“Oh my gosh, we got pushback,” says Benjamin Hart, a professor emeritus at the University of California at Davis School of Veterinary Medicine. In 2013, a team led by Hart and his wife and collaborator, Lynette Hart, published a study that found higher rates of joint disorders in golden retrievers spayed or neutered before one year of age and of certain cancers in female golden retrievers that were spayed early. It immediately caused an uproar. “This is irresponsible,” Hart recalls critics saying. “You’re looking at just one breed. You can’t generalize.”

So they started looking at other breeds. The Harts have since published two follow-up papers, on Labrador retrievers and German shepherds, also finding an elevated risk of joint disorders but not of cancers after early spaying and neutering. And they have just finished another study, on 35 different dog breeds as well as mixed breeds. The risks of cancers and joint disorders appear to vary significantly by breed and sex, Hart says, with small dogs generally less affected by early neutering.