>> reporter: researcher mark beckoff who spent years analyzing dog's play, every nip, wag and bark in slow motion says there's more going on. he says he's discovered dogs share with us a sense of fairness and might even know right from wrong. >> you can say they're just playing. but, boy, they are rapidly exchanging signals. >> reporter: watch as this great dane invites a dog half its size to play. this move is called a play bow. it means i want to be your friend. >> there is no doubt about it. okay, saying you are okay. let's play. >> reporter: take a look at this, that bite looks fierce. but this husky is holding back. beckoff calls it self handicapping, a way of playing fair. and what happens when a dog doesn't play by the rules? this little guy tries to push his way into the action, but is left barking on the sidelines. >> animals learn very fast if they violate the rules, they're ostracized.