http://www.apa.org/monitor/2012/04/spanking.aspx
Physical discipline is slowly declining as some studies reveal lasting harms for children.
Many studies have shown that physical punishment — including spanking, hitting and other means of causing pain — can lead to increased aggression, antisocial behavior, physical injury and mental health problems for children. Americans’ acceptance of physical punishment has declined since the 1960s, yet surveys show that two-thirds of Americans still approve of parents spanking their kids.
Physical punishment can work momentarily to stop problematic behavior because children are afraid of being hit, but it doesn’t work in the long term and can make children more aggressive, Graham-Bermann says.