This skews study results, making it appear that pets protect against allergies when they actually don't. Although some studies have tried to circumvent this potential bias by stratifying results based on hereditary risk, "until there is a 'randomized distribution of cats trial,'" says Columbia's Perzanowski, "there will always be some chance of confounding by who chooses to own a cat,"—or a dog, for that matter.
So should parents get a pet if they want to minimize their child's risk of developing allergies? "That's the million dollar question," says Litonjua, and the short answer, he says, is no. "If you want to get to get a pet to try and prevent allergies, that's probably not a good reason," he explains. But "if the kids really want the pet—if you want the pet—then go ahead, as long as you're not having any symptoms when you get exposed." Flea bites and poop aversion, of course, don't count.