I think the American Psychiatric Association [http://www.apa.org/pi/cyf/res_punish.html] sums up the arguments against quite succinctly.
Psychologist Elizabeth Gershoff found links between corporal punishment and aggression, anti-social behavior and mental health problems. [source] [http://findarticles.com/p/articles/mi_m1355/is_3_102/ai_89388984]
Researchers from universities in six different countries found children punished with physical discipline had more aggression and anxiety. [source] [http://www.medicalnewstoday.com/articles/33612.php]
Researchers at McMaster's University found people who were spanked as children were "twice as likely to report alcohol abuse, illicit drug use or anti-social behaviour". [source] [http://www.cbc.ca/health/story/1999/10/04/spank991004.html]
I can cite more sources if needed, but they generally repeat the same basic problems: increased risk of anxiety and anxiety disorders, aggression, and substance abuse. And yes, I'm aware not everyone physically disciplined exhibits these problems. That's no excuse.
In the end though, beyond the risks listed above and even beyond moral issues, corporal punishment is simply attacking the symptom and not the problem. Temporarily stopping undesirable behavior through pain and/or fear does nothing to resolve the underlying cause of the behavior. It also does not give the child the necessary understanding of how to peacefully resolve problems as an adult, where frequently one cannot (or should not) use pain and fear to control the actions of others. It's a cop-out, and a child deserves better than that, especially from a parent.
Psychologist Elizabeth Gershoff found links between corporal punishment and aggression, anti-social behavior and mental health problems. [source] [http://findarticles.com/p/articles/mi_m1355/is_3_102/ai_89388984]
Researchers from universities in six different countries found children punished with physical discipline had more aggression and anxiety. [source] [http://www.medicalnewstoday.com/articles/33612.php]
Researchers at McMaster's University found people who were spanked as children were "twice as likely to report alcohol abuse, illicit drug use or anti-social behaviour". [source] [http://www.cbc.ca/health/story/1999/10/04/spank991004.html]
I can cite more sources if needed, but they generally repeat the same basic problems: increased risk of anxiety and anxiety disorders, aggression, and substance abuse. And yes, I'm aware not everyone physically disciplined exhibits these problems. That's no excuse.
In the end though, beyond the risks listed above and even beyond moral issues, corporal punishment is simply attacking the symptom and not the problem. Temporarily stopping undesirable behavior through pain and/or fear does nothing to resolve the underlying cause of the behavior. It also does not give the child the necessary understanding of how to peacefully resolve problems as an adult, where frequently one cannot (or should not) use pain and fear to control the actions of others. It's a cop-out, and a child deserves better than that, especially from a parent.