts of different breeds.
So the take home is pretty clear: the 7:1 ratio is a gross oversimplification of how dogs age. But it will probably be dog's years before that popular myth goes away.