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.
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.