That ought to do it, right? Wrong!
Unfortunately, what makes perfect sense to us, means something drastically different to the growling dog.
Dogs growl for a lot of reasons. Not all of them are bad, either. Take a look at Nemo on the left. He's politely telling Talos (but a mere pup at the time) to back off. Perfectly appropriate.
It doesn't look particularly pleasant – Nemo's pearly whites on display for everyone to see. But that's exactly why he's done that! He wants Talos to take note of those teeth. And to back the play down a notch.
Why such a display? Because this visual display is a much kinder, much easier way for Nemo to get his point across. This beautiful display didn't work, though, as I remember. Not 15 seconds after I snapped this picture, Nemo gave a menacing growl and got up and walked away from Talos. Game over!
Here's the trouble with punishing our dogs for growling. Growling is just a symptom. There's a whole lot more going on than "just a growl." Growling is one way that dogs communicate. If we punish them for trying to tell us (or another dog) to back off, we've just dug ourselves a mighty big hole.
Because I'm betting that we, the "superior species," aren't going to take the time to figure out why the dog is growling in the first place. Maybe it's because the child is squeezing the dog too tightly, or because the dog is scared that his toy will be taken, or that he'll not be able to get away from the reach of another dog…
The point is this: your dog is growling for a reason. If you don't stop to figure out why — and to fix the underlying problem — you're setting yourself up for trouble (and very possibly a bite) down the road. So instead of smacking the dog when he growls, examine the situation. Was another dog too close? Did your dog seem scared or nervous? Take a look at his body language: where was his tail when he was growling? Where were his ears? Was he crouched down? Or was he standing very tall? These are all very valuable signals.
Punish a growl and you will only get rid of a very important warning signal. Punish a growl and you're much more likely to get bitten. If you take away your dog's ability to say "back off, I'm a little nervous," he'll then have to go a step further to get you to back off. Instead of a growl, he'll go to snapping. If that gets punished, he'll bark and snap. If that gets punished, your dog has no choice but to bite.
Your dog has no choice but to bite.
That doesn't even make sense, does it? It may not be intuitive to us, but it's the truth. And if you take the time to look at why your dog is growling, and then use dog-friendly training to help your dog over his fear or dislike, you're much more likely to have a happy, pleasant dog.
So while your first reaction may be to correct your dog for growling, I urge you to smack yourself instead. Smack yourself and then take a good hard look at the entire situation and figure out how you can better equip your dog to handle the situation better. (Hint: the solution may be to avoid that situation completely until you talk to a positive reinforcement trainer who can help you make your dog more comfortable.)