We used to have terrible problems with our son when he was very young.... he had a phase where he would have temper tantrums for no apparent reason and I'm afraid that on more than one occasion I had to resort to smacking him. I was getting to the end of my tether and was seriously worried that I might really hurt him one day.
Then we met a woman, purely by chance, who witnessed one of his tantrums. She asked if we had tried taking him off artificial colours...... I had no idea what she meant, and she explained about artificial additives and how certain artificial colours - notabley tartrazine (E102) - which could cause behaviour problems in children. We were by now willing to try anything, and were horrified to discover that this additive was present in his favourite drink, orange squash! He used to drink gallons of it.... also various sweets, especially Smarties, had lots of colour additives too.
We explained to him what we were doing, and he accepted it very well..... and within about 2 weeks he was like a different person - the change in him was astounding! We never had any more problems after that, and he knew himself what he could eat/drink and what he couldn't.... if someone offered him some Smarties, he would say No, he couldn't eat them.
I'm not totally against smacking, sometimes it's needed.... but sometimes it seems the answer is totally unexpected. He has since grown to be a very well-behaved and polite young man, of whom I am very proud, and I am really glad we met that lady one day when he was young.