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Thanks for your answers madamspud and katangel. I have been putting him on the naughty step but he still seems to be continuing with the biting. I talk to him and tell him that he is hurting his sister and I show him the bite , telling him that he has hurt her but still he does it again. He has given her four bad bites now and that is four too many as far as I am concerned. As I said , I know that my three year old winds him up and taunts him as my eldest daughter tells me what goes on when my back is turned. I may only be one room away but I have to come running when I hear the screaming and my eldest daughter fills me in on what has happened , accompanied by wailing from my three year old and a petted lip from my son.
I have been talking to my three year old too and telling her that my son bites when he gets frustrated and/or angry because he can't communicate with her properly to tell her not to pinch his toys but I fear that because of the reaction he gets when he bites her , ie , her screaming , releasing his toy and running away from him , then he knows that he gets his own way by behaving like this and I must break this cycle of bad behaviour. I'm just not sure that the naughty step is enough of a deterrant.
Earlier today he bit my daughter again and I immediately put him on the naughty step. I comforted my daughter and deflected any attention away from him. When his two minutes were up , I brought him to his sister and showed him her arm , telling him how he made her cry because it was sore when he bit her. He looked quite ashamed and didn't really want to look at her arm. Instead he had a petted lip.