People used to think it was necessary to "spank" adult members of the community, military trainees, and prisoners. In some countries they still do. In our country, it is considered sexual baiting if a person over the age of 18 is "spanked".

Firstly it is because the buttocks are so close to the sex organs, anal region, and so linked in various ways to sexual nerve centers and striking them can trigger powerful and involuntary sexual stimulus in some people. There are numerous physiological ways in which it can be intentionally or unintentionally sexually abusive, but I won't list them all here. One can read the testimony, documentation, and educational resources available from the website of Parents and Teachers Against Violence In Education .

Child bottom-slapping vs. discipline

Child bottom-slapping/battering [euphemistically labeled "spanking","swatting","switching","smacking", "paddling",or other cute-sounding names] for the purpose of gaining compliance is nothing more than an inherited bad habit.

Its a good idea for people to take a look at what they are doing, and learn how to discipline instead of hitting.

I think the reason why television shows like "Super nanny" and "Dr. Phil" are so popular is because they precisely depict what many people do.

There are several reasons why child bottom-slapping isn't a good idea. Here are some good, quick reads recommended by professionals:

Plain Talk About Spanking
by Jordan Riak,

The Sexual Dangers of Spanking Children
by Tom Johnson,

NO VITAL ORGANS THERE, So They Say
by Lesli Taylor M.D. and Adah Maurer Ph.D.

Just a handful of those helping to raise awareness of why child bottom-slapping isn't a good idea:

American Academy of Pediatrics,
American Academy of Child and Adolescent Psychiatry,
American Psychological Association,
Center For Effective Discipline,
Churches' Network For Non-Violence,
Nobel Peace Prize recipient Archbishop Desmond Tutu,
Parenting In Jesus' Footsteps,
Global Initiative To End All Corporal Punishment of Children,
United Nations Committee on the Rights of the Child.

In 26 countries, child corporal punishment is prohibited by law (with more in process). In fact, the US was the only UN member that did not ratify the Convention on the Rights of the Child.