It is currently legal to smack children in England. The legal defense of Reasonable Chastisement directly contravenes a child's right to respect for human dignity and physical integrity. The harms of smacking are well established and can lead to increased aggression and mental health disorders.
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The use of punishment and physical punishment has repeatedly been shown to be ineffective and results in overall worsening of behaviour, not better.
Long term studies show that people subjected to this type of abuse have a dramatically increased risk of developing mental health disorders and to develop drug and alcohol problems.
Using violence against children can cause them to develop interpersonal and relationship problems. It can also lead to bullying and many more negative outcomes.