A new study from the University of Leeds in the UK finds that children as young as three "can bite with the same force as a dog." No mention about what kind of dog is the reference point. There's a big difference between the bite of a chihuahua and a pit bull. The research was mainly geared toward child safety, showing that kids can bite things into pieces creating a choking risk. But the article caught my attention because of my son.
When he was around two we had a significant problem with him biting. He'd be sitting on your lap, or next to you, and all the sudden he would lunge and bite. And he did bite pretty hard. Finally my wife got tired of it and started biting him back (gently, but enough to send a message). That brought the biting to a halt.